Sprinting as a clinical tool for the prevention of exercise-mediated

hypoglycaemia in Type 1 Diabetes Mellitus

Vanessa Anne Bussau

Bachelor of Science (Honours)

This Thesis is presented for the degree of Doctor of Philosophy at the University of Western Australia

School of Sport Science, Exercise and Health

2015

ii

Statement of Candidate Contribution

The work involved in designing and conducting the studies described in this thesis has

been carried out primarily by Vanessa Bussau (the candidate). The thesis outline and

experimental design of the studies was developed and planned by the candidate in

consultation with Professor Paul A. Fournier (the candidate’s primary supervisor) and

Professor Timothy W. Jones (co-supervisor). All participant recruitment and

management was carried out entirely by the candidate, along with the actual organisation,

implementation and performance of the experiments. In addition, the candidate was

responsible for all data analysis and original drafting of the thesis and peer-reviewed

publications. Professor Paul Fournier (and Dr Luis Ferreira) have provided feedback for

further drafts and completion of the thesis and manuscripts.

Signed:

Vanessa Bussau Paul Fournier

(Candidate) (Supervisor)

iii

Abstract

Despite the numerous physiological and psychological health benefits of a physically

active lifestyle for individuals with type 1 diabetes, the risk of hypoglycaemia increases

both during and after exercise. It is important to note, however, that not all types of

exercise result in an elevated risk of hypoglycaemia. For instance, prolonged high-

intensity aerobic exercise in these individuals result in an increase in glycaemia during

and after exercise. This raises the intriguing possibility that this type of exercise might be

beneficial if adopted to counter a fall in glycaemia in complication-free individuals with

type 1 diabetes, thus helping to prevent or delay hypoglycaemia if no carbohydrate is

readily available. Unfortunately, this type of exercise modality to prevent hypoglycaemia

is unlikely to be well tolerated by most individuals with type 1 diabetes due to the

impractical duration of this type of exercise. This raises the primary aim at the core of

this thesis to determine whether a much shorter bout of exercise lasting only 10 sec and

performed at maximal intensity could be adopted to prevent glycaemia from falling. For

this reason, the primary goal of this thesis was to determine whether a 10-sec maximal

sprint effort performed after (Chapter 3) or before (Chapter 4) moderate intensity exercise

provides a possible means other than carbohydrate intake to prevent glycaemia from

falling when exercise is performed under hyperinsulinaemic conditions by complication-

free individuals with type 1 diabetes. Also, given that for this type of study, it is common

practice to subject participants to a graded exercise test to set exercise intensity relative

to V O2peak, a secondary objective of this thesis was to determine whether the risk of

hypoglycaemia is increased early during recovery from this type of exercise protocol

(Chapter 2). Finally, since the counterregulatory response to sprinting has not been

examined in hyperinsulinaemic individuals with type 1 diabetes, thus making it difficult

to compare the findings of Chapters 3 and 4 with the literature, our last aim was to

examine the counterregulatory responses to sprinting in type 1 diabetic individuals under

hyperinsulinaemic conditions (Chapter 5). The first study of this thesis (Chapter 2) examines whether the risk of hypoglycaemia

increases in response to graded exercise testing in individuals with type 1 diabetes. Eight

non-diabetic male participants and seven complication-free type 1 diabetic male

individuals in good glycaemic control were recruited. On the morning of testing, the

diabetic participants followed their normal insulin regimen, and both groups ate their

usual breakfast. Then, participants were subjected to graded exercise testing

iv

approximately four hours later. We found that this type of exercise result in a rapid post-

exercise increase in blood glucose levels (> 2 mM), which remain elevated for the first

two hours of recovery. On clinical grounds, these findings suggest for the first time that

the early post-exercise risks of hypoglycaemia associated with graded exercise testing are

minimal when performed under near basal plasma insulin levels, with no carbohydrate

administration required soon before or after testing to prevent hypoglycaemia.

The primary goal of our next study (Chapter 3) was to determine whether a short 10-sec

maximal sprint effort is preferable to only resting as a means to counter a further fall in

glycaemia during recovery from moderate intensity exercise in hyperinsulinaemic

individuals with type 1 diabetes. To meet our objective, seven healthy complication-free

male participants with type 1 diabetes injected their normal insulin dose and ate their

usual breakfast. Then, when their postprandial glycaemia fell to ~11 mM they pedalled

at 40% peak 2OV for 20 min on a cycle ergometer followed immediately by either a

maximal 10-sec sprint or a rest. Our results show that, during exercise, blood glucose

levels fell rapidly. However, sprinting immediately after exercise opposes a further fall

in blood glucose levels for at least 120 min while glycaemia decreases significantly (p <

0.05) by ~ 3.5mM when no sprint was performed. We also found that sprinting is likely

to counter the exercise-mediated decrease in blood glucose levels through an increase in

catecholamine, lactate, and growth hormone levels. Interestingly, these glucoregulatory

benefits of sprinting are remarkable considering the sprint trial was performed when

insulin levels were elevated, a time when exercise is not usually recommended. On the

basis of these findings, one might tentatively recommend that in order to minimise the

risk of early hypoglycaemia post-moderate intensity exercise, it is preferable for

complication-free young individuals with type 1 diabetes to engage in a 10-sec maximal

sprint effort before resting than to only rest during recovery, particularly if a source of

dietary carbohydrate is not readily available.

Given the glycaemia stabilising effect of sprinting performed after moderate-intensity

exercise (Chapter 3), the study described in Chapter 4 examines whether performing a

short sprint effort immediately prior to moderate-intensity exercise may offer a novel way

of preventing glycaemia from falling both during and after moderate-intensity exercise.

To this end, seven complication-free type 1 diabetic males injected their normal morning

insulin dose and ate their usual breakfast. When post-meal glycaemia fell to ~11 mM,

they were asked to perform a 10 sec all-out sprint (sprint trial) or to rest (control trial)

v

immediately before cycling at 40% of peak rate of oxygen consumption for 20 min. We

found, against expectations, that sprinting for 10 sec immediately before moderate-

intensity exercise performed under hyperinsulinaemic conditions does not affect the rapid

decline in glycaemia during exercise. However, sprinting rather than resting before

moderate intensity exercise did prevent glycaemia from falling for at least the first 45 min

of recovery in individuals with type 1 diabetes. This suggests that including a short sprint

as part of the warm-up routine of individuals with type 1 diabetes before they engage in

sustained moderate-intensity exercise might provide another means of temporarily

stabilising glycaemia during early recovery.

Unfortunately, one difficulty with comparing the counterregulatory responses described

in Chapters 3 and 4 with the literature is the lack of information on the effect of short-

duration sprinting per se on the responses of counterregulatory hormones in

hyperinsulinaemic diabetic individuals. For this reason, the purpose of the study

described in Chapter 5 was to investigate the effect of a single 10-sec sprint on the levels

of the counterregulatory hormones in type 1 diabetic individuals under hyperinsulinaemic

conditions designed to approach those reported in Studies 3 and 4. In this study, we found

that performing a 10-sec maximal sprint resulted in patterns of change in plasma

catecholamines, growth hormone, cortisol and glucagon levels comparable to those

observed when a sprint is performed immediately after a bout of moderate intensity

exercise in individuals with type 1 diabetes (Study 2) and also comparable to those

observed in response to a sprint performed after an overnight fast (Fahey et al., 2012).

What remains to be established in future studies is the extent to which the changes in the

levels of these counterregulatory hormones contribute to the glucoregulatory benefits of

sprinting.

In conclusion, although a number of issues must be addressed before recommending the

adoption of short duration sprinting as a safe and reliable tool for the short-term

management of blood glucose levels in individuals with type 1 diabetes, this thesis shows

that sprinting has the potential to help individuals with type 1 diabetes to exercise more

safely and take advantage of the many physiological and psychological health benefits

of a physically active lifestyle.

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Acknowledgements

In writing my acknowledgements, I am overwhelmed with a sense of gratitude and

appreciation to all the people who have made this thesis possible. I have a huge smile as

I reminisce about the amazing people I have met and worked with along the way. Words

can not express how thankful I am for help and support of so many amazing people.

During the course of my PhD it seems like I have experienced almost every major life

event and the some of the absolute best and worst of times. Throughout the journey I

have had the support the best family anyone could wish for together with wonderful

colleagues who have become lifelong friends. I look forward to thanking you in person

but please realise how extremely grateful I am for your help, support and friendship

during the course of my PhD. In particular, I would like to sincerely thank the following

incredible people for their contribution towards my thesis:

My study participants - Thank you for your invaluable contribution. Recruitment was

such a challenge for this thesis so an extra huge thank you for your time and effort. It

was great to get to know you all and I wish you all the very best always. I hope you can

safely enjoy a more active life as a result of the research in this field.

Professor Paul Fournier (and Angeline) - for your passion, professionalism, infinite

knowledge and expertise, commitment, work ethic, endless enthusiasm and support (not

only with my PhD but with my career and life in general).

Dr Luis Ferreira (and Daniela) - for your research knowledge and expertise, balanced

advice, encouragement (together with your friendship and sharing my love of the Eagles,

sport and the important things in life).

Dr Kym Guelfi - for always being there throughout the journey with practical help,

advice, knowledge, friendship and huge morale support, you are an amazing friend and

researcher.

Alex D’Vauz (nee Baptista) - for your amazing friendship, encouragement, help, sense of

fun and for always bringing a smile to my face.

vii

Dr Ray Davey - from ‘Little Aths’ in Margaret River as kids to PhD buddies in the same

research team - thanks for your friendship, encouragement, advice and football banter.

Scott, Lian, Jen, Rob D, Hans (& Emma), Rob M, James (& Bree), Tim (& Sarah), Les,

Pete, Elisa, Tom, Jonas, Si, Nat, Stephen, Sani, Brad & the ‘Postgrad team’ - to each and

every one of you for your friendship, help, advice, camaraderie, fun times and great

memories (from corridor cricket & Friday tennis to our Annual Cocktail Party and so

many celebrations). Hans - it still seems surreal that you are not with us but your memory

will remind us to always cherish each day and we will never forget your love for your

family, friends and your passion for Exercise Physiology and Muscle Metabolism.

UWA Type 1 Diabetes Team – Paul, Luis, Kym, Alex, Tim, Avril, Katherine, Chee,

Harris & the PMH Team - Prof. Tim Jones, A/Prof. Liz Davis, Niru, Leanne, Michael,

Sarah, Vanessa. Thank you to a fantastic team of passionate researchers. It has been a

long journey but I have learnt so much from ‘being there at the start’ and seeing our

research team and field grow.

Our Nurses – Alisha, Nurse Bettye, Niru and Christy - thanks for your professionalism,

friendship and ability to make ‘testing days’ more enjoyable for all.

SSEH Team – Bec, Kerry, Prof. ‘Daws’, Karen, Danny, Brenda, Inga, Pat, Margaret,

Robin, Don & the Technical Staff for your help and support. Heads of Schools (Prof.

Brian Blanksby, Prof. Bruce Elliot, Prof. Tim Ackland) – for your support and exciting

career, research and teaching opportunities. To Prof. Tim Ackland, Sharon Gam and Jo

Francis for your huge help in this final phase.

Past Teachers, Lecturers and Mentors – from tiny Karridale Primary to Margaret River

High and UWA - to everyone who helped nurture my love of learning and science.

Inspiring Researchers, specialists, diabetes educators, endocrinologists and the incredible

people with type 1 diabetes I was privileged to meet. Hearing your thoughts and

discussing ideas at various conferences around the world was a huge privilege and

highlight of my PhD journey.

viii

To my awesome friends - a huge thank you for your friendship, inspiration and support

in so many ways. To Marshy & ‘The Marsh Family’, Michelle & Sean, Jas & Jamie,

Petrina & Rohan, Kym & Mark, Seaton, Amy & Dave, Jules & Matt, Carmen & Ross,

Ali & Mike, Alicia & Scott, Liv & Ian, Lise & Rod, Dan & Harriet, Fiona, Jaye, Krista

& Charles, Mia & Peter, Kylz & Dane, Erynne, Dale & Paul, Dyl, Joel, Waz, Col, Kat &

Guy, Sharne, Steph, K, Ames, Suz, Thalina, Alana & Ant, SuAnne, Bek, Mel, Paul,

Renae, Sarah, Anna-Mieke, Julia, Father Phillip & Our Guildford Grammar School &

Freeth House ‘Family’.

Mum, Dad, Daniel, Natalie - throughout life, school, ‘Undergrad’, Honours, PhD and

now life with my boys and business. Thanks for your unwavering love and support

always. Mum and Dad, thank you for always encouraging me to work hard and to pursue

my dreams. Thank you for all the opportunities you gave me throughout my education

and for encouraging me to follow my dreams and create a career in a field that I love. I

feel so incredibly grateful every time I see a patient and think how much I love helping

people and promoting exercise and health. Mum, Dad, Daniel (& Lucy) and Nat (&

Mike) – thank you for all your never-ending support along the journey – from school,

study, career and life. I love you all so unbelievably much and will never forget your

endless love and support always. Your contribution to this thesis is immense and will

always be cherished.

Dad - I miss you so much and wish you were here in person to firstly meet Marcus & my

‘boys’ and to see this PhD completed. I remember how proud you were when my 1st paper

was accepted for publication in your final days with us and hope you can enjoy a

celebration from ‘above’ with us.

To my extended family and friends, thank you to each and every one of you. In particular,

thank you to Ann, Noelle, Carrina for inspiring me in your own way. To Stu and Kay,

Rayma (& Geoff) and George for welcoming me into your family and for all your help

and support with our boys.

Finally, to Marcus, Owen, Max (and Lexie). Thanks for making me so unbelievably

happy and bringing so much joy and happiness into my life. I love you all more than

words can say. Marcus – thank you for your immense support. I will be eternally grateful

for the huge effort and sacrifices you and the boys have made so I could finish my PhD.

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Table of Contents

Statement of candidate contribution…………………………………………..

ii

Abstract………………………………………………………………………..

iii

Acknowledgements……………………………………………..…………….

vi

Table of contents………………………………………………………………

ix

List of figures.........……………………………………………………………

xiii

List of abbreviations…………………….........................………………….....

xiv

Publications arising from this thesis..………………………………………….

xvi

Chapter 1 - Introduction and Review of the Literature………… 1

1.1 Introduction and type 1 diabetes mellitus……….................................... 2

1.2 Treatment of type 1 diabetes mellitus and associated hypoglycaemia…... 3

1.3 Counterregulatory response to hypoglycaemia in non-diabetic individuals 5

1.3.1 Insulin………………………………………………………………. 5

1.3.2 Glucagon……………………………………………………...…….. 6

1.3.3 Catecholamines…………………………………………………...… 7

1.3.4 Growth hormone……………………………………………………. 8

1.3.5 Cortisol……………………………………………………………... 9

1.3.6 Other factors………………………………………………............ 10

1.4 Counterregulatory response to hypoglycaemia in type 1 diabetes……… 12

1.5 Glucoregulatory responses to moderate intensity exercise in non-diabetic

individuals...………………………………… ………………………...

15

x

1.6 Glucoregulatory responses to moderate intensity exercise in individuals

with type 1 diabetes…………………………………………..……………

20

1.7 Glucoregulatory responses to high-intensity aerobic exercise in non-

diabetic individuals…………………………………………………...……

23

1.8 Glucoregulatory responses to high-intensity aerobic exercise in

individuals with type 1diabetes……………………………………………

29

1.9 Glucoregulatory responses to intermittent high intensity exercise in non-

diabetic individuals. ………………………………………………..…

31

1.10 Glucoregulatory responses to intermittent high intensity exercise in

individuals with type 1 diabetes……………………………………….…

32

1.11 Glucoregulatory responses to a single short sprint in non-diabetic

individuals…………………………………………………….……………

34

1.12 Glucoregulatory response to a single short sprint in individuals with type

1 diabetes……………………………………………………..…..………..

40

1.13 Statement of the problem and aims………………………………...……... 41

1.14 Significance of the thesis………………………………...………...…...… 43

Chapter 2 – Glycaemic Response to Graded Exercise in

Individuals with Type 1 Diabetes………………………………...

44

2.1 Abstract……………………………………………………………….…. 45

2.2 Introduction…………………………………………………………….... 46

2.3 Research design and methods………………………………………..….. 48

2.3.1 Participants………………………………………………………… 48

2.3.2 Experimental trials and assays………………………………......… 48

2.3.3 Statistical analyses………………………………….……………… 50

2.4 Results……………………………………………………………………. 51

2.4.1 Blood metabolite response to graded exercise…………………...... 51

2.4.2 Hormonal response to graded exercise………………………..….... 53

2.5 Discussion…………………………………………………..……………. 55

2.6 Acknowledgements…………………………………………..…………... 59

xi

Chapter 3 – A 10-second Maximal Sprint Effort: A Novel

Approach to Counter an Exercise-Mediated Fall in Glycaemia in

Individuals with Type 1 Diabetes…………………………………

60 3.1 Abstract…………………………………………………………………… 61

3.2 Introduction………………………………………………………………. 62

3.3 Research design and methods…………………………………………….. 63

3.3.1 Participants………………………………………………………. 63

3.3.2 Experimental trials …………………………………………… 63

3.3.3 Hormones and metabolite assays……………………………… 65

3.3.4 Statistical analyses……………………………………………… 65

3.4 Results…………………………………………………………………... 66

3.4.1 Blood metabolite response……………………………………... 66

3.4.2 Hormonal response……………………………………………... 68

3.5 Discussion……………………………………………………………. 70

3.6 Acknowledgement………………………………………………………... 72

Chapter 4 – A 10-second Sprint Performed Prior to Moderate-

Intensity Exercise Prevents Early Post-Exercise Fall in

Glycaemia in Individuals with Type 1 Diabetes…………………

73

4.1 Abstract……………………………………………………………………. 74

4.2 Introduction……………………………………………………………... 75

4.3 Methods……………………………………………………………………. 77

4.3.1 Participants………………………………………………………. 77

4.3.2 Experimental Trials and Assays…………………………………. 77

4.3.3 Statistical Analyses……………………………………………… 78

4.4 Results ………………………………………………………. 79

4.4.1 Blood metabolite response 79

4.4.2 Hormonal response 79

4.5 Discussion………………………………………………………. 84

4.6 Acknowledgements………………………………………………… 86

xii

Chapter 5 – Counterregulatory Response to a 10-second Sprint

in Young Individuals with Type 1 Diabetes Mellitus ……………… 87

5.1 Abstract…………………………………………………………………. 88

5.2 Introduction……………………………………………………………... 89

5.3 Methods…………………………………………………………………. 91

5.3.1 Participants………………………………………………………. 91

5.3.2 Experimental Trials …………………………………………….. 91

5.3.3 Hormones and Metabolite Assays…………… 92

5.3.4 Statistical Analyses……………………………… 92

5.4 Results…………………………………………………………………... 93

5.4.1 Hormonal Response to a 10-sec Sprint………………………….. 93

5.4.2 Blood Metabolite Response to a 10-sec Sprint………………… 95

5.4.3 Work Load and Peak Power Associated with a 10-sec Sprint…... 95

5.5 Discussion………………………………………………………………. 97

5.6 Acknowledgements……………………………………………………... 100

Chapter 6 - General Discussion…………………………………. 101 6.1 General discussion……………………………………………………... 102

6.2 Clinical implications, limitations with our findings and direction for

future studies……………………………………………………………………

106

Chapter 7 – References…………………………………….......... 110

xiii

List of Figures

2.1 Effect of graded exercise on the levels of glucose (A), lactate (B), pH (C) and

free fatty acids (D)………………………………………………….................

52

2.2 Effect of graded exercise on the levels of epinephrine (A), norepinephrine (B),

growth hormone (C), cortisol (D), glucagon (E) and free insulin (F) ………....

54

3.1 Effect of a 10-sec sprint on blood glucose levels after moderate intensity

exercise………………………………………………………………...……...

67

3.2 Effect of a 10-sec sprint on the levels of lactate (A), free fatty acids (B),

norepinephrine (C), epinephrine (D), growth hormone (E), cortisol (F),

glucagon (G) and free insulin (H) after moderate intensity exercise ………….

69

4.1 Effect of a 10-sec sprint on blood glucose levels during and after moderate-

intensity exercise……………………………………………………...………

81

4.2 Effect of a 10-sec sprint on the levels of lactate (A), NEFA (B), norepinephrine

(C), epinephrine (D), growth hormone (E), cortisol (F), glucagon (G) and free

insulin (H) during and after moderate-intensity exercise……………………...

82

5.1 Effect of a single 10-sec sprint on the levels of norepinephrine (A), epinephrine

(B), growth hormone (C), cortisol (D), free insulin (E), and glucagon (F) ……

94

5.2 Effect of a single 10-sec sprint on blood glucose (A), free fatty acids (B), pH

(C) and blood lactate (D) levels………………………………………….........

96

xiv

List of Abbreviations

ADA American Diabetes Association

AIHW Australian Institute of Health and Welfare

ANOVA Analysis of variance

BMI Body mass index

CON Control

CGMS Continuous glucose monitoring system

DCCT Diabetes Control and Complications Trial

Direcnet Diabetes Research in Children Network

FFA Free fatty acids

GIR Glucose infusion rate

GH Growth hormone

GLUT Glucose transport protein

HbA1c Glycosylated haemoglobin

IHE Intermittent high-intensity exercise

IL-6 Interleukin 6

J Joule

JDRF Juvenile Diabetes Research Foundation

LOPEH Late onset post-exercise hypoglycaemia

Min Minute

mM Millimoles per litre

MOD Moderate-intensity exercise

n Number of participants

nmol/L Nanomoles per litre

N/A Not applicable

Ra Rate of appearance

xv

Rd Rate of disappearance

RIA Radioimmunoassay

SD Standard deviation

Sec Second

SEM Standard error of the mean

SPSS Statistical Package for the Social Sciences

T1DM Type 1 diabetes mellitus

µL Microlitres

O2 Rate of oxygen consumption

V O2max Maximal rate of oxygen consumption

V O2peak Peak rate of oxygen consumption

xvi

Publications Arising from this Thesis

Peer-Reviewed Publications

Bussau, V.A., Ferreira, L.D., Jones, T.W. & Fournier, P.A. (2007). A 10-s sprint

performed prior to moderate-intensity exercise prevents early post-exercise fall in

glycaemia in individuals with type 1 diabetes. Diabetologia 50 (9): 1815-1818.

Bussau, V.A., Ferreira, L.D., Jones, T.W. & Fournier, P.A. (2006). The 10-s Maximal

Sprint: A Novel Approach to Counter an Exercise-Mediated Fall in Glycemia in

Individuals with Type 1 Diabetes. Diabetes Care 29 (3): 601-606.

Conference Proceedings

Bussau, V.A., D’Vauz, A., Ferreira, L.D., Jones, T.W. & Fournier, P.A. (2010). Does

VO2 max testing increase risk of hypoglycaemia in individuals with type 1 diabetes?

Combined Biological Sciences Meeting, Perth, Australia.

Bussau, V.A., D’Vauz, A., Ferreira, L.D., Jones, T.W. & Fournier, P.A. (2009).

Glycaemic response to graded exercise in individuals with type 1 diabetes. Australian

Diabetes Society National Conference, Adelaide, Australia.

Bussau, V.A., D’Vauz, A., Ferreira, L.D., Jones, T.W. & Fournier, P.A. (2009).

Glycaemic response to graded exercise in individuals with type 1 diabetes. Australian

Diabetes Educators Association State Conference, Mandurah, Australia.

Fournier, P.A., Bussau, V.A., Fahey, A., Ferreira, L.D., Jones, T.W. (2008). Sprinting

as a novel approach to preventing exercise-mediated hypoglycemia. Diabetes,

Exercise & Sports Association International Conference, Toronto, Canada.

Bussau, V.A., Ferreira, L.D., Jones, T.W. & Fournier, P.A. (2006). A 10-second

sprint performed prior to moderate intensity exercise decreases the risk of post-

exercise hypoglycaemia in individuals with type 1 diabetes. International Diabetes

Federation Congress, Cape Town, South Africa.

xvii

Bussau, V.A., Ferreira, L.D., Youngs, L.M., Jones, T.W. & Fournier, P.A. (2005). A

10-second sprint performed prior to moderate intensity exercise decreases the risk of

post-exercise hypoglycaemia in individuals with type 1 diabetes. Australian Diabetes

Society Conference, Perth.

Bussau, V.A., Ferreira, L.D., Youngs, L.M., Jones, T.W. & Fournier, P.A. (2004). A

10-second sprint acutely prevents an exercise-mediated decrease in glycaemia in

individuals with type 1 diabetes mellitus. European College of Sports Science

Conference, Clermont- Ferrand, France.

Bussau, V.A., Ferreira, L.D., Youngs, L.M., Jones, T.W. & Fournier, P.A. (2004). A

10-second sprint acutely prevents an exercise-mediated decrease in glycemia in

individuals with type 1 diabetes. American Diabetes Association Conference,

Orlando, USA.

Chapter 1

Introduction and Review of the Literature

2

1.1 Introduction and Type 1 Diabetes Mellitus

Type 1 diabetes mellitus, formerly known as insulin-dependent or juvenile-onset diabetes

mellitus, is an endocrine disease that occurs commonly in childhood and adolescence, but

can be recognised and become symptomatic at any age (Dinneen and Rizza, 2001). Type

1 diabetes affects millions of people worldwide including approximately 140 000

Australians (IDF, 2009). The International Diabetes Federation found Australia to be one

of the top ten countries with the highest incidence of type 1 diabetes in children (Soltesz

et al., 2009), with over 8,000 new cases between 2000 and 2008, an average of two new

cases every day (AIHW, 2010). In individuals over 15 years, 9000 new cases of type 1

diabetes were diagnosed in Australia between 2000-2006 or three new cases per day

(AIHW, 2008).

Type 1 diabetes is characterised by the absence of insulin secretion due to the autoimmune

destruction of the beta cells of the Islets of Langerhans of the pancreas (Rizza et al., 2001).

The small proportion of individuals that appear not to have an autoimmune basis for their

beta cell destruction have a sub-type of diabetes referred to as idiopathic type 1 diabetes

(Dinneen and Rizza, 2001). As a consequence of the destruction of the beta cells, the

body loses its capacity to produce insulin. Given that insulin promotes the storage of

carbohydrates and fat, inhibits ketone body production, and stimulates a decrease in blood

glucose levels by both inhibiting hepatic glucose production and stimulating peripheral

glucose uptake, it is not surprising that the absence of insulin leads to an increase in blood

glucose (hyperglycaemia) and ketone body levels (Lernmark, 2001). This results in the

many symptoms typical of untreated type 1 diabetes including glucose and ketone body

loss in the urine (glucosuria and ketonuria), excessive urine production (polyurea),

extreme thirst and consumption of large quantities of water (polydipsia), excessive

consumption of food (polyphagia), and rapid weight loss (Eisenbarth et al., 2008). Other

symptoms may include nausea, vomiting, blurred vision, confusion, shortness of breath

and extreme fatigue (Eisenbarth et al., 2008). Severe hyperglycaemia and elevated ketone

body levels increase markedly the risk of septicaemia and ketoacidosis, respectively. This

explains why, without treatment, death generally occurs within 1-2 years of the onset of

type 1 diabetes (Campaigne and Lampman, 1994).

The discovery of insulin in 1921 and the initiation of treatments based on regular insulin

injections have extended considerably the life expectancy of individuals with type 1

3

diabetes. Currently, insulin is delivered by injection or constant subcutaneous infusion

via a pump (Sherr et al., 2009; Bergenstal et al., 2010). A number of types of insulin have

been developed, each with different absorption rates, onset, time to peak action and

duration of action. The dosage of insulin is often adjusted in response to self-monitored

blood glucose levels, nutritional intake and physical activity (Eisenbarth et al., 2008).

1.2 Treatment of Type 1 diabetes Mellitus and Associated Hypoglycaemia

The main challenge in the treatment of type 1 diabetes is to maintain blood glucose levels

close to a normal physiological range as this reduces the risk of developing a number of

severe diabetic complications including microvascular and macrovascular diseases,

nephropathy (kidney disease), retinopathy (damage to the retina of the eye) and

neuropathy (disease of the nervous system) (Brownlee et al., 2008; Eisenbarth et al.,

2008). This link between low risk of developing such complications and the importance

of maintaining blood glucose levels as close to normal as possible were demonstrated by

The Diabetes Control and Complications Trial in 1993 (Diabetes Control

and Complications Trial Research Group, 1993) and more recent studies (Nathan et al.,

2009). There is also evidence that, irrespective of glycaemic control, severe glycaemic

excursions also contribute to the development of these complications (Soupal et al.,

2014).

Unfortunately, the treatment of type 1 diabetes to maintain blood glucose levels within

the narrow physiological range found in non-diabetic individuals increases considerably

the risk of hypoglycaemic episodes (Cryer, 2010a). This is because of the absence of

feedback mechanisms between blood glucose levels and insulin secretion normally found

in non-diabetic individuals (Galassetti and Riddell, 2013). If too much insulin is

administered, the state of relative hyperinsulinaemia that ensues decreases hepatic blood

glucose production and increases glucose uptake, thus causing glucose utilisation to

exceed glucose production rate and, as a result, blood glucose levels decrease below

normal levels, a condition referred to as hypoglycaemia (Gerich, 2001).

In general, when blood glucose levels fall below 3 mM, warning symptoms alert an

individual to the presence of hypoglycaemia (McAulay et al., 2001). These include

“neuroglycopaenic” symptoms due to insufficient glucose reaching the brain (Cryer,

1999) and may include a loss in concentration, fatigue, weakness, confusion, difficulty in

4

thinking and speaking (Hepburn et al., 1991; Towler et al., 1993). Hypoglycaemia-

induced stimulation of the sympatho-adrenal system also triggers neurogenic (autonomic)

symptoms. These include “adrenergic” symptoms including tremor, heart palpitations

and anxiety as well as “cholinergic” symptoms such as sweating, hunger and paraesthesia

(Towler et al., 1993). Perception of these neurogenic symptoms is essential in one’s

awareness of hypoglycaemia and self-recognition that blood glucose levels are low

(Towler et al., 1993). Unfortunately, some individuals, diagnosed as “hypoglycaemia

unaware” experience minimal symptoms when their blood glucose levels fall to

dangerously low levels, with a late response to warning symptoms increasing their

possibility of experiencing severe hypoglycaemic episodes (Gold et al., 1994; McAuley

et al., 2001; Cryer et al., 2009; Candace et al., 2013).

The increased risk of hypoglycaemia associated with insulin-based therapy is a major

concern because severe hypoglycaemia can lead to central nervous system (CNS)

damage, and in extreme cases coma and even death (Ben-Ami et al., 1999; Cryer, 2007).

This is because a continuous supply of glucose is essential for normal cerebral function,

since the brain uses glucose as its main source of fuel under normal conditions (Gerich,

2000). Moreover, the brain stores little carbohydrate as glycogen (Herzog et al., 2010).

Given the importance of supporting the glucose requirements of the brain, it comes as no

surprise that most of the glucose synthesised in the body is utilised by this organ.

Unfortunately, hypoglycaemia is a common problem in the everyday life of most

individuals with type 1 diabetes. In fact, individuals with type 1 diabetes experience on

average two episodes of symptomatic hypoglycaemia per week, which equates to

thousands of hypoglycaemic episodes during their lifetime (Alsahli and Gerich, 2008). It

is important to remember that these rates are likely to underestimate the true number of

hypoglycaemic events due to many missed asymptomatic and symptomatic episodes

together with incorrect reporting (Cryer et al., 2009). In contrast, rates of severe episodes

of hypoglycaemia that require extended intervention to treat are more reliable, with

individuals with type 1 diabetes likely to suffer one such event per year often involving

coma or seizures (Cryer, 2008a, 2010b). Even more alarming is the fact that people with

type 1 diabetes die from hypoglycaemia (Laing et al., 1999). For these reasons, it is

understandable that hypoglycaemia is the primary and most feared complication of

insulin therapy (Amiel, 2009). It is not surprising, therefore, that such a therapy-induced

hypoglycaemia, or iatrogenic hypoglycaemia, is considered as a major limiting factor in

5

the glycaemic management of type 1 diabetes as it causes recurrent morbidity in most

individuals with type 1 diabetes (Cryer, 2005; 2010b). For this reason, any strategy aimed

at reducing the risk of hypoglycaemia is likely to be well received by individuals with

type 1 diabetes.

1.3 Counterregulatory Response to Hypoglycaemia in Non-Diabetic Individuals

Fortunately, the body possesses a range of mechanisms to counter hypoglycaemia. In

order to best appreciate how this is achieved in individuals with type 1 diabetes, it is

important in the first instance to examine how hypoglycaemia is countered in non-diabetic

individuals. Firstly, a number of highly specialised regions in the body including the beta

cells of the pancreas, ventromedial hypothalamus, and glucose sensing neurons in the

mouth, gut, portal/mesenteric vein and carotid body detect glucose changes or

hypoglycaemia (McCrimmon, 2009; Watts and Donovan., 2010). In healthy non-diabetic

individuals, a fall in blood glucose levels triggers complex but highly effective

physiological mechanisms known as counterregulatory responses that aim to reverse

falling blood glucose levels to restore euglycaemia (Cryer, 1993, 2008b). The magnitude

of such counterregulatory responses is determined in part by the depth and duration of

hypoglycaemia as well as by other factors including age, gender, rate of decline in blood

glucose before the onset of hypoglycaemia, antecedent exercise (Galassetti et al., 2001a),

and antecedent hypoglycaemia (Davis et al., 1991, 2000b).

1.3.1 Insulin

The first line of defence against hypoglycaemia is suppression of insulin secretion (Bolli,

1999) when blood glucose falls to a level of approximately 4.5 mM (Cryer et al., 2003).

The resultant decrease in circulating insulin levels stimulates an increase in hepatic

glucose production (Cherrington et al., 1998) and a decrease in glucose utilisation rate

(Cryer, 2001), which as a result counter declining glycaemia. However, if blood glucose

levels continue to fall to between 3.6 to 3.9 mM, the body responds by increasing the

secretion of counterregulatory hormones (Cryer et al., 2003). These hormones act to

restore glucose levels within a normal, safe physiological range by increasing glucose

production and inhibiting peripheral glucose uptake. It is important to note that the

contribution of each counterregulatory hormone is different, with some hormones being

more important than others as discussed below (Rizza et al., 1979b; Schwartz et al., 1987;

Mitrakou et al., 1991; Fanelli et al., 1994).

6

1.3.2 Glucagon

One of the key counterregulatory hormones is glucagon. Glucagon is secreted by the

alpha cells of the Islets of Langerhans of the pancreas (Quesada et al., 2008) when blood

glucose levels are low, with this hormone having an immediate effect on glucose kinetics

(Alsahli and Gerich, 2008). The secretion of glucagon by the alpha cells of the pancreas

is mainly influenced by blood glucose levels, with an increased secretion rate when blood

glucose levels are low (Jiang and Zhang, 2003; Porcellati et al., 2003). Other factors that

stimulate glucagon secretion are catecholamines (Gerich et al., 1973a), amino acids

(Rocha et al., 1972; Schmid et al., 1989) and short-term exposure to fatty acids (Hong et

al., 2005). In contrast, high levels of insulin and somatostatin inhibit glucagon secretion

(Ito et al., 1995; Hauge-Evans et al., 2009). Acting independently of these factors,

autonomic neural activation of the islet is thought to influence glucagon response to

hypoglycaemia (Havel et al., 1993).

Glucagon stimulates an increase in glucose levels firstly by activating hepatic

glycogenolysis and gluconeogenesis (Lecavalier et al., 1989; Roden et al., 1996;

Camacho et al., 2005). Even small changes in glucagon levels can increase glucose

production (Myers et al., 1991). Glucagon may also prevent blood glucose levels from

falling by inhibiting glycogen synthesis in the liver (Jiang and Zhang, 2003). Glucagon

is considered the most important counterregulatory hormone as demonstrated by studies

where recovery from hypoglycaemia has been shown to be impaired by a deficiency in

glucagon (Rizza et al., 1979b; Boyle et al., 1989). In particular, the importance of

glucagon is best shown by the observation that when glucagon response to hypoglycaemia

is prevented, this results in a blunted compensatory increase in endogenous glucose

production despite an increase in epinephrine secretion (De Feo et al., 1991b). In contrast,

glucagon does not affect renal glucose production or utilisation (Stumvoll et al., 1998a).

It is important to note that it is the level of portal glucagon relative to that of insulin as

expressed by the portal venous glucagon to insulin ratio that plays a central role in the

control of glycaemia (Quesada et al., 2008) as these hormones acutely increase or

decrease glucose production and utilisation to maintain blood glucose levels within a

narrow physiological range (Cryer, 2008b). A number of studies have highlighted the

importance of the portal venous glucagon-to insulin ratio in the regulation of hepatic

glucose production (Ferrannini et al., 1982; Lins et al., 1983; Steiner et al., 1990). When

7

this ratio is high, blood glucose levels increase as a result of decreased glucose utilisation

and increased glucose production (Boyle et al., 1989). The opposite occurs when this

ratio is low (Boyle et al., 1989).

1.3.3 Catecholamines

Catecholamines, including epinephrine and norepinephrine, are also thought to play a

major role in counterregulation in non-diabetic individuals. Epinephrine is released by

the chromaffin cells of the adrenal medulla, while norepinephrine is released by both the

adrenal medulla and the sympathetic nerve endings (Deschenes et al., 1991; Zouhal et al.,

2008). Hypoglycaemia, in its early stage, induces an elevation in sympathetic activity in

non-diabetic individuals, which results in an increase in catecholamine levels (Sotsky et

al., 1989; Bolli, 1999; Davis et al., 2000b). Although this increase in epinephrine and

norepinephrine levels is well established, there has been a long standing controversy

regarding the relative importance of catecholamines versus glucagon in counterregulation

(Cryer, 1981; Bolli, 1999). An important role for catecholamines in the stimulation of

glucose production during prolonged hypoglycaemia is highlighted by the observation

that pharmacological blockage of catecholamine action results in severe hypoglycaemia

despite increases in other counterregulatory hormones including glucagon (De Feo et al.,

1991a). In contrast, others have concluded that when glucagon response to

hypoglycaemia is normal, catecholamines play only a minor role (Rizza et al., 1979b;

Cryer et al., 2003). However, when glucagon response is impaired or absent,

catecholamines, in particular epinephrine, play a critical role in counterregulation (Rizza

et al., 1979b; Boyle et al., 1989; Cryer et al., 2003).

Catecholamines increase glycaemia by acting on multiple organs (Meyer et al., 1999;

Alsahli and Gerich, 2008). An increase in epinephrine and norepinephrine levels results

in an elevation in blood glucose levels due to an increase in hepatic (Sacca et al., 1980)

and renal (Stumvoll et al., 1998b) glucose production together with a fall in insulin-

mediated stimulation of glucose utilisation (Rizza et al., 1979a; Nonogaki, 2000; Coker

& Kjaer, 2005). Catecholamines increase the rate of hepatic glucose production by

stimulating an increase in both glycogenolysis and gluconeogenesis (Barth et al., 2007).

This increase in hepatic glucose production is initially due to the stimulation of

glycogenolysis, while hepatic gluconeogenesis later becomes the predominant

contributor to sustained hepatic glucose production (Sacca et al., 1983). Catecholamines

also decrease the rate of glucose utilisation with both epinephrine (Rizza et al., 1979a;

8

Deibert and Defronzo, 1980; Lager et al., 1986) and norepinephrine (Lembo et al., 1994)

inhibiting insulin-mediated stimulation of glucose uptake by skeletal muscles. As

discussed later, however, there are conditions where catecholamines and other

adrenoceptor agonists have been reported to increase muscle glucose uptake (Abe et al.,

1993; Nonogaki, 2000; Ngala et al., 2013).

Catecholamines also indirectly increase blood glucose levels. High levels of

catecholamines suppress endogenous insulin release (Clutter et al., 1980; Sherwin et al.,

1980; Sacca et al., 1983) and stimulate the supply of gluconeogenic substrates (Sacca et

al., 1983; Stumvoll et al., 1998b) including lactate from resting muscles (Sacca et al.,

1983). Catecholamines also stimulate glucagon (Gerich et al., 1973a) and growth

hormone release (Blackard and Heidingsfelder, 1968). Finally, catecholamines stimulate

lipolysis (Clutter et al., 1980) and therefore increase the levels of plasma free fatty acids

(Sherwin et al., 1980; Sacca et al., 1983), decreasing carbohydrate utilisation and

increasing hepatic glucose production.

1.3.4 Growth hormone

Hypoglycaemia also results in the activation and release of GH, a group of

counterregulatory hormones produced by the pituitary gland and which also plays some

role in the defence against hypoglycaemia. Indeed, growth hormone is a heterogeneous

class of protein hormones consisting of a series of related isoforms (Baumann, 2009).

When the hypothalamus senses hypoglycaemia, growth hormone releasing hormone and

somatostatin are thought to be released together with other growth hormone-releasing

factors resulting in the pulsatile secretion of growth hormone from the anterior pituitary

gland (Reiter and Rosenfeld, 2008).

Growth hormone is not considered important in opposing acute hypoglycaemia, but it

may play a role during prolonged hypoglycaemia. This is based on the observation that

the pharmacological blockage of growth hormone release or action during prolonged

hypoglycaemia impairs the increase in glucose production, thus resulting in more severe

hypoglycaemia (De Feo et al., 1989b; Boyle and Cryer, 1991). This effect of growth

hormone on glucose turnover takes several hours to take place (De Feo et al., 1989b;

Boyle and Cryer, 1991). Growth hormone has also an indirect effect on counterregulation

by stimulating lipolysis. The resulting increase in glycerol and free fatty acids levels

provide gluconeogenic substrates and fuels, respectively, with the capacity to spare

9

glucose by inhibiting glucose utilisation (De Feo et al., 1989b). Prolonged growth

hormone exposure has been shown to have insulin-antagonistic effects (Fowelin et al.,

1991) whereas in contrast short duration growth hormone infusion does not invoke insulin

resistance (Djurhuus et al., 2004).

It is noteworthy that some studies have shown that growth hormone has an acute effect

on glucose metabolism. Indeed, local growth hormone exposure results in a rapid

decrease in forearm glucose uptake (Zierler and Rabinowitz, 1963; Rabinowitz et al.,

1965; Gibney et al., 2007). Furthermore, the administration of a physiological growth

hormone pulse in non-diabetic individuals has been reported to result in a rapid fall in

muscle glucose uptake (Moller et al., 1990, 1992b, 2003) and a 1-2 hour delayed increase

in lipolysis, circulating free fatty acid levels, and fat oxidation rates (Moller et al., 1990,

1992b; Gravholt et al., 1999; Møller et al., 2003; Djurhuus et al., 2004), which altogether

could contribute further to lowering glucose utilisation rates (Møller et al., 1992b) and

stimulating hepatic glucose production.

1.3.5 Cortisol

Another counterregulatory hormone implicated in prevention of hypoglycaemia is

cortisol. When the hypothalamus senses hypoglycaemia, this results in the release of

corticotrophin-releasing factor in the pituitary portal vessels which in turn stimulates the

secretion of adrenocorticotrophic (ACTH) hormone by the anterior pituitary gland.

ACTH then activates the secretion of cortisol by the cortex of the adrenal glands

(Macdonald and King, 2007), which acts indirectly in the acute counterregulatory

response by further stimulating catecholamine secretion by an intra-adrenal effect.

Like GH, cortisol is not considered important in the acute response to hypoglycaemia

(Heller, 2011). Despite this, cortisol plays an important role in long term

counterregulation when hypoglycaemia is prolonged (De Feo et al., 1989a; Boyle and

Cryer, 1991). Under these conditions, cortisol increases glucose production and inhibit

glucose utilisation after approximately 3 hours (De Feo et al., 1989a; Boyle and Cryer,

1991). Prolonged infusion of cortisol increases glycaemia and reduces the required

glucose infusion rate to maintain euglycaemia by increasing glucose production and

decreasing glucose uptake by the peripheral tissues, partly by inducing hepatic and

peripheral insulin resistance (Rizza et al., 1982; Rooney et al., 1993).

10

During prolonged hypoglycaemia, cortisol has also an indirect effect on blood glucose

levels by stimulating systemic and regional lipolysis (Djurhuus et al., 2002, 2004) . If

growth hormone is present, the lipolytic effect of both hormones is additive (Djurhuus et

al., 2004). This increase in lipolysis, in turn, has independent insulin-resistant or glucose

sparing effects (De Feo et al., 1989b; Corral et al., 1998).

1.3.6 Other factors

In recent years, the potential role that the cytokine interleukin-6 (IL-6) plays in the

regulation of blood glucose levels and hepatic glucose production has received some

attention (Glund and Krook, 2008; Hoene and Weigert, 2008; Pedersen, 2009). IL-6 is a

pro-inflammatory cytokine that is involved in mediating many inflammatory processes,

brain function, fatigue and immune response (Glund and Krook, 2008). The relatively

recent discovery that skeletal muscle can produce and release cytokines such as IL-6 is at

the origin of the term, myokine. This class of proteins expressed and released by skeletal

muscle is associated with endocrine and/or paracrine effects (Pedersen, 2009). In fact,

IL-6 was the first discovered muscle contraction-induced “exercise factor” or myokine

(Pedersen, 2009).

Hypoglycaemia is associated with an increase in plasma IL-6 levels (Dotson et al., 2008).

IL-6 infusion in vivo increases glucose uptake potentially due to increased translocation

of GLUT4 from intracellular compartments to the plasma membrane (Carey et al., 2006).

Indeed, in healthy men subjected to a hyperinsulinaemic, euglycaemic clamp,

recombinant human IL-6 infusion increases the glucose infusion rate without affecting

the total suppression of endogenous glucose production (Carey et al., 2006), thus

suggesting an increase in glucose uptake. Although these effects of IL-6 would not be

expected to prevent hypoglycaemia, IL-6 might be indirectly beneficial given that

exogenous IL-6 administration has been shown to increase cortisol and glucagon levels

together with a rise in blood glucose levels, thus probably helping with opposing

hypoglycaemia (Dotson et al., 2008; Glund & Krook, 2008).

Other than IL-6, the levels of circulating lactate, glycerol and amino acids may also play

some role in counterregulation. These metabolites act as substrates for gluconeogenesis

to increase glucose production and modulate the secretion of hormones from the pancreas

(Roden and Bernroider, 2003). They may also act as fuels for peripheral tissues to spare

glucose or inhibit insulin-mediated glucose uptake. Indeed, both oral (Rossetti et al.,

11

2008) and intravenous (Porcellati et al., 2007) amino acids enhance the response of

glucagon to hypoglycaemia (Porcellati et al., 2007). Since lactate is a significant fuel

source, gluconeogenic precursor (Gerich, 1988), and may have a role in increasing insulin

resistance (Harmer et al., 2008), it has the potential to play a role in both increasing

glucose production and decreasing glucose utilisation. Some have proposed that

increased muscle lactate utilisation together with decreased muscle glucose uptake make

major contributions to glucose counterregulation in response to hypoglycaemia humans

(Meyer et al., 2005).

There is evidence that changes in circulating blood glucose concentrations have the

capacity to affect hepatic glucose production and thus contribute to the counterregulatory

response to hypoglycaemia. Such a non-hormonal autoregulatory mechanism accounts

for approximately 25% of the rise in net hepatic glucose production during

hypoglycaemia (Connolly et al., 1992). On the other hand, hyperglycaemia exerts a direct

inhibitory effect on endogenous glucose production via a glycogen phosphorylase-

mediated inhibition of glycogenolysis (Tonelli et al., 2005; Yki-Järvinen, 1993). In

addition, animal models have shown that net hepatic gluconeogenesis is reduced under

hyperglycaemic conditions when glycogen levels are depleted (Tonelli et al., 2005). The

mechanisms underlying the aforementioned autoregulation are only partly understood

(Moore et al., 1998; Tonelli et al., 2005).

1.4 Counterregulatory Response to Hypoglycaemia in Individuals with Type 1

Diabetes

In individuals with type 1 diabetes, many of the counterregulatory responses described

above are either absent or impaired, increasing the risk of severe and potentially life-

threatening episodes of hypoglycaemia (Galassetti and Riddell, 2013). Firstly, when

blood glucose levels begin to fall, the levels of insulin do not decrease, which is the initial

response to a decrease in glycaemia in non-diabetic individuals (Briscoe et al., 2007).

This is because the level of circulating insulin in insulin-treated individuals with type 1

diabetes is determined primarily by the rate of passive absorption from the site of insulin

injection together with the pharmokinetics of the particular type of insulin administered

(Cryer et al., 2003), with no feedback existing between blood glucose levels and insulin

release.

12

Early after the onset of type 1 diabetes, patients generally have normal insulin-

independent counter-regulatory responses to hypoglycaemia. However, this defence

mechanism against hypoglycaemia deteriorates thereafter (Richter and Galbo, 1986;

Gerich, 1988). Reduced glucagon response to a fall in glycaemia is the first and possibly

most important counterregulatory response to be impaired (Gerich et al., 1973b) despite

normal glucagon secretion in response to other stimuli such as exercise (Cryer et al.,

1989; Shilo et al., 1990). It is well established that this deficient glucagon response to

hypoglycaemia is strongly correlated to duration of diabetes (Bolli et al., 1983); however

the mechanism(s) involved is still unclear. Recent studies have provided evidence of beta

cell regulation of alpha cell glucagon secretion (Cooperberg and Cryer, 2009), with an

increase in insulin level signalling a decrease in glucagon secretion in response to

hypoglycaemia (Banarer et al., 2002; Cooperberg and Cryer, 2010). Therefore, current

evidence suggests defective glucagon response may be due to beta cell failure.

Furthermore, there may also be a central nervous system component to the impaired or

absent glucagon response since insulin's inhibitory effect on glucagon release is partly

mediated by the ventromedial hypothalamus under both normoglycaemic and

hypoglycaemic conditions (Paranjape et al., 2010).

As a result of their attenuated or lack of glucagon response to hypoglycaemia (Gerich et

al., 1973b; Bolli et al., 1983; Hirsch and Shamoon, 1987; Meyer et al., 1998), individuals

with type 1 diabetes become more dependent on catecholamines (Marker et al., 1991), in

particular epinephrine (Bolli et al., 1982; Cryer et al., 1989), together with other

counterregulatory hormones to overcome hypoglycaemia (De Feo et al., 1983).

Unfortunately, individuals with type 1 diabetes have an attenuated epinephrine (Bolli et

al., 1983; Amiel et al., 1988; Dagogo-Jack et al., 1993; Meyer et al., 1998) and

norepinephrine (Meyer et al., 1998) response to hypoglycaemia compared to non-diabetic

individuals. This is exacerbated by the fact that the rate of fall of glycaemia can affect

epinephrine response to hypoglycaemia, with a rapid rate of fall resulting in a smaller

epinephrine response compared with a slower rate of fall. In diabetic individuals with

autonomic neuropathy, catecholamine response to hypoglycaemia is further impaired

(Bolli et al., 1983; Bottini et al., 1997; Meyer et al., 1998). In summary, the three key

normal responses that play an important role in the defence against hypoglycaemia when

glycaemia is falling (decreased insulin levels, increased glucagon and increased

epinephrine levels) are often deficient in individuals with type 1 diabetes (Cryer, 2003).

13

This is highly problematic for those individuals with both glucagon and epinephrine

deficiency as they are at an increased risk of severe hypoglycaemia (White et al., 1983).

The aforementioned impaired counterregulatory responses to hypoglycaemia also

implicate the glycaemic threshold for the release of counterregulatory hormones, which

is often different in type 1 diabetic individuals compared to non-diabetic individuals

(Cryer et al., 2003). The glycaemic threshold for individuals with poorly controlled type

1 diabetes is higher (Amiel et al., 1988; Boyle et al., 1988) than in non-diabetic

individuals. In contrast, the plasma glucose level threshold is generally lower in tightly

controlled type 1 diabetes (Amiel et al., 1988). The degree of attenuation, absence or

lowering of the glycaemic threshold for these counterregulatory responses appears to be

associated with a longer duration of type 1 diabetes (Bolli et al., 1983). Strict glycaemic

control and intensive insulin therapy (Amiel et al., 1987; 1998) are associated with

impaired counterregulation, thus increasing the risk of severe hypoglycaemia (DCCT,

1997).

One key factor that can alter the counterregulatory response to hypoglycaemia in

individuals with type 1 diabetes is a recent previous episode of hypoglycaemia. Recent

antecedent hypoglycaemia reduces the glycaemic thresholds for the activation of

counterregulatory responses to subsequent hypoglycaemia (Amiel et al., 1988). This is

because recurrent hypoglycaemia interferes with glucose sensors and neural networks that

detect hypoglycaemia (McCrimmon, 2009). Antecedent hypoglycaemia also blunt the

magnitude of the response of counterregulatory hormones to subsequent hypoglycaemia

and increases the risk of future episodes of hypoglycaemia (Davis and Shamoon, 1991;

Heller and Cryer, 1991; Widom and Simonson, 1992; Dagogo-Jack et al., 1993; Hvidberg

et al., 1996; Davis et al., 1997, 2000b, 2000c; Shum et al., 2001; Cryer et al., 2003). Even

mild episodes of hypoglycaemia can attenuate this counterregulatory response; however

the depth of hypoglycaemia is an important determinant of the impaired

counterregulatory response to a subsequent hypoglycaemic episode with a dose-response

effect of antecedent hypoglycaemia (Davis et al., 1997). For instance, mild

hypoglycaemia of ~3.9 mM can reduce epinephrine, muscle sympathetic nervous system

and glucagon responses to hypoglycaemia by ~30% the next day (Davis et al., 1997).

Lower glycaemia of ~3.3 mM also reduces the response of these glucoregulatory

hormones together with blunting norepinephrine, growth hormone, endogenous glucose

production, pancreatic polypeptide and lipolytic responses to hypoglycaemia (Davis et

14

al., 1997). Interestingly, an even lower antecedent blood glucose level of ~ 2.9 mM

elicits a similar impaired counterregulation to ~3.3 mM (Davis et al., 1997). Together

with this dose-response effect of antecedent hypoglycaemia on subsequent

counterregulatory response to hypoglycaemia, the duration and frequency of

hypoglycaemic episodes can also influence this counterregulatory response. Indeed, there

seems to be a hierarchical effect of the duration of antecedent hypoglycaemia on the

reduction of the counterregulatory responses to subsequent hypoglycaemic episodes

(Davis et al., 2000b).

Recurring episodes of hypoglycaemia are also associated with marked attenuation of the

warning symptoms of hypoglycaemia (Bolli, 2003). This leads to a situation where

individuals with type 1 diabetes often suffer from a reduced ability or failure to recognise

hypoglycaemia, a condition referred to as hypoglycaemia unawareness (Cryer et a., 2003;

Alsahli and Gerich, 2008). As a result, these patients are unable to detect and therefore

correct hypoglycaemia (Bolli, 2003), increasing markedly their risk of severe

hypoglycaemia (Gold et al., 1994).

Antecedent exercise can also impair one’s capacity to counter a subsequent episode of

hypoglycaemia in individuals with type 1 diabetes as exercise impairs the

counterregulatory response to subsequent hypoglycaemia (Galassetti et al., 2001a;

Sandoval et al., 2004, 2006). Although exercise and hypoglycaemia can blunt

counterregulation to a similar level (Sandoval et al., 2006), the increase in insulin

sensitivity that occurs as a result of exercise can elevate the risk for hypoglycaemia during

subsequent hypoglycaemic episodes (Briscoe et al., 2007, Galassetti and Riddell, 2013).

Finally, one important factor that can aggravate the risk of hypoglycaemia in individuals

with type 1 diabetes is physical activity. Participation in regular physical activity

provides numerous health benefits for individuals with type 1 diabetes, including weight

control, improvement of muscle strength, lowering atherosclerosis risk factors, and

overall improvement in cardiovascular function (Chimen et al., 2012). Unfortunately,

exercise for these individuals increases the risk of hypoglycaemia during and for several

hours after exercise (MacDonald, 1987; Tsalikian et al., 2005; McMahon et al., 2007).

For this reason, many individuals with type 1 diabetes are often reluctant to be physically

active and thus miss out on the many physical and psychological benefits of an active

lifestyle (Ludvigsson et al., 1980). In order to fully appreciate how exercise increases the

15

risk of hypoglycaemia, we must examine how blood glucose levels are regulated during

exercise in healthy individuals. Here we will focus on sustained aerobic exercise of

moderate intensity, aerobic/anaerobic exercise of high intensity, intermittent high

intensity exercise, and maximal sprint effort, but not on other forms of exercise such as

resistance exercise.

1.5 Glucoregulatory Responses to Moderate Intensity Exercise in Non-Diabetic

Individuals

During moderate intensity exercise in healthy non-diabetic individuals, stable blood

glucose levels are maintained by feedback mechanisms that allow the increase in glucose

utilisation by exercising muscles to be precisely matched by an equal increase in glucose

production rate (Richter and Galbo, 1986; Marliss and Vranic, 2002). What is still a

source of some debate is the mechanisms that coordinate liver and muscle responses to

exercise and the role of insulin, glucagon, catecholamines together with other

counterregulatory hormones and metabolites in this precise matching of glucose kinetics.

At the start of moderate intensity exercise, it is well accepted that the contraction of

skeletal muscle induces a rapid increase in glucose uptake (Wahren and Ekberg, 2007).

At rest, only 15-20% of peripheral glucose utilisation is attributed to skeletal muscle, but

during moderate intensity exercise at 55-60% V O2max skeletal muscle accounts for up to

80-85% of whole body glucose disposal (Hargreaves and Spriet, 2006). This marked

increase in muscle glucose uptake during exercise (Camacho et al., 2005; Hargreaves and

Spriet, 2006) is explained by an exercise-mediated stimulation of the translocation of a

non-insulin responsive pool of the glucose transporter GLUT4 (Douen et al., 1989;

Coderre et al., 1995), together with increased blood flow, capillary recruitment, glucose

extraction and improved glucose delivery to skeletal muscle.

Although there is a significant increase in the rate of glucose removal from the blood

during moderate intensity exercise, non-diabetic individuals do not become

hypoglycaemic. Instead, their blood glucose levels remain stable as a result of a matched

increase in the rate of hepatic glucose production (Camacho et al., 2005). The activation

of hepatic glycogenolysis and gluconeogenesis increases hepatic glucose production, with

the relative contributions of each metabolic pathway changing with exercise duration

16

(Trimmer et al., 2002; Wahren and Ekberg, 2007) and intensity (MacRae et al., 1995;

Staehr et al., 2007) and dietary state (Staehr et al., 2007), with both prolonged fasting and

exercise being favourable to hepatic gluconeogenesis (Trimmer et al., 2002; Staehr et al.,

2007). At the start of exercise, hepatic glycogenolysis accounts for most of the increase

in glucose produced by the liver. As time progresses, the rate of glycogenolysis declines

with decreasing hepatic glycogen stores, and gluconeogenesis becomes increasingly more

important (Richter & Galbo, 1986; Camacho et al., 2005; Hargreaves & Spriet, 2006;

Wahren & Ekberg, 2007). If the duration of exercise is more than two hours, the

depletion of hepatic glycogen stores together with an inadequate compensatory increase

in gluconeogenesis can result in declining glycaemia and even hypoglycaemia (Trimmer

et al., 2002; Camacho et al., 2005).

The increase in hepatic glucose production during exercise results to some extent from

the combined fall in insulin level and rise in glucagon concentration (Marliss and Vranic,

2002; Camacho et al., 2005). A fall in insulin level is required for a full increase in hepatic

glycogenolysis (Wasserman et al., 1989b), whereas elevation in glucagon level is

necessary for both increased hepatic glycogenolysis and gluconeogenesis (Wasserman et

al., 1989a). Although such a fall in insulin and increase in glucagon levels are often

observed during moderate intensity exercise, some studies have reported that the levels

of these hormones, in particular glucagon levels, do not change or only change slightly

(Wasserman et al., 1993; Wasserman, 2009). It is important to note, however, that the

observation that the levels of these hormones change little or not at all (Wasserman et al.,

1993; Wasserman, 2009) informs us little about their importance as ultimately it is the

glucagon/insulin ratio (Richter & Galbo, 1986) and the portal levels of these hormones

that determine their effects on hepatic glucose production (Wasserman et al., 1989a). The

small changes in peripheral glucagon levels during exercise are due to the hepatic

extraction of glucagon released by the pancreas, leading to lesser rise in peripheral

glucagon levels and the underestimation of the physiological importance of glucagon in

blood glucose regulation (Wasserman et al., 1993). It has been shown that even a small

increase in glucagon level can have a marked effect on hepatic glucose production as the

potency of a given glucagon level is enhanced considerably during exercise compared to

rest (Wasserman et al., 1989a; Wasserman, 2009).

One powerful approach to evaluate the relative contributions of insulin and glucagon in

the activation of glucose production during exercise without the confounding effect of the

17

counterregulatory response to falling blood glucose level is to manipulate portal glucagon

and insulin levels by infusing an inhibitor (e.g. somatostatin, octreotide) of their

pancreatic release together with the infusion of insulin, glucagon and glucose to maintain

euglycaemia, a technique known as pancreatic islet clamp. Using this technique, insulin

and glucagon have been reported to account for ~55% and ~60% of the exercise-mediated

increase in hepatic glucose production in dogs, respectively (Wasserman et al., 1989a,

1989b). Similar techniques in humans have shown both hormones play an important role

in increasing hepatic glucose production during exercise since the absence of changes in

insulin and glucagon concentrations has been shown by many to prevent glucose

production rate from increasing (Wolfe et al., 1986; Hirsch et al., 1991; Kjaer et al.,

1993a; Lavoie et al., 1997). However, others have reported that the increase in hepatic

glucose production during moderate exercise is little or not affected when plasma insulin

and glucagon levels are held constant in humans (Bjorkman et al., 1983; Coker et al.,

2001). Therefore, despite playing some role, changes in glucagon and insulin do not

totally explain the increase in hepatic glucose production that takes place during moderate

intensity exercise in humans, thus indicating that other factors are involved (Hargreaves

and Spriet, 2006).

Catecholamines have been proposed to play some role in the activation of hepatic glucose

production during moderate intensity exercise. However, most studies have failed to

show a significant role for the sympathoadrenergic system although the increase in

epinephrine and norepinephrine levels has led some researchers to suggest the opposite.

Indeed, there is a strong correlation in humans between the exercise-induced rise in

hepatic glucose production and increases in catecholamines levels, with the amount of

active muscle mass influencing the magnitude of the rise in catecholamines (Kjaer et al.,

1991). Hypoxic exercise is another condition which results in a greater rise in both

hepatic glucose production and catecholamines (Cooper et al., 1986). Adrenal medulla

removal in rats decreases hepatic glycogenolysis (Richter et al., 1981) and hepatic glucose

production (Sonne et al., 1985) during exercise. However, some studies in rats have

shown no effect of epinephrine on hepatic glycogen breakdown during exercise

(Hargreaves and Spriet, 2006).

The small contribution of plasma catecholamines in mediating the increase in glucose

production during exercise is suggested by the studies which have shown that alpha- and

beta-adrenergic blockades have little or no effect on the rise in hepatic glucose production

18

during moderate exercise in dogs (Coker et al., 1997) and humans (Simonson et al, 1984;

Marker et al., 1991). Also, direct adrenergic stimulation at physiological dose has little

effect on the rate of hepatic glucose production during exercise even in the absence of

changes in glucagon and insulin levels (Coker et al., 2002). The small role played by

circulating catecholamines is not this surprising when one considers that epinephrine

levels in the portal circulation are much lower than in peripheral blood where

measurements are normally performed. This is because the gut extracts a large proportion

of blood epinephrine before it reaches portal circulation (Coker and Kjaer, 2005).

The role of hepatic nerves in the activation of hepatic glucose production has also been

investigated, with most studies suggesting that stimulation of hepatic nerves plays a

minimal role. For instance, hepatic denervation does not affect the rise in hepatic glucose

production during exercise in rats (Richter et al., 1980; Sonne et al., 1985) or dogs

(Wasserman et al., 1990). Similarly, blockade of sympathoadrenergic activity in healthy

males during exercise by local anaesthesia of the celiac ganglion that usually innervate

the liver and adrenal medulla, together with the infusion of glucagon and insulin

hormones and physiological doses of epinephrine to mimic usual responses to exercise,

has no effect on the increase in hepatic glucose production rate during exercise, indicating

that sympathetic liver nerve activity is unlikely to be involved (Kjaer et al., 1993a).

Likewise, patients with denervated liver transplants experience similar increases in

hepatic glucose production during exercise to those of control individuals, indicating

hepatic nerve activity is not an important glucoregulatory factor during exercise (Kjaer et

al., 1995).

Other than the sympathoadrenal system and pancreatic hormones, it has been proposed

that glucose itself plays some role in the regulation of hepatic glucose production during

exercise. In support of this view, a fall in glucose level during moderate intensity exercise

stimulates hepatic glucose production without changes in pancreatic hormone levels or

catecholamine release (Coker et al., 2002). These results support the view that

decrements in glycaemia may stimulate hepatic glucose production during moderate

exercise and therefore maintain euglycaemia. A small decline in glycaemia can also

indirectly stimulate hepatic glucose production by stimulating counterregulatory

hormone response (Wasserman et al., 1984, 1991), with an increase in growth hormone

(Shilo and Shamoon, 1990; Davis et al., 2000e) and cortisol levels being observed (Shilo

and Shamoon, 1990; Davis et al., 2000e; Horton et al., 2002). However, growth hormone

19

and cortisol are unlikely to play an important role as the levels of these hormones change

little during moderate intensity exercise, and experimentally induced growth hormone or

cortisol deficiency does not affect blood glucose levels during exercise (Hoelzser et al.,

1986a, 1986b; Wasserman, 1995). It is important to note, however, that when insulin and

glucagon levels are kept constant at basal levels during mild to moderate intensity

exercise, glucose production increases rapidly with a decrease in blood glucose level

before plateauing despite a decrease in glycaemia (Kjaer et al., 1993a). Hepatic glucose

production after prolonged low blood glucose levels thus appears reasonably insensitive

to small deceases in glycaemia (Kjaer et al., 1993a). In contrast, exogenous glucose

infusion or glucose ingestion affects insulin and glucagon secretion and markedly inhibits

hepatic glucose production during exercise (Manzon et al., 1998; Jeukendrup et al., 1999)

Jenkins et al., 1985 even in responses to very small changes in blood glucose levels

(Berger et al., 1994). This shows that hepatic glucose production is very sensitive to a

rise in blood glucose levels.

There is evidence that afferent neural reflex activity originating from exercising muscles

may be important for increasing hepatic glucose production during exercise. This is

illustrated by the observation that electrical stimulation of cut muscle branches of the

femoral nerves increases hepatic glucose production and glycaemia in cats (Vissing et al.,

1994). However, in humans, epidural blockade to evaluate the effect of neural feedback

does not change hepatic glucose production during moderate intensity exercise (Kjaer et

al., 1989). Therefore, although afferent neural reflex activity can increase glucose

mobilisation during exercise, it is likely to be secondary or only play a minor role

comparative to other mechanisms in healthy individuals.

There are other important factors that influence the glucoregulatory response to exercise.

In particular, counterregulatory responses to moderate intensity exercise are attenuated

by prior hypoglycaemia (Davis et al., 2000d), with blunting of glucagon, catecholamines,

GH, cortisol, endogenous glucose production, ketogenesis and lipolytic responses (Davis

et al., 2000d). Similarly, antecedent morning exercise of moderate intensity can

significantly impair metabolic and neuroendocrine responses during moderate exercise

performed three hours later in a gender-specific manner (Galassetti et al., 2001b).

Given that skeletal muscles release IL-6 during exercise, it has been proposed that this

might provide a means whereby muscle activity and associated fuel utilisation is related

20

to the regulation of hepatic glucose production. In support of this view, IL-6 increases

hepatic glucose production during exercise (Febbraio et al., 2004), and IL-6 levels

increase during exercise, following an almost exponential pattern reaching peak

concentration of up to 100-fold from basal levels immediately following exercise

(Pedersen and Fischer, 2007). The magnitude of IL-6 response is influenced by the

muscle mass recruited during exercise, endurance capacity, and both exercise intensity

and duration (Pedersen, 2009). IL-6 also increases both whole-body and intramuscular

fatty acid oxidation, thus contributing to the sparing of carbohydrate (Pedersen and

Febbraio, 2008). It must be stressed, however, that IL-6 also promotes fuel uptake and

utilisation in working muscle, and as a result increases whole body glucose disposal

during exercise (Febbraio et al., 2004), thus raising the question of its importance in

counterregulation.

1.6 Glucoregulatory Responses to Moderate Intensity Exercise in Individuals with

Type 1 Diabetes

In insulin-treated individuals with type 1 diabetes, moderate intensity exercise stimulates

an increase in muscle glucose utilisation that is similar in magnitude to non-diabetic

individuals (Richter and Galbo, 1986), but that increases with plasma insulin levels

(Chokkalingam et al., 2007). However, most but not all studies have reported that hepatic

glucose production does not rise to the same degree (Simonson et al., 1984; Richter and

Galbo, 1986), resulting in a decrease in blood glucose levels and a rise in hypoglycaemia

risk (Schiffrin et al., 1984; Simonson et al., 1984; Zinman et al., 1984; Hübinger et al.,

1985; Campaigne et al., 1987; Sonnenberg et al., 1990; Oskarsson et al., 1999; Riddell et

al., 1999; Francescato et al., 2004; Petersen et al., 2004). Furthermore, during recovery

from moderate intensity exercise in type 1 diabetic individuals, blood glucose levels

continue to decrease (Hübinger et al., 1985; Campaigne et al., 1987) or stabilise

(Simonson et al., 1984; Oskarsson et al., 1999) while both hepatic glucose production and

glucose utilisation decreasing back to basal levels.

The inability of hepatic glucose production rate to rise to a level that matches glucose

utilisation rate during moderate intensity exercise in individuals with type 1 diabetes is

due in part to the absence of a fall in circulating insulin levels. As discussed earlier, non-

diabetic individuals have the capacity to decrease the secretion of insulin during exercise,

thus favouring the stimulation of hepatic glucose production. In contrast, individuals with

21

type 1 diabetes are unable to control the rate of passive absorption of the insulin from the

previously administered insulin bolus, thus increasing their likelihood of exercising in a

hyperinsulinaemic state. Hyperinsulinaemia has been reported to increase the rate of

exercise-mediated glucose utilisation in type 1 diabetic individuals while having no effect

on hepatic glycogen breakdown (Chokkalingam et al., 2007).

Another factor which increases the mismatch between glucose production and utilisation

and thus increase further the risk of hypoglycaemia during exercise is the

hyperinsulinaemia that may occur as a result of increased insulin mobilisation from

subcutaneous depots, especially if insulin is injected near an exercising muscle (Koivisto

and Felig, 1978), with the rates of insulin absorption often increasing during exercise

(Richter & Galbo, 1986). Significant increases in insulin concentrations is also observed

during exercise in insulin-infused diabetic individuals, thus suggesting reduced insulin

clearance (Chokkalingam et al., 2007). Overall, since exercise not only increases insulin

absorption from subcutaneously administered injection depots, but also decreases insulin

clearance, these factors lead to elevated circulating insulin levels during exercise. This

state of over-insulinisation combined with exercise itself results in a profound increase in

insulin action (Camacho et al., 2005). Indeed, increased insulin level and action can

negate the effect of glucagon on hepatic glucose production and amplify glucose

utilisation to greater than required levels, thus resulting in a drop in blood glucose level

(Camacho et al., 2005). Whether the resulting fall in glycaemia reaches dangerous

hypoglycaemic levels greatly depends on pre-exercise blood glucose levels (Richter and

Galbo, 1986). Thus, in general, if insulin concentration is elevated, the lack of exercise-

induced decrease in insulin levels results in a marked activation of glucose utilisation

(Camacho et al., 2005). This together with decreased hepatic glucose production results

in a fall in blood glucose levels (Zinman et al., 1977) and a rise in risk of hypoglycaemia.

This risk is further exacerbated by the symptoms of hypoglycaemia that may alert an

individual to an impending episode such as sweating and tachycardia being difficult to

notice as they may be similar to normal responses to exercise.

Aside from the marked difference in insulin response to moderate intensity exercise

between diabetic and non-diabetic individuals, the response of counterregulatory

hormones to moderate intensity exercise in insulin-treated individuals with type 1

diabetes is relatively similar to that of non-diabetic individuals. Glucagon levels are

increased (Oskarsson et al., 1999; Galassetti et al., 2002, 2003, 2004, 2006) or unchanged

22

(Simonson et al., 1984; Hübinger et al., 1985; Sonnenberg et al., 1990), catecholamines

levels are increased (Simonson et al., 1984; Hübinger et al., 1985; Sonnenberg et al.,

1990; Oskarsson et al., 1999; Galassetti et al., 2002, 2003, 2004, 2006), and growth

hormone levels are elevated (Hübinger et al., 1985; Shilo and Shamoon, 1990;

Sonnenberg et al., 1990; Oskarsson et al., 1999; Galassetti et al., 2002, 2004). In contrast,

cortisol levels either remain stable (Hübinger et al., 1985; Sonnenberg et al., 1990) or

increase later (Oskarsson et al., 1999; Galassetti et al., 2002, 2003, 2004, 2006) during

moderate intensity exercise. It is noteworthy that despite the exercise-induced increase

in the levels of the circulating counterregulatory hormones being similar between diabetic

and non-diabetic individuals, hypoglycaemia risk is increased in insulin-treated diabetic

individuals, thus highlighting the glucoregulatory importance of insulin in glucose

homeostasis during moderate intensity exercise (Galassetti, et al., 2006).

There are a number of important factors that affect the counterregulatory response to

exercise in individuals with type 1 diabetes. In particular, this response is attenuated by

prior hypoglycaemia (Galassetti et al., 2003, 2006). This blunting effect appears to be

dose-dependent, with hypoglycaemia of increasing depth progressively inducing more

acute counterregulatory failure, with a reduction in glucagon, catecholamine, cortisol,

endogenous glucose production, and lipolytic responses (Galassetti et al., 2006). Prior

aerobic exercise also impairs the counterregulatory response associated with a subsequent

bout of moderate intensity exercise (Galassetti et al., 2001b).

Another important factor that influences glycaemic response to moderate intensity

exercise and associated risk of hypoglycaemia is the increase in insulin sensitivity that

occurs in response to exercise (Richter et al., 1982; Holloszy, 2005; Jensen and Richter,

2012). This together with the exercise-induced increase in glucose utilisation in insulin-

treated type 1 diabetic individuals can lead to a rapid decline in blood glucose levels not

only during exercise, but also for several hours afterwards (MacDonald, 1987b; Hirsch et

al., 1991; Tsalikian et al., 2005; McMahon et al., 2007). For this reason, it is generally

recommended that insulin dose should be reduced and carbohydrate intake increased prior

to and after exercise to reduce the aforementioned risk of hypoglycaemia (ADA, 2007;

Dube et al., 2005; Zinman et al., 2004).

It must be stressed, however, that in cases of severe insulin deficiency exercise provokes

a rise in blood glucose levels that increase further during recovery (Wahren et al., 1975).

23

The increase in blood glucose levels is explained in part on the basis that the magnitude

of the increase in muscle glucose utilisation is reduced due to the small additive effect of

exercise and insulin. In addition, low insulin is conducive to increased hepatic glucose

production at rates greater than rates of glucose utilisation, thus resulting in

hyperglycaemia (Riddell and Perkins, 2006). Finally, severe insulin deficiency is

accompanied by elevated glucagon levels, thus favouring the activation of hepatic glucose

production. Therefore, exercising in a state of severe insulin deficiency is not

recommended as the resulting lower insulin to glucagon ratio not only increases

glycaemia, but also ketone body levels, thus substantially increasing the risk of severe

ketoacidosis (Berger et al., 1980; Richter and Galbo, 1986) and ketoacidotic coma.

1.7 Glucoregulatory Responses to High-Intensity Aerobic Exercise in Non-Diabetic

Individuals

Although moderate intensity exercise increases risk of hypoglycaemia in insulin-treated

type 1 diabetic individuals, it is important to note that not all types of exercise increase

this risk. In fact, there are conditions where exercise is actually conducive to

hyperglycaemia not only in insulin-treated individuals with diabetes but also in non-

diabetic individuals. For instance, blood glucose levels in non-diabetic (Stokes et al.,

2013) and diabetic individuals (Turner et al., 2014) increase in response to resistance

exercise or when exercise is performed for at least 10 min at an intensity greater than 80%

of maximum rate of oxygen consumption ( V O2max; Mitchell et al., 1988; Marliss et al.,

1991,1992b; Purdon et al., 1993; Sigal et al., 1994b; Kreisman et al., 2000a; Marliss et

al., 2000; Sigal et al., 2000; Harmer et al., 2008; Manzon et al., 1998; Marliss & Vranic,

2002; Wahren & Ekberg, 2007). This rise is glycaemia is followed by a period of

hyperglycaemia that can persist for up to an hour post-exercise (Marliss et al., 1991, 2000;

Sigal et al., 2000).

The glycaemia-rising effect of intense aerobic exercise has been explained on the basis

that this type of exercise results in a seven- to eight-fold increase in glucose production

(Marliss et al., 1991,1992b; Purdon et al., 1993; Sigal et al., 1994b), the greatest increase

in glucose production observed under any physiological or pathophysiological condition

(Marliss and Vranic, 2002). This is accompanied by a lesser increase in glucose

utilisation rates of approximately 3-4 fold (Marliss et al., 1991, 1992b; Purdon et al.,

1993; Sigal et al., 1994b). This greater rate of glucose production in comparison to

24

glucose utilisation results in an increase in plasma glucose levels. A further increase in

glycaemia also occur immediately after the cessation of prolonged intense aerobic

exercise in non-diabetic individuals (Mitchell et al., 1988; Marliss et al., 1991, 1992b;

Sigal et al., 1994a, 1994b; Kreisman et al., 2000a; Marliss et al., 2000; Sigal et al., 2000),

before blood glucose levels return to pre-exercise levels. This is due to glucose utilisation

rate initially decreasing more rapidly than glucose production rates at the onset of

recovery, thus contributing further to greater hyperglycaemia (Marliss et al., 1992b; Sigal

et al., 1994b). This increase in glycaemia is only of short duration, with blood glucose

returning to pre-exercise levels by 60 min post-exercise.

Unlike moderate intensity exercise, the significant increase in hepatic glucose production

during high intensity exercise is unlikely to be due to changes in pancreatic hormone

levels. Insulin levels actually remain unchanged (Kjaer et al., 1986; Mitchell et al., 1988;

Marliss et al., 1991, 1992b; Kreisman et al., 2000a), or decline only slightly (Sigal et al.,

1994b; Marliss et al., 2000; Sigal et al., 2000), while glucagon levels increase minimally

(Marliss et al., 1991, 1992b; Purdon et al., 1993; Sigal et al., 1994b), resulting in a small

increase in glucagon to insulin ratio (Sigal et al., 1996; Manzon et al., 1998; Sigal et al.,

2000). As mentioned previously, it is important to note that changes in plasma glucagon

levels may underestimate the portal levels of this hormone due to hepatic extraction thus

suggesting glucagon could play an important role than that suggested by the

aforementioned studies. However, experiments involving islet clamps where insulin and

glucagon are kept at stable and basal levels show that this has no effect on hepatic glucose

production during intense aerobic exercise, thus indicating that these hormones are

unlikely to play a major role in increasing hepatic glucose production during high

intensity exercise (Sigal et al., 1996; Marliss and Vranic, 2002). A minor role for

pancreatic hormones is also suggested by the observation that the pattern of rise in glucose

production in response to high intensity exercise persists in glucose-infused individuals

despite high plasma insulin levels and insulin/glucagon ratios (Manzon et al., 1998).

The marked catecholamine response to high intensity exercise is thought to play a major

role in the activation of hepatic glucose production and rise in blood glucose levels in

non-diabetic individuals (Marliss and Vranic, 2002). This is supported by the observation

that the increase in glucose production associated with high intensity aerobic exercise is

accompanied by a 14- to 18-fold increase in both epinephrine and norepinephrine levels

(Kjaer et al., 1986; Marliss et al., 1991, 1992b; Sigal et al., 1994b; Manzon et al., 1998;

25

Kreisman et al., 2000a; Marliss et al., 2000; Sigal et al., 2000). However, this relationship

between catecholamine levels and an increase in hepatic glucose production during high

intensity exercise only suggests causality.

One approach to establish causality is to determine whether adrenergic blockade inhibits

the rise in hepatic glucose production during intense exercise. Although there is a well

demonstrated relationship between exercise-induced changes in glucose output and

circulating catecholamine levels, a role for catecholamines is challenged by the

observation that the administration of alpha or beta adrenergic blockers during high

intensity exercise has no inhibitory effect on the rise in hepatic glucose production (Sigal

et al., 1994b; Coker et al., 1997; Sigal et al., 1999, 2000). Indeed, the infusion of

propanolol (beta-adrenergic blocker) in non-diabetic individuals (Sigal et al., 1994a)

results in an unexpected higher rise rather than a lesser increase in hepatic glucose

production, and only a minor inhibition of glucose production is achieved in response to

phentolamine infusion (alpha-adrenergic blocker) during intense exercise in humans

(Sigal et al., 2000). Moreover, intraportal infusion of propanol and phentolamine to

selectively block hepatic and adrenergic receptors in dogs has no effect on endogenous

glucose production or net hepatic glucose output during intense exercise (Coker et al.,

1997). Overall, these findings do not exclude the possibility that other factors may

contribute to the regulation of glucose production during intense exercise, particularly

considering the lack of any effect of adrenergic blockade on glucose production (Sigal et

al., 1994a, 1999, 2000)” One limitation with these studies, however, is that it remains to

be determined whether the dose of adrenergic blockers administered was sufficient to

counter the effect of the large rise in catecholamines associated with intense exercise.

One of the most convincing pieces of evidence that catecholamines play a role in

stimulating hepatic glucose production during intense aerobic exercise is the fact that

when epinephrine and norepinephrine are infused during moderate intensity exercise to

mimic the levels attained during higher intensity exercise there is a further increase in

hepatic glucose production and rise in blood glucose levels that matches these observed

during high intensity exercise, therefore suggesting an important role for epinephrine and

norepinephrine. When infused alone, epinephrine (Howlett et al., 1999a; Kreisman et al.,

2000b) and norepinephrine (Kreisman et al., 2001) are each considered capable of

producing a portion of the increase in hepatic glucose production associated with intense

exercise. Moreover, blockade of sympathetic nerve activity to liver and adrenal medulla

26

by local anaesthesia of the celiac ganglion followed by infusion of high physiological

doses of epinephrine during exercise resulted in enhanced glucose production in healthy

males (Kjaer et al., 1993a). Also, when both catecholamines are infused together to mimic

the usual epinephrine and norepinephrine response to high intensity exercise in healthy

men, there is a marked and progressive increase in hepatic glucose production and

increase in glycaemia, suggesting a role for catecholamines in the regulation of hepatic

glucose production during high intensity exercise (Kreisman et al., 2003). However, it is

noteworthy that in epinephrine-deficient, bilaterally adrenalectomised humans, the

absence of epinephrine does not impair glucose production during high intensity exercise

(Howlett et al., 1999b), thus suggesting that norepinephrine might play a more important

role here and that other glucoregulatory factors might be involved.

Acute changes in the levels of other glucoregulatory hormones such as growth hormones

and cortisol are unlikely to have any effect on the acute increase in blood glucose levels

associated with high intensity exercise. This is, in part, because the levels of these

hormones change little during a short bout of intense exercise, with their levels increasing

mainly during recovery (Marliss & Vranic, 2002). Also, the infusion of octreotide to

inhibit insulin, glucagon and growth hormones secretion while glucagon and insulin

levels are replaced and maintained at stable levels has no effect on the increase in glucose

production during intense exercise (Marlis & Vranic, 2002; Sigal et al., 1996).

In order to understand how intense exercise increases blood glucose level it is important

to focus not only on the potential effect of catecholamines and other factors on hepatic

glucose production, but also on the factors that influence the rates of glucose utilisation

during exercise. On the basis of the findings by some that catecholamines inhibit insulin-

stimulated glucose utilisation in resting skeletal muscles (Aftab-Guy et al., 2005;

Wasserman, 1995) and glucose Rd during exercise (Howlett et al., 1999; Watt &

Hargreaves, 2002; Watt et al., 2001), in part via a glycogenolytically-mediated increase

in glucose-6-phosphate level (Watt et al., 2001), a potent inhibitor of hexokinase

(Nonogaki, 2000), it is possible that the lesser rise in glucose utilisation relative to glucose

production during intense exercise is due to a catecholamine-mediated inhibition of the

stimulatory effect of muscle contraction on peripheral glucose utilisation. This issue was

investigated in epinephrine-deficient, bilaterally adrenalectomised humans with

epinephrine infusion inhibiting glucose clearance during exercise (Howlett et al., 1999b).

Similarly, studies in rodents have also shown that epinephrine infusion decreases muscle

27

glucose transport despite an elevation in glucose transporter (GLUT4) translocation to

the plasma membrane, which suggests a reduction in GLUT4 intrinsic activity (Bonen et

al., 1992). Despite these findings, other researchers have found that elevated

catecholamines in humans stimulate rather than inhibit glucose utilisation (Kreisman et

al., 2000b, 2001, 2003), and these investigators proposed that the increase in blood

glucose levels during intense aerobic exercise could simply be the result of a greater

stimulation of glucose production by catecholamines compared to glucose utilisation rates

(Kreisman et al., 2003).

The apparent contradiction with respect to the role of catecholamines on glucose

utilisation rate is not surprising given the evidence that the stimulation of muscle

adrenoceptors by catecholamines and other adrenoceptor agonists can stimulate glucose

transport in skeletal muscle (Abe et al., 1993; Ngala et al., 2013), with the effect of

adrenoceptor antagonists having little or a marked effect depending on muscle fibre

compositions (Liu et al., 1995). Also, predicting the effect of catecholamines on

peripheral glucose utilisation rate is further complicated by the observation that

circulating epinephrine inhibits glucose uptake in skeletal muscle by insulin-dependent

mechanisms, whereas norepinephrine released from sympathetic nerves increases glucose

uptake via insulin-independent mechanisms (Nonogaki, 2000).

The post-exercise transient increase in blood glucose levels and its subsequent fall are

explained on the basis of the pattern of change in plasma insulin, glucose and

catecholamines levels. Immediately after intense exercise, catecholamine levels are

elevated and inhibit insulin secretion, thus contributing to a further transient increase in

blood glucose levels. Afterwards, the rapid decrease in catecholamine levels and high

blood glucose levels result in a marked rise in plasma insulin level (Marliss et al., 1991;

Purdon et al., 1993; Sigal et al., 1994a, 1994b; Kreisman et al., 2000a; Marliss et al.,

2000) that helps to decrease glucose back to basal level (Marliss and Vranic, 2002).

Indeed, elevated blood glucose and insulin levels enhance the rate of glucose transport

during recovery from intense exercise while inhibiting hepatic glucose production. It is

noteworthy that this rapid decline in epinephrine and norepinephrine levels during the

first few minutes of recovery (Marliss et al., 1991; Kreisman et al., 2000a; Marliss et al.,

2000; Sigal et al., 2000) is closely matched with the post-exercise decrease in hepatic

glucose production (Marliss et al., 2000; Marliss and Vranic, 2002), supporting the

opinion that declining catecholamine levels may potentially mediate the post-exercise fall

28

in glycaemia in non-diabetic individuals. Other factors might contribute to the fall in

hepatic glucose production during recovery. During recovery from high intensity aerobic

exercise, lactate (Marliss et al., 1991; Sigal et al., 1994b; Kreisman et al., 2000a; Marliss

et al., 2000) levels decline. Since lactate is a potential glucoregulatory precursor, lower

lactate levels would be expected to result in a decrease in hepatic gluconeogenesis.

Likewise, glucagon levels decline (Marliss et al., 1991; Sigal et al., 1994b) or remain

relatively stable during recovery is taking place during high intensity exercise (Marliss et

al., 2000), thus producing a further fall in hepatic glucose production. Other

counterregulatory hormones such as growth hormone and cortisol are unlikely to be

important in the fall in glycaemia observed during recovery from prolonged high intensity

exercise.

It is noteworthy that the glucoregulatory response to prolonged high intensity exercise is

affected by a number of factors. Firstly, the state of feeding seems to be one of these

factors, with a smaller increase in glycaemia observed during high intensity exercise

performed postprandially as opposed to in a postabsorptive state (Kreisman et al., 2000a).

Unlike moderate intensity exercise, glucose production during high intensity exercise is

not as sensitive to the inhibitory effect of hyperglycaemia (Manzon et al., 1998; Wiersma

et al., 1993). Gender may also play a role, since women experience a greater rise in

glycaemia in the post-exercise period due to a lesser increment in glucose utilisation rate

(Marliss et al., 2000) despite men and women experiencing a similar glucoregulatory

response to high-intensity exercise. This is also associated with higher insulin levels in

women compared to men post-exercise, although glucose levels return to basal within a

similar time-frame (Marliss et al., 2000). Finally, training may influence the

glucoregulatory response to high intensity exercise, with an increase in exercise-induced

epinephrine levels contributing to a greater increase in blood glucose levels due to

increased hepatic glucose production in exercise-trained individuals (Kjaer et al., 1986).

However, recently it has been reported that the rise in blood glucose levels in response to

intense exercise is less in sprint-trained individuals (Harmer et al., 2007).

29

1.8 Glucoregulatory Responses to High-Intensity Aerobic Exercise in Individuals

with Type 1 Diabetes

Not all forms of exercise result in an acute fall in blood glucose levels. For instance,

resistance exercise has been shown in some studies to result in a post-exercise increase in

blood glucose level (Turner et al., 2014). Also, when exercise is performed at high

intensity (>80% V O2max) for at least 10 min in insulin-treated patients with type 1

diabetes, blood glucose levels increase (Mitchell et al., 1988; Purdon et al., 1993; Sigal

et al., 1994a) in a manner similar to non-diabetic individuals. One major difference,

however, is that during recovery, blood glucose level rises and remains elevated for up to

several hours post-exercise. This is in marked contrast to non-diabetic individuals where

blood glucose levels return to pre-exercise levels within one hour.

The increase in blood glucose levels during intense aerobic exercise in diabetic

individuals is due to the approximately 7-fod increase in hepatic glucose production rate

(Purdon et al., 1993) being higher than the approximately 4-fold increase in glucose

utilisation (Purdon et al., 1993; Sigal et al., 1994a). During early recovery, blood glucose

levels continue to rise for a few minutes in these individuals due to glucose production

rate being comparatively higher than glucose utilisation (Purdon et al., 1993; Sigal et al.,

1994a). Then glucose production and utilisation rate decline, with these processes

matching each other as hyperglycaemia remains for at least an hour (Sigal et al., 1999)

and up to several hours after the cessation of exercise (Mitchell et al., 1988; Purdon et al.,

1993; Sigal et al., 1994a).

The glucoregulatory mechanisms responsible for the increase in blood glucose level

during high intensity exercise are similar in both diabetic and non-diabetic individuals.

A large catecholamine response to high intensity exercise is also reported in individuals

with diabetes, with a similar relative increase of approximately 14-fold for both

epinephrine and norepinephrine levels (Purdon et al., 1993). Likewise, glucagon

response to high intensity exercise is similar in both diabetic and non-diabetic individuals,

together with similar increase in lactate (Mitchell et al., 1988) and cortisol levels (Purdon

et al., 1993). However, the pattern of free fatty acid response is different, with an increase

during recovery from high intensity exercise in individuals with diabetes, while free fatty

levels decrease in non-diabetic individuals (Mitchell et al., 1988).

30

The sustained rise in glycaemia throughout recovery in diabetic compared to non-diabetic

individuals results mainly from the absence of a post-exercise increase in insulin levels

(Mitchell et al., 1988; Purdon et al., 1993; Sigal et al., 1994a, 1999). Such a role for

insulin is suggested by the observation that the administration of insulin during recovery

from high intensity exercise accelerates the return of blood glucose to pre-exercise levels

and prevents prolonged hyperglycaemia (Purdon et al., 1993; Sigal et al., 1994a). Due to

the absence of a post-exercise increase in insulin release, there is no stimulus to promote

glucose uptake by skeletal muscles or to inhibit hepatic glucose production by the liver.

For these reasons, blood glucose levels in individuals with type 1 diabetes remain elevated

or continues to rise well after the cessation of exercise (Mitchell et al., 1988; Purdon et

al., 1993; Sigal et al., 1994a, 1999; Marliss and Vranic, 2002).

1.9 Glucoregulatory Responses to Intermittent High Intensity Exercise in Non-

Diabetic Individuals

Although the glucoregulatory responses to both moderate and high intensity exercise have

been researched comprehensively as described above, this is not the case for intermittent

high intensity, a pattern of activity where moderate intensity is combined with multiple

sprints performed in succession. This is unfortunate given that this pattern of physical

activity characterises most team sports such as soccer, rugby, Australian Rules football,

netball and basketball (Docherty et al., 1988; Bangsbo et al., 1991; Spencer et al., 2004;

Spencer et al., 2005; Bishop and Wright, 2006; Davidson and Trewartha, 2008).

This combination of multiple sprints interspersed with active or passive recovery

generally results in an increase in blood glucose levels and glucoregulatory responses

similar to high intensity exercise. In particular, multiple prolonged sprints of 45-60 sec

duration each interspersed with 3-5 min periods result in a progressive rise in glycaemia

after each work period in non-diabetic individuals (Hermansen et al., 1970). Blood

glucose often continues to rise following the cessation of exercise (Hermansen et al.,

1970; Näveri et al., 1985) with a peak in glycaemia reported ~5 min into recovery

(Hermansen et al., 1970) before blood glucose declines to pre-exercise levels. Such

multiple sprints of 45-60 sec duration are associated with a several-fold increase in the

levels of epinephrine, norepinephrine, lactate, glucagon, growth hormone and insulin

levels in non-diabetic individuals (Näveri et al., 1985). It is important to note, however,

that these prolonged sprints are physically demanding and are not representative of those

31

observed in team sports where each sprint lasts generally less that 10 sec (Spencer et al.,

2005). Sports such as soccer, rugby, hockey, netball, basketball and Australian Rules

football have average sprint duration of 2-4 sec (Docherty et al., 1988; Bangsbo et al.,

1991; Spencer et al., 2004, 2005; Bishop and Wright, 2006; Davidson and Trewartha,

2008).

Several studies have investigated on the glucoregulatory response to multiple sprint

protocols that more closely mimic team sport with shorter duration sprints of 10 sec or

less (Brooks et al., 1990; Hamilton et al., 1991; Gaitanos and Williams, 1993; Nevill et

al., 1993; Goto et al., 2007; Trapp et al., 2007; Bracken et al., 2009). These patterns of

exercise result in an overall increase in glycaemia post-exercise in non-diabetic

individuals (Holmyard et al., 1988; Brooks et al., 1990; Hamilton et al., 1991; Gaitanos

and Williams, 1993; Nevill et al., 1993). Blood glucose levels typically peak early during

recovery (3-5 min; Brooks et al., 1990; Gaitanos and Williams, 1993), with blood glucose

levels increasing by approximately 1-2.5 mM (Brooks et al., 1990; Gaitanos and

Williams, 1993; Nevill et al., 1993).

This increase in glycaemia in non-diabetic individuals after multiple short sprints is

associated with a several-fold increase in epinephrine (Brooks et al., 1990; Gaitanos and

Williams, 1993; Goto et al., 2007; Trapp et al., 2007; Bracken et al., 2009),

norepinephrine (Brooks et al., 1990; Gaitanos and Williams, 1993; Goto et al., 2007;

Trapp et al., 2007; Bracken et al., 2009) and blood lactate levels (Brooks et al., 1990;

Hamilton et al., 1991; Gaitanos and Williams, 1993; Nevill et al., 1993; Goto et al., 2007;

Trapp et al., 2007; Bracken et al., 2009). When multiple short sprints of less than 10 sec

duration are performed in non-diabetic individuals, a decrease in pH is also observed

(Brooks et al., 1990; Gaitanos and Williams, 1993; Goto et al., 2007; Bracken et al., 2009)

together with an increase in growth hormone (Goto et al., 2007), and glycerol levels (Goto

et al., 2007; Trapp et al., 2007).

1.10 Glucoregulatory Responses to Intermittent High Intensity Exercise in

Individuals with Type 1 Diabetes

Until quite recently, there has been very limited information on the glucoregulatory

responses to intermittent high intensity exercise in individuals with type 1 diabetes. The

information that was available came from studies focusing on multiple long sprints of 20

32

to 60 sec duration. One study involved 30 min of cycling interspersed with successive

one-min periods of high intensity cycling at 100% of maximal oxygen uptake, each

separated by a one-min rest between (Sills and Cerny, 1983). This resulted in a decline

in blood glucose levels during exercise in individuals with type 1 diabetes, but glycaemia

stabilised during early recovery (Sills and Cerny, 1983). An increase in growth hormone

was also observed while glucagon and cortisol levels remained stable in those individuals

(Sills and Cerny, 1983). Another study investigating the glycaemic response of type 1

diabetic individuals to multiple 20-sec sprints at 120% V O2max performed every 2 min,

with active recovery in between high intensity bouts, shows no significant change in

blood glucose levels during the multiple sprints or recovery (Ford et al., 1999). As

mentioned above, multiple high intensity bouts of prolonged duration are both physically

demanding and do not reflect the activity patterns of repeated sprints in most sports where

a typical sprint is usually very short in duration.

Recently, some studies have been performed in individuals with type 1 diabetes to

investigate their responses to the multiple short duration sprints typical of many team

sports (Guelfi et al., 2007a). Twenty min of repeated 4-sec sprints each interspersed with

2 min of passive recovery result in a decline in glycaemia, but blood glucose levels

stabilise during recovery (Guelfi et al., 2005c). This stabilisation was associated with

elevated catecholamine and growth hormone levels (Guelfi et al., 2005c). In another

study, 30 min of repeated 4-sec sprint cycling were once again performed, each separated

by 2 min of active recovery at 40% V O2peak was compared to constant moderate intensity

cycling at 40% V O2max (Guelfi et al., 2005a). In this study, the rate of decline in

glycaemia was lower during the multiple sprint protocol compared to sustained exercise,

with no further decrease during early recovery. In contrast, blood glucose levels declined

during early recovery from sustained exercise (Guelfi et al., 2005a). The lesser rate of

fall in blood glucose levels during intermittent compared to sustained exercise was

associated with increased catecholamine and growth hormone levels (Guelfi et al.,

2005a). Such a multiple short sprint protocol induces a greater increase in glucose

production rate during exercise than moderate intensity alone, with glucose utilisation

rate being similar in response to both exercise protocols (Guelfi et al., 2007b). The

findings of the aforementioned studies and of more recent ones (Dube et al., 2013;

Campbell et al., 2014) suggest that the risk of exercise-mediated hypoglycaemia during

exercise are lower if moderate intensity exercise is interspersed with repeated short sprints

33

(Guelfi et al., 2005a). It is noteworthy that resistance exercise also results in a lesser initial

rate of fall in blood glucose level than aerobic exercise (Yardley et al., 2012, 2013).

However, the effect of intermittent high intensity exercise on the risk of late onset

hypoglycaemia was not examined in these studies.

Two recent studies performed on individuals with type 1 diabetes have investigated the

risk of late-onset post-exercise hypoglycaemia (LOPEH) following intermittent high

intensity exercise (Maran et al., 2010; Iscoe and Riddell, 2011), but with conflicting

findings. One study reported an increased risk of LOPEH with repeated 5-sec sprints

compared to moderate intensity exercise (Maran et al., 2010). However, since energy

intake post-exercise was not measured and high intensity exercise can suppress appetite

(King et al., 1994), it is possible these findings could be explained by a lower

carbohydrate intake post-exercise as opposed to the exercise protocol itself. In contrast,

another study found that repeated 15-sec sprints as part of an intermittent high intensity

exercise protocol was associated with a lower risk of LOPEH in athletes with type 1

diabetes (Iscoe and Riddell, 2011). The discrepancy between the two studies may be

explained by differences in fitness levels and training status of the participants together

with different sprint durations and the nature of the exercise protocol being performed.

1.11 Glucoregulatory Responses to a Single Short Sprint in Non-Diabetic

Individuals

In non-diabetic individuals, the performance of a single short-duration maximal sprint

effort generally results in a post-exercise increase in blood glucose level (Hermansen et

al., 1970; Schnabel et al., 1983, 1984; Cheetham et al., 1985; Lavoie et al., 1987; Brooks

et al., 1988; Nevill et al., 1989; Allsop et al., 1990; Langfort and Zarzeczny, 1997; Bell

et al., 2001; Moussa et al., 2003; Vincent et al., 2004; Saraslanidis et al., 2009; Zouhal et

al., 2009) across a broad range of sprint duration. For instance, a 6-sec sprint in untrained

males results in an increase in glycaemia of approximately 1 to 2 mM, peaking 20 to 30

min after sprinting (Moussa et al., 2003). Similarly, a longer 30-sec sprint results in an

increase in blood glucose levels of approximately 1 to 2 mM, but with maximal levels

attained approximately 5 to 10 min after the sprint (Cheetham et al., 1985; Brooks et al.,

1988; Nevill and Boobis, 1989; Allsop et al., 1990; Langfort and Zarzeczny, 1997; Bell

et al., 2001; Moussa et al., 2003; Vincent et al., 2004; Zouhal et al., 2009). Sprinting of

even longer duration (40 to 60 sec) also increases blood glucose by approximately 1 to 2

34

mM, with maximal levels attained 5 to 10 min post-sprint with an increase of (Schnabel

et al., 1983, 1984; Saraslanidis, 2009). Likewise, a 90-sec sprint increases glycaemia by

approximately 1.5 to 2 mM (Lavoie et al., 1987). Although most studies show that a short

sprint increases blood glucose level, one study has reported that glycaemia remains stable

during recovery from a single short sprint (Vincent et al., 2004).

Interestingly, Moussa and colleagues (2003) reported significantly higher blood glucose

levels following a 6-sec sprint compared to a 30-sec sprint at both 20 and 30 min of

recovery in untrained non-diabetic males. The authors could not explain these findings

on the basis of changes in the levels of the glucoregulatory hormones measured in their

study, so they hypothesised a higher rate of muscle glucose utilisation following the 30-

sec sprint than the 6-sec sprint to replenish muscle glycogen stores may explain the

smaller increase in glycaemia during recovery (Moussa et al., 2003).

The glucoregulatory responses associated with a single maximal sprint efforts are

qualitatively comparable to those associated with high-intensity aerobic exercise. A short

duration maximal sprint effort results in a strong sympatho-adrenal response resulting in

a several-fold increase in epinephrine and norepinephrine levels in non-diabetic

individuals (Kindermann et al., 1982; Schnabel et al., 1983, 1984; Lavoie et al., 1987;

Brooks et al., 1988; Nevill et al., 1989; Allsop et al., 1990; Langfort et al., 1997; Zouhal

et al., 1998; Bell et al., 2001; Zouhal et al., 2001; Jacob et al., 2002; Moussa et al., 2003;

Vincent et al., 2003; Jacob et al., 2004; Vincent et al., 2004; Botcazou et al., 2007;

Bracken et al., 2009; Zouhal et al., 2009). This large increase in blood catecholamine

levels is observed irrespective of sprint duration. Epinephrine levels double immediately

following a 6-sec sprint, with maximal levels of approximately 1.2 nmol/l and 0.6 nmol/l

in men and women, respectively (Moussa et al., 2003; Botcazou et al., 2006, 2007;

Bracken et al., 2009). The duration of the sprint is one factor influencing the

sympathoadrenal activity with significantly higher epinephrine levels recorded after a 30-

sec sprint than a 6-sec sprint in untrained males (Moussa et al., 2003). In response to a

30-sec sprint, most studies report a 3 to 7 fold increase in epinephrine levels from resting

values (Brooks et al., 1988; Nevill et al., 1989; Langfort et al., 1997; Moussa et al., 2003;

Jacob et al., 2004; Vincent et al., 2004) with a peak concentration of approximately 1.4

to 4.5 nmol/l (Brooks et al., 1988; Nevill et al., 1989; Langfort et al., 1997; Zouhal et al.,

1998, 2001; Jacob et al., 2002; Moussa et al., 2003; Vincent et al., 2003; Jacob et al.,

2004; Vincent et al., 2004; Zouhal et al., 2009). Trained individuals attain even higher

35

peak epinephrine levels, ranging from approximately 7 to 8 nmol/l (an ~ 8 to 10 fold

increase), or at least double those of untrained individuals (Zouhal et al., 1998, 2001).

One study in active individuals reported an approximately 25-fold increase in epinephrine

levels, peaking at 10.2 nmol/l immediately after a 30-sec treadmill sprint (Allsop et al.,

1990). Longer sprint lasting 40 to 60 sec also results in a large increase in epinephrine

levels (Schnabel et al., 1983, 1984; Ohkuwa et al., 1984; Schwarz et al., 1990) as is the

case for a 90-sec sprint (Kindermann et al., 1982; Lavoie et al., 1987).

Plasma norepinephrine levels also increase markedly in response to a single short sprint,

with the magnitude of the increase linked to sprint duration. A single 6-sec sprint results

in an approximately 2-fold elevation in norepinephrine levels in men (Moussa et al., 2003;

Botcazou et al., 2006, 2007; Bracken et al., 2009) and women (Botcazou et al., 2007),

with maximal levels of approximately 5 nmol/l. Significantly higher norepinephrine

levels are recorded after a 30-sec sprint, with a 5-fold increase (Moussa et al., 2003) to

approximately 13.5 nmol/l in untrained males (Moussa et al., 2003). Numerous other

studies have shown that a 30-sec sprint is associated with a 5-6 fold increase in

norepinephrine levels (Brooks et al., 1988; Nevill et al., 1989; Zouhal et al., 1998, 2001;

Moussa et al., 2003; Vincent et al., 2003, 2004; Jacob et al., 2004; Zouhal et al., 2009) to

levels around 11 to 15 nmol/l (Brooks et al., 1988; Nevill et al., 1989; Langfort et al.,

1997; Zouhal et al., 1998, 2001; Moussa et al., 2003; Vincent et al., 2003; Jacob et al.,

2004; Vincent et al., 2004; Zouhal et al., 2009) in untrained individuals. Trained

individuals attain even higher peak norepinephrine levels, ranging from approximately

16 to 18 nmol/l (~ 7 to 10 fold increase) in endurance-trained individuals (Zouhal et al.,

2001, 2009) and up to 30 nmol/l (~ 14 fold increase) in sprint-trained individuals (Jacob

et al., 2002). One study of active individuals reported an approximately 30-fold increase

in norepinephrine levels, which reached 37.1 nmol/l immediately after a treadmill-sprint

(Allsop et al., 1990). Longer sprints lasting 40-60 sec (Schnabel et al., 1983, 1984;

Ohkuwa et al., 1984; Schwarz et al., 1990) and 90 sec (Kindermann et al., 1982; Lavoie

et al., 1987) also result in a large increase in norepinephrine levels. This large rise in

catecholamines immediately after short sprints of different durations has been proposed

to activate hepatic glucose production and inhibit insulin-mediated glucose uptake in

skeletal muscles (Nonogaki, 2000).

The pattern of change in plasma insulin levels in response to a sprint varies across studies

in non-diabetic individuals. Plasma insulin levels are unchanged following 6-sec and 30-

36

sec sprints in one study of untrained males (Moussa et al., 2003). Other studies have also

reported no change in insulin levels after a single 30-sec sprint in young active male

participants (Vincent et al., 2004; Zouhal et al., 2009). In contrast, a 30-sec sprint has

been reported to increase insulin levels during recovery in young active female

participants (Vincent et al., 2004), untrained 21 year old males, and trained 34 year old

males (Zouhal et al., 2009). Longer duration 45 to 50-sec sprints also result in an increase

in insulin levels (Schnabel et al., 1984) as do 90-sec sprints (Kindermann et al., 1982;

Lavoie et al., 1987). More recently, a decrease in insulin after a 30-sec sprint was reported

in young trained participants (Zouhal et al., 2009). The lack of increase in insulin in some

studies in response to the rise in blood glucose levels post-sprint may be due to the

inhibition of glucose-mediated stimulation of insulin secretion by the elevated levels of

circulating catecholamines (Marliss and Vranic, 2002). In most studies where plasma

insulin level increases, insulin helps to bring glucose back to pre-exercise levels by

stimulating glucose uptake by skeletal muscles and/or inhibiting hepatic glucose

production (Marliss and Vranic, 2002).

Changes in growth hormone level also occur in response to sprinting. Growth hormone

levels increases after a sprint, peaking approximately 20 to 60 min post- print (Nevill et

al., 1996; Stokes et al., 2002, 2003, 2004, 2005, 2006; Gilbert et al., 2008). Growth

hormone levels after sprinting remain elevated for at least 60 min (Nevill et al., 1996;

Stokes et al., 2002, 2004, 2005) and even for up to 90 to 120 min in some individuals

(Stokes et al., 2002). It should be noted that many studies have reported marked inter-

individual variability in growth hormone response to sprinting (Stokes et al., 2002; Stokes

et al., 2003). Sprint duration may be a factor influencing growth hormone response

(Stokes et al., 2003), with the increase in growth hormone levels being significantly less

following a 6-sec compared to a 30-sec sprint (Stokes et al., 2002). A sprint duration of

30 to 90-sec also increases markedly growth hormone levels (Gordon et al., 1994; Nevill

et al., 1996; Stokes et al., 2002, 2003, 2005; Gilbert et al., 2008).

There are a number of factors that may influence growth hormone response to a single

sprint effort. Firstly, the mode of exercise may influence growth hormone response with

the larger muscle mass recruited during treadmill sprinting resulting in greater increases

in growth hormone than sprint cycling (Stokes et al., 2002). There is also a trend towards

faster pedaling rates in cycling at a given workload resulting in greater growth hormone

concentrations (Stokes et al., 2002). The increase in growth hormone in response to

37

sprinting is also larger in sprint-trained compared to endurance-trained athletes (Nevill et

al., 1996). However, it is noteworthy that 6 weeks of combined speed- and speed-

endurance training blunts the growth hormone response to a single sprint (Stokes et al.,

2004). Finally, age may also influence the magnitude of growth hormone response, with

the rise in growth hormone levels after a 30-sec sprint being attenuated in older males

(Stokes et al., 2006; Gilbert et al., 2008).

There is evidence that the rise in growth hormone level may contribute to the acute

increase in blood glucose level post-sprint. Indeed, the administration of a physiological

growth hormone pulse in non-exercised individuals results in a rapid fall in muscle

glucose uptake (Moller et al., 1990, 1992b, 2003). If this were to be the case in resting

individuals recovering from a sprint, growth hormone could potentially contribute to the

post-sprint rise in blood glucose levels. However, evidence against this view is the

finding that growth hormone levels increase mainly during recovery (Kinderman, et al.,

1982; Schnabel et al., 1984; Stokes et al., 2002) and the infusion of octreotide (a

somatostatin analogue), an inhibitor of growth hormone release, has no acute effects on

the magnitude of the hyperglycaemic effect of high intensity exercise (Sigal et al., 1996).

Another counterregulatory hormone affected by sprinting is cortisol. Some studies have

shown that a sprint of 30 to 90-sec results in an increase in plasma cortisol levels which

peak approximately 15 to 20-min after sprinting (Kindermann et al., 1982; Buono et al.,

1986; Nevill et al., 1996). In contrast, 45 to 60 sec (Schnabel et al., 1984; Schwarz and

Kindermann, 1990) sprints have been reported not to change cortisol levels. Although

there is some published evidence that a rise in cortisol levels may play a role in stabilising

glycaemia due to its potential acute inhibitory effect on glucose utilisation in skeletal

muscles (Shamoon et al., 1980), this hormone is unlikely to contribute to early changes

in glycaemia post-sprint because cortisol levels increase after the post-exercise rise in

blood glucose level and the effects of cortisol on hepatic glucose production and blood

glucose levels require several hours to take place (De Feo et al., 1989a; Heller and Cryer,

1991; Marker et al., 1991; Mitrakou et al., 1991).

Another key glucoregulatory factor that increases after a single sprint effort in non-

diabetic individuals is lactate. Sprinting, irrespective of its duration, results in an increase

in blood lactate levels that peak approximately 5 to 7 min into recovery. A 6-sec sprint

results in an increase in blood lactate levels (Stokes et al., 2002; Moussa et al., 2003;

38

Botcazou et al., 2006; Bracken et al., 2009). Likewise, a 30-sec sprint causes a large

increase in blood lactate levels (Cheetham et al., 1985; Jones et al., 1985; Hardman et al.,

1986; Brooks et al., 1988; Nevill et al., 1989; Allsop et al., 1990; Bogdanis et al., 1994,

1995; Granier and et al, 1995; Bogdanis et al., 1996; Nevill et al., 1996; Langfort et al.,

1997; Weinstein et al., 1998; Bell et al., 2001; Zouhal et al., 2001; Jacob et al., 2002;

Stokes et al., 2002; Moussa et al., 2003; Vincent et al., 2003; Jacob et al., 2004; Gilbert

et al., 2008; Chiappa et al. 2009; Zouhal et al., 2009). This increase in lactate is of greater

magnitude after a 30-sec sprint, peaking at around 12 to 16 mM (Brooks et al., 1988;

Moussa et al., 2003). Longer sprints of 40 to 60 sec (Fujitsuka et al., 1982; Schnabel et

al., 1983; Ohkuwa et al., 1984; Schnabel et al., 1984; Saraslanidis et al., 2009) and up to

90 sec (Kindermann et al., 1982; Lavoie et al., 1987) are also associated with very large

increases in blood lactate levels. Not surprisingly, the increase in lactate is more

pronounced in sprint-trained athletes (Cheetham et al., 1985; Granier et al., 1995).

Middle distance runners who complete approximately three anaerobic training sessions

per week reach significantly higher peak lactate concentrations of approximately 15 mM

after a single sprint than long distance runners and untrained individuals (Jacob et al.,

2004). This increase in lactate levels may contribute to the early post-sprint increase in

glycaemia by providing gluconeogenic precursors for hepatic glucose production (Miller

et al., 2002) and by increasing peripheral insulin resistance (Vettor et al., 1997).

A short duration maximal sprint effort also results in a decrease in blood pH in non-

diabetic individuals (Brooks et al., 1988; Nevill et al., 1989; Allsop et al., 1990; Bogdanis

et al., 1995; Bogdanis et al., 1996; Stokes et al., 2002; Bracken et al., 2009). This decrease

in blood pH occurs after a single sprint of both 6 sec (Stokes et al., 2002; Bracken et al.,

2009) and 30 sec (Allsop et al., 1990; Bogdanis et al, 1995; Bogdanis et al., 1996; Stokes

et al., 2002). There is evidence that low blood pH can inhibit glucose utilisation by

skeletal muscles (Gordon et al., 1994), thus contributing to the early rise in blood glucose

levels post-sprint.

Changes in free fatty acid levels are also associated with sprinting. An increase in free

fatty acid levels has been reported to continue for at least 4 hours after a single 30-sec

cycle sprint (Stokes et al., 2005, 2008). In contrast, a decrease in FFA was recorded early

immediately after a longer approximately 45-sec treadmill sprint (Schnabel et al., 1984).

Since increases in free fatty acid levels occur hours after a sprint, they are unlikely to play

a role in the increase in glycaemia early in recovery from a short maximal sprint effort.

39

The mechanisms underlying the glycaemic rising effect of a short sprint differ markedly

from those associated with high intensity aerobic exercise. Recently, it has been shown

that the rise in blood glucose level following a 10-sec sprint results from a transient

decline in the rate of glucose utilisation together with the absence of changes in

endogenous rates of glucose production (Fahey et al., 2012). This is in marked contrast

to the mechanism underlying the post-exercise increase in blood glucose level following

a bout of intense aerobic exercise where the rise in blood glucose level results from a

disproportionate increase in endogenous glucose production relative to that of glucose

utilisation rate (Marliss & Vranic, 2002). Maybe the increases in plasma catecholamines

and growth hormone levels associated with short duration sprinting are too small to affect

endogenous glucose production rate, whereas the transient fall in peripheral glucose

utilisation may result from an exercise-mediated intramuscular rise in glucose 6-

phosphate levels resulting in an inhibition of glucose utilisation via a glucose 6-

phosphate-meidated inhibition of hexokinase (Fahey et al., 2013).

1.12 Glucoregulatory Response to a Single Short Sprint in Individuals with Type 1

Diabetes

Despite the large amount of research described above on the glucoregulatory responses

to a single sprint in non-diabetic individuals, there is a lack of information on the effect

of sprinting in individuals with type 1 diabetes. To the best of our knowledge, only a

handful of studies have examined the effect of sprinting on blood glucose levels in

insulin-treated individuals with type 1 diabetes (Harmer et al., 2006; Fahey et al., 2012).

One of these studies showed that a constant load sprint to exhaustion at 130% V O2peak

lasting approximately 60 to 80 sec increases blood glucose levels, with a peak change in

plasma glucose of approximately 3.8 mM after 60 min of recovery in individuals with

type 1 diabetes (Harmer et al., 2006). The performance of such a prolonged sprint in

these individuals is associated with a sharp elevation in norepinephrine (Harmer et al.,

2000, 2006), epinephrine (Harmer et al., 2000, 2006) and plasma lactate (Harmer et al.,

2000, 2006, 2008) levels immediately post-sprint, whereas immunoreactive free insulin

and glucagon levels did not significantly change during recovery (Harmer et al., 2006).

Interestingly, seven weeks of sprint-training did not prevent the rise in blood glucose

levels, but reduced its magnitude, and had little effect on catecholamine, insulin or

40

glucagon response to an identical 60 to 80 sec sprint matched for total work (Harmer et

al., 2006).

To the best of our knowledge, only one study other than those described in this thesis has

examined the effect of maximal sprint effort on blood glucose levels in individuals with

type 1 diabetes (Fahey et al., 2012). This study shows that sprinting for 10 sec in these

individuals under basal insulinaemic conditions results in a 1.2 mM increase in blood

glucose levels within 30 minutes of recovery, with blood glucose remaining at stable and

elevated levels afterwards. The mechanisms underlying this rise in blood glucose levels

differ markedly from those associated with prolonged high intensity aerobic exercise

(Marliss and Vranic, 2002). Indeed, the rise in blood glucose level in response to

sprinting results from a transient decline in the rate of glucose utilisation, with no change

in endogenous rate of glucose production (Fahey et al., 2012). This is markedly different

from intense aerobic exercise where the post-exercise rise in blood glucose level in type

1 diabetic individuals results from a disproportionate increase in endogenous glucose

production relative to the increase in glucose utilisation rate (Marliss and Vranic, 2002).

Since the levels of plasma epinephrine, norepinephrine and growth hormone rose

transiently and to only a small extent post-sprinting compared to intense aerobic exercise,

this suggests they reached levels insufficient to stimulate hepatic glucose production.

Interestingly, the effects of such a short sprint on blood glucose levels and

counterregulatory hormones are not affected by an episode of antecedent hypoglycaemia

(Davey et al., 2014).

1.13 Statement of the Problem and Aims

Individuals with Type 1 diabetes are encouraged to participate in regular physical activity

(Zinman et al., 2004) due to the numerous physiological and psychological health benefits

associated with a physically active lifestyle (Norris et al., 1990; Moy et al., 1993;

Laaksonen et al., 2000; Riddell and Iscoe, 2006; Chimen et al., 2012). Unfortunately,

exercise increases the risk of hypoglycaemia due, in part, to a contraction-mediated

activation of glucose utilisation rate by skeletal muscle (Peirce, 1999) and an increase in

insulin sensitivity (Wasserman and Zinman, 1994). This increased risk occurs both

during exercise (Riddell et al., 1999; Tuominen et al., 1995; Rabasa-Lhoret et al., 2001),

and for several hours during recovery (MacDonald, 1987a; Tsalikian et al., 2005;

McMahon et al., 2007; Maran et al., 2010; Iscoe and Riddell, 2011; Davey et al., 2013a).

41

This is of concern because severe hypoglycaemia can cause brain damage (Suh et al.,

2007; Puente et al., 2010), cognitive dysfunction (Northam et al., 2009; Asvold et al.,

2010) and even sudden death (Tanenberg et al., 2010). Approximately 6-10% of patients

with type 1 diabetes die from hypoglycaemia (Skrivarhau et al., 2006; Jacobson et al.,

2007; Feltbower et al., 2008). Consequently, it is not surprising that one of the biggest

barriers to regular physical activity in individuals with type 1 diabetes is the fear of

hypoglycaemia (Brazeau and et al., 2008). As a result of this legitimate fear, many

individuals with type 1 diabetes have been known to avoid physical activity (Ludvigsson

et al., 1980; Guelfi et al., 2007a; Brazeau et al., 2008), with ~60-65% of individuals with

type 1 diabetes reported to be inactive (Thomas et al., 2004; Plotnikoff et al., 2006).

It is important to remember, however, that not all types of exercise result in elevated risk

of hypoglycaemia. In fact, although exercise of low to moderate intensity increases the

risk of hypoglycaemia both during and after exercise (MacDonald, 1987a; Tuominen et

al., 1995; Riddell et al., 1999; Rabasa-Lhoret et al., 2001), exercise performed at high

intensity (>80% of V O2max) for approximately 10-15 min is accompanied by an increase

in glycaemia during and after exercise (Mitchell et al., 1988; Marliss et al., 1992a; Purdon

et al., 1993; Sigal et al., 1994c, 1999; Marliss and Vranic, 2002). The hyperglycaemic

effect of prolonged high intensity exercise in individuals with Type 1 diabetes raises the

intriguing possibility that this type of exercise might be beneficial if adopted to counter a

fall in glycaemia in complication-free individuals with type 1 diabetes, and thus might

help to prevent or delay hypoglycaemia if no carbohydrate is readily available. The

problem here is that 10-15 min of exercise at intensities above 80% V O2max would be

unlikely to be well tolerated by most individuals with type 1 diabetes due to the very

intense nature of such exercise combined with the impractical duration of the exercise

bout. This raises the question of whether a much shorter bout of exercise performed at a

higher intensity could be adopted to prevent glycaemia from falling. Although, as

discussed above, a 30-90-sec sprint can increase blood glucose levels in individuals with

type 1 diabetes, sprint efforts lasting 30-sec or more are associated with physical

discomfort (Marquardt et al., 1993; Laurent et al., 2007), undesirable physiological

consequences such as nausea, vomiting and dizziness (Inbar et al., 1996; 1998; Laurent

et al., 2007) and therefore may be unsafe or impractical for many individuals (Little et

al., 2010). However, a maximal sprint effort lasting only 10 sec is known to be well

tolerated. For this reason, it was the primary goal of this thesis to determine for the first

time whether a 10-sec maximal sprint effort provides a potential means other than

42

carbohydrate intake to oppose an insulin- or exercise- mediated fall in glycaemia, thus

decreasing the risk of hypoglycaemia in complication-free individuals with type 1

diabetes. Also, given that studies concerned with investigating response of individuals

with type 1 diabetes to exercise typically determine the V O2max of their participants using

graded exercise testing, another objective of this thesis is to determine whether risk of

hypoglycaemia is increased with this type of exercise protocol.

More specifically, our aims are to test the following hypotheses;

1) Graded exercise testing in complication-free individuals with type 1 diabetes will

result in a post-exercise increase in blood glucose level, therefore not increasing

the risk of hypoglycaemia during early recovery.

2) The performance of a single 10-sec maximal sprint effort after 20 min of moderate

intensity exercise performed under hyperinsulinaemic conditions will oppose the

exercise-mediated fall in glycaemia, thus decreasing the risk of hypoglycaemia

during early recovery in complication-free individuals with type 1 diabetes.

3) The performance of a single 10-sec maximal sprint effort before 20 min of

moderate intensity exercise performed under hyperinsulinaemic conditions will

lessen the subsequent exercise-mediated fall in glycaemia, thus helping to

decrease the risk of hypoglycaemia during exercise in complication-free

individuals with type 1 diabetes.

4) The performance of a single 10-sec maximal sprint under hyperinsulinaemic

conditions will result in a counterregulatory response typical of sprinting.

43

1.14 Significance of the Thesis

Unfortunately, individuals with type 1 diabetes are on average less physically active than

recommended due to their fear of hypoglycaemia and a lack of information and guidelines

on how to safely participate in various sports and recreational activities. It is important

for these individuals to enjoy the numerous health and lifestyle benefits of regular

physical exercise. Regrettably, there is little research in this area, resulting in a lack of

comprehensive guidelines and practical advice on how to safely manage blood glucose

levels while performing various types of physical activity. Our findings might identify

sprinting as a novel tool for the prevention of hypoglycaemia in individuals with type 1

diabetes.

44

Chapter 2

Glycaemic Response to Graded Exercise in

Individuals with Type 1 Diabetes

45

2.1 Abstract

Objective: To investigate whether the risk of hypoglycaemia in individuals with type 1

diabetes increases in response to the type of graded exercise testing used to determine

V O2peak

Research Design and Methods: Eight non-diabetic male participants (CON) and seven

complication-free type 1 diabetic male individuals in good to moderate glycaemic control

(HbA1c = 7.6 ± 0.5%) were recruited for this study. On the morning of testing, the type

1 diabetic participants followed their normal insulin regimen, and both groups ate their

usual breakfast to transiently increase their blood glucose levels. Approximately four

hours later, graded exercise was commenced on a cycle ergometer. Blood metabolites

and hormones were sampled at rest and regularly during recovery.

Results: During graded exercise, there were no significant changes in glycaemia in either

group, but blood glucose levels in type 1 diabetic participants increased by more than 2

mM during recovery (p < 0.05) and remained elevated for 120 min, while remaining at

stable levels in the control group. In both groups, catecholamine and lactate levels

increased significantly early in recovery and fell thereafter (p < 0.05). Glucagon and

cortisol levels also increased during recovery in both groups, while free fatty acid levels

increased only in the type 1 diabetic participants (p < 0.05). In contrast, the levels of

insulin remained stable in both groups (p > 0.05).

Conclusions: Our graded exercise protocol performed in the post-absorptive state while

plasma insulin levels are low increases glycaemia early post-exercise, with no

carbohydrate ingestion required for hypoglycaemia prevention early during that time.

This type of exercise, therefore, does not increase the risk of early post-exercise

hypoglycaemia in individuals with type 1 diabetes. The mechanisms underlying the post-

exercise increase in glycaemia remain to be determined.

46

2.2 Introduction

Regular participation in physical activity is an important component in type 1 diabetes

management because it is associated with numerous physiological and psychological

health benefits (Norris et al., 1990; Moy et al., 1993; Laaksonen et al., 2000; Steppel and

Horton, 2004; Zinman et al., 2004; Riddell and Iscoe, 2006; Chimen et al., 2012, Tonoli

et al., 2012). Unfortunately, an active lifestyle may make acute management of blood

glucose levels more difficult (Steppel and Horton, 2004; Camacho et al., 2005; Riddell

and Perkins, 2006; Guelfi et al., 2007c; Younk and Davis, 2012). For instance, exercise

of low to moderate intensity increases the risk of hypoglycaemia both during and after

exercise (Camacho et al., 2005; Guelfi et al., 2007c) due, in part, to a contraction-

mediated activation of glucose utilisation rate by skeletal muscle and an increase in

insulin sensitivity (Steppel and Horton, 2004; Camacho et al., 2005). In contrast, exercise

performed at high intensity increases blood glucose levels (Marliss and Vranic, 2002).

Given the numerous benefits of an active lifestyle for individuals with type 1 diabetes

together with the difficulties and challenges associated with managing blood glucose

levels during and after exercise, it is not surprising that these issues have attracted a large

volume of research (Chapter 1).

One feature shared by most of the research on exercise in diabetes is the common practice

of standardising exercise intensity relative to maximal or peak rate of oxygen

consumption ( V O2 max or V O2peak) or lactate threshold (Chapters 3, 4, and 5 of this thesis;

Fremion et al., 1987; Wanke et al., 1992; Purdon et al., 1993; Sigal et al., 1994c, 1996;

Nugent et al., 1997; Ford et al., 1999b; Galassetti et al., 2001b; Rabasa-Lhoret et al., 2001;

Galassetti et al., 2003; Heyman et al., 2007). The determination of V O2peak or lactate

threshold requires the performance of a graded exercise test whereby exercise intensity

increases in a stepwise fashion or continuously until maximal intensity is achieved

(volitional exhaustion or attainment of criteria for V O2max/peak).

Despite the importance and prevalence of graded exercise testing for V O2/peak

determination, it is unclear whether this testing protocol increases the risk of

hypoglycaemia in type 1 diabetic individuals, and whether insulin reduction as well as

increasing food intake before or immediately after exercise is required to prevent blood

glucose levels from falling post-exercise. All studies addressing this issue have performed

blood glucose measurements before and immediately after V O2peak testing (Fremion et

47

al., 1987; Wanke et al., 1992; Nugent et al., 1997; Ford et al., 1999; Heyman et al., 2007),

with some reporting either the absence of changes in blood glucose levels (Fremion et al.,

1987; Wanke et al., 1992; Nugent et al., 1997; Ford et al., 1999) or a decrease (Heyman

et al., 2007). Since there are conditions where blood glucose level can change markedly

after exercise (Purdon et al., 1993; Sigal et al., 1996; Marliss and Vranic, 2002) it is

important to assess whether the risk of hypoglycaemia increases during recovery from

graded exercise testing. To the best of our knowledge, only one study has examined this

issue, but found no significant changes in blood glucose levels 20 min post-exercise (Ford

et al., 1999b). However, as pointed out by the authors themselves, the main limitation

with their study is that insulin dosage was adjusted and food was ingested one hour prior

to exercise (Ford et al., 1999b), thus leaving unanswered the important question of

whether blood glucose levels would have fallen during recovery had no food been

ingested.

Given that a short bout of intense aerobic exercise under basal insulinaemic conditions

can increase blood glucose levels (Marliss and Vranic, 2002), this raises the possibility

that graded exercise testing performed when plasma insulin is at near basal level might

also be associated with a post-exercise increase in glycaemia, with no carbohydrate

ingestion required prior to or after exercise to prevent hypoglycaemia. The primary

objective of this study is to test this hypothesis by examining the glycaemic and hormonal

response to the type of graded exercise test used for V O2peak determination under

conditions where food is not ingested soon prior to testing and without adjusting insulin

dosage. This is an important issue to address given the widespread use of graded exercise

testing in diabetes research and fitness testing, and the absence of information about the

risk of hypoglycaemia associated with this exercise protocol.

48

2.3 Research Design and Methods

2.3.1 Participants

Seven young males with type 1 diabetes (aged 20.4 ± 1.5 years; BMI 27.4 ± 1.0 kg/m2;

peak 2OV 45.9 ± 2.4 ml/kg/min; duration of diabetes 5.8 ± 1.8 years) and eight age-matched

non-diabetic male (CON; aged 20.1 ± 0.8 years; BMI 25.3 ± 1.5 kg/m2; peak 2OV 44.6 ±

3.6 ml/kg/min) participants were recruited from Princess Margaret Hospital and the

University of Western Australia, respectively. All participants with type 1 diabetes were

in moderate glycaemic control (HbA1c = 7.6 ± 0.5%), free from diabetic complications,

hypoglycaemia aware, had undetectable levels of C-peptide and were not taking any

prescribed medication other than insulin. Participants with type 1 diabetes were on a

multiple-injection insulin regime that had not changed for at least three months prior to

testing. Following a familiarisation session during which their informed consent was

obtained together with anthropometric measurements, participants were required to attend

our laboratory on another occasion for testing. Both the University of Western Australia

and the Princess Margaret Hospital Ethics Committees approved all the procedures

described in this study.

2.3.2 Experimental trials and assays

Participants were not allowed to exercise for 48 h prior to the experimental trial since

antecedent exercise has the potential to affect the endocrine response to a subsequent bout

of exercise (Galassetti et al., 2001b). Also, testing was rescheduled if they had

experienced a hypoglycaemic episode over the previous 48 h because prior

hypoglycaemia can also affect the counter-regulatory response to exercise (Galassetti et

al., 2003). Each participant was also required to maintain a normal diet and to avoid

alcohol for 24 h prior to testing. On the morning of testing, participants were instructed

to monitor their blood glucose regularly. They arrived in the laboratory at ~ 7:30 am, and

all type 1 diabetic participants were instructed to self-administer their usual dose of

morning insulin into their abdomen. Both the control and type 1 diabetes groups were

then asked to consume a breakfast, with the meal choice and nutritional content reflecting

that normally ingested by the participant.

49

After breakfast, a catheter was inserted in one of the superficial veins at the back of the

subject’s hand prior to heating the hand for the sampling of arterialised venous blood. No

physical activity was allowed and blood glucose levels were measured every 15 min. In

an effort to comply with the recommendations that exercise should be avoided if insulin

levels are peaking, commencement of testing was delayed until four hours after insulin

injection, at a time when insulin was near basal levels. During that time, post-prandial

blood glucose levels fell, and when glycaemia approached 7 mM (7.24 ± 0.45 mM in the

type 1 diabetic participants; 5.27 ± 0.20 mM CON), the graded exercise test to determine

V O2peak was initiated on an Evolution cycle ergometer (Evolution, Geelong, Australia).

The graded exercise test described here required participants to initiate cycling at an

intensity of 50 watts while breathing through a mouthpiece into an oxygen analyser

system. Briefly, every 3 min the exercise intensity was raised progressively by 50 watts

until the subject reached his maximal rate of oxygen consumption. A plateau in oxygen

consumption (an increase of < 150 mL·min-1) and/or a respiratory exchange ratio greater

than 1.15 during the last min of exercise were the criteria for the achievement of V O2peak.

The participant’s expired air was monitored continuously, and both oxygen uptake and

carbon dioxide production were calculated every 15 sec using a computerized on-line gas

analysis system. This comprised of a Morgan Ventilation Monitor (Morgan, Reinham,

Kent, U.K.), Ametek S3A Oxygen Analyser and Ametek CD3A Carbon Dioxide

Analyser (Ametek, Paoli, PA).

Blood was sampled prior to exercise and then at 0, 5, 10, 15, 30, 60, 90 and 120 min post-

exercise. Some of the blood was assayed immediately for glucose, pO2, pH and lactate

levels using an ABL 625 blood gas system (Radiometer, Copenhagen) and the remainder

of the blood was combined with sodium metabisulphite, polyethylene glycol or aprotinin

(Trasylol) for the assays of catecholamines, insulin and glucagon, respectively (Bussau

et al., 2006). All samples were centrifuged at 720g for 5 min and the plasma stored at -

80C for later analyses of catecholamines, free fatty acids, insulin, glucagon, cortisol, GH

and C-peptide levels.

Heparinized plasma treated with sodium metabisulphate was used for the determination

of catecholamine levels by reverse phase high-performance liquid chromatography using

a Waters Novapak C18 reverse phase column and a model 5200A Coulochem detector

(ESA Biosciences Inc., Chelmsford, MA, USA). Free fatty acids levels were measured

50

in EDTA-treated plasma using the Roche Half Micro Test Free Fatty Acids Assay Kit

(Roche Diagnostic, Mannheim, Germany). Heparinized plasma treated with polyethylene

glycol was assayed for free insulin using the Coat-a-Count Insulin Kit (Diagnostic

Products Corporation, Los Angeles, CA, USA). Glucagon levels in plasma collected with

trasylol were measured from EDTA-treated plasma by radioimmunoassay using a Linco

Glucagon RIA Kit (Linco Research, St Charles, Missouri, USA). Cortisol levels were

assayed from venous serum by competitive immunoassay on an Immulite 2000 Analyser

using the Immulite Cortisol Assay Kit (Diagnostic Products Corporation, Los Angeles,

CA, USA). Growth hormone levels were determined from serum by immunometric assay

on an Immulite 2000 Analyser using the Immulite Growth Hormone Assay Kit

(Diagnostic Products Corporation, Los Angeles, CA, USA). Finally, C-peptide levels

were determined by solid-phase competitive chemiluminescent enzyme immunoassay on

an Immulite 2000 analyser using the Immulite C-peptide Assay Kit (Diagnostic Products

Corporation, Los Angeles, CA, USA).

2.3.3 Statistical analyses

The results within each experimental group were analysed using a mixed model repeated

measure analysis of variance (ANOVA) and Fisher’s least significant differences test for

a posteriori analysis using SPSS 13.0 for Windows software (SPSS, Chicago, IL, USA).

Statistical significance was accepted at p < 0.05. Participants’ characteristics are

expressed as means ± S.D. whereas all other results are expressed as means ± S.E.M.

51

2.4 Results

2.4.1 Blood metabolite response to graded exercise

Before the graded exercise test, blood glucose levels in both experimental groups fell

significantly (p < 0.05) (Fig. 2.1). When glycaemia reached ~7 mM in diabetic

participants (7.24 ± 0.45 mM in type 1 diabetic participants; 5.27 ± 0.20 mM control

participants), the test was initiated. In response to graded exercise (exercise duration ~

12 min), there were no significant changes in glycaemia in either experimental group.

Immediately following exercise, glycaemia increased significantly (p < 0.05) and

remained elevated for the whole duration of the 120-min recovery period (p < 0.05) in the

type 1 diabetic participants (Fig. 2.1). In contrast, blood glucose levels remained

relatively stable in the control group (Fig. 2.1). Blood lactate levels reached maximal

levels at 5 min post-exercise in both experimental groups and decreased throughout

recovery (Fig. 2.1). In contrast, blood pH reached minimal levels after 5 min of recovery

in both experimental groups before returning to pre-exercise levels within 60 min of

recovery (Fig. 2.1). During recovery, free fatty acid levels increased gradually over time

and reached maximal levels after 90-120 min of recovery (Fig. 2.1).

52

Figure 2.1: Effect of graded exercise on the levels of blood glucose (A), lactate (B), pH

(C) and free fatty acids (D). The exercise testing period is represented by the shaded box.

All results are shown as mean standard error. Black circles refer to the type 1 diabetic

participants whereas white circles refer to the non-diabetic control group. a represents a

statistically significant difference (p < 0.05) compared to the rest time point in the type 1

diabetic participants, b represents a statistically significant difference (p < 0.05) compared

to the rest time point in the control group.

53

2.4.2 Hormonal response to graded exercise

In response to graded exercise, catecholamines increased to maximal levels at the onset

of recovery in both experimental groups (p < 0.05; Fig. 2.2), with the levels attained being

not significantly different between experimental groups (p > 0.05). Norepinephrine levels

returned to pre-exercise levels within 15 and 30 min after exercise in the control and type

1 diabetic groups, respectively (Fig. 2.2), whereas epinephrine returned to pre-exercise

levels within 5 min post-exercise in both experimental groups (Fig. 2.2). Growth

hormone (GH) levels displayed a non-significant transient increase in the type 1 diabetic

and control groups, and reached significantly elevated levels in the control group after 15

min of recovery (p < 0.05; Fig. 2.2). Following graded exercise, plasma cortisol levels

increased significantly (p < 0.05) and reached maximal levels within 10-30 min post-

exercise in both groups (Fig. 2.2). Likewise, glucagon levels increased significantly in

the type 1 diabetic group by 30 min post-exercise and by 90 min in the control group (p

< 0.05; Fig. 2.2). During exercise, plasma insulin levels remained at relatively stable low

levels in both experimental groups (Fig. 2.2).

54

Figure 2.2: Effect of graded exercise on the levels of epinephrine (A), norepinephrine

(B), growth hormone (C), cortisol (D), glucagon (E), and free insulin (F). The exercise

testing period is represented by the shaded box. All results are shown as mean standard

error. Black circles refer to the type 1 diabetic group whereas white circles refer to the

control group. a represents a statistically significant difference (p < 0.05) compared to the

rest time point in the type 1 diabetic group, b represents a statistically significant difference

(p < 0.05) compared to the rest time point in the control group.

55

2.5 Discussion

Graded exercise testing is integral to V O2max/peak and lactate threshold determination in

basic, applied and clinical research. What is still unclear, however, is whether the risk of

hypoglycaemia increases in response to this type of exercise in individuals with type 1

diabetes. This is an important issue given that most laboratory-based studies on exercise

in diabetes involve the performance of graded exercise for V O2max/peak determination.

Here we show for the first time that graded exercise performed when plasma insulin is at

near basal level in young adult males with type 1 diabetes increases blood glucose level

post-exercise and thus does not increase the risk of early hypoglycaemia. In particular,

graded exercise under these conditions results in a rapid post-exercise increase in blood

glucose levels (> 2 mM) which remains elevated for the first two hours of recovery (Fig.

2.1), thus suggesting that no carbohydrate ingestion before or after this type of exercise

is required to prevent hypoglycaemia. Our findings thus suggest this exercise protocol is

safe provided it is performed some time after the injection of rapid acting insulin.

The absence of a significant fall in glycaemia observed here during graded exercise

corroborates the findings of most (Fremion et al., 1987; Wanke et al., 1992; Nugent et al.,

1997; Ford et al., 1999), but not all studies (Heyman et al., 2007) on graded exercise in

individuals with type 1 diabetes, whereas the post-exercise increase in blood glucose

levels is described here for the first time. Although differences in experimental design

make it difficult to compare our findings with those of others, most earlier studies reported

that graded exercise does not affect blood glucose levels, but participants were fasted and

omitted their morning insulin to reduce the risk of exercise-induced hypoglycaemia in

one of these studies (Nugent et al., 1997) or were fed with excess of food before exercise

with (Ford et al., 1999) or without (Fremion et al., 1987) adjusting their insulin dosage

prior to exercise, thus making it less likely for blood glucose levels to fall. Although one

study reported a fall in glycaemia (Heyman et al., 2007) during V O2peak testing, exercise

in that study was performed when plasma insulin levels were near maximal, a time when

exercise is usually not recommended for individuals with type 1 diabetes (Rabasa-Lhoret

et al., 2001; Steppel and Horton, 2004).

The finding here that blood glucose levels increase during recovery from graded exercise

has never been reported before. To the best of our knowledge, only one other study has

examined how blood glucose levels change during recovery from graded exercise testing

56

in individuals with type 1 diabetes, but it only measured glycaemia at 0 and 20 min post-

exercise and found no significant change in blood glucose levels (Ford et al., 1999b).

However, the main limitation with that study is that insulin was adjusted and food was

ingested one hour prior to exercise, thus limiting the relevance of these findings and

leaving unanswered the question of whether graded exercise increases the risk of early

hypoglycaemia post-exercise if insulin dosage is not reduced or if no carbohydrate is

ingested soon before exercise (Ford et al., 1999b). Our findings are consistent with those

examining blood glucose response to intense aerobic exercise (>80% of V O2peak)

performed under basal insulinaemia. Indeed, 10-15 min of sustained high intensity

exercise at near 80% of V O2peak in individuals with type 1 diabetes results in a sustained

post-exercise increase in blood glucose levels (Purdon et al., 1993; Sigal et al., 1994c;

Marliss and Vranic, 2002) similar to that observed here where blood glucose levels

increase and remain elevated or continue to rise well after the cessation of exercise

(Purdon et al., 1993; Sigal et al., 1994c; Marliss and Vranic, 2002). Similarly, the

relatively stable blood glucose levels during graded exercise in the control group (Fig.

2.1) were similar to those reported in previous studies (Nugent et al., 1997; Heyman et

al., 2007).

The marked rise in epinephrine and norepinephrine levels immediately following graded

exercise might explain, at least in part, the rapid increase in glycaemia during early

recovery in the type 1 diabetic group. In support of this view, the increased catecholamine

levels during that time coincided with the rapid initial rise in blood glucose levels, with

epinephrine and norepinephrine levels increasing by ~10-fold in the type 1 diabetic group

(Fig. 2.2). A comparable increase in catecholamines levels (14-18-fold) has also been

reported to occur in response to sustained high intensity aerobic exercise in individuals

with type 1 diabetes (Purdon et al., 1993; Marliss and Vranic, 2002). It is important to

note, however, that others have reported that catecholamines increase by less than 5-fold

immediately following graded exercise (Nugent et al., 1997). Although elevated

catecholamine levels are acknowledged to oppose insulin-mediated fall in glycaemia

(Marliss and Vranic, 2002) via activation of hepatic glucose production and inhibition of

insulin-mediated glucose uptake in skeletal muscles (Marliss and Vranic, 2002;

Nonogaki, 2000), it is important to note that the importance of catecholamines in the

activation of hepatic glucose production in response to intense exercise has been the

object of some dispute (Camacho et al., 2005; Coker and Kjaer, 2005). Also, and more

importantly, the fact that blood glucose levels remained stable in the control group despite

57

experiencing a comparable rise in catecholamine levels and no changes in plasma insulin

levels in response to graded exercise suggests that other mechanisms both mediate the

post-exercise rise in glycaemia in type 1 diabetic participants and explain the different

glycaemic responses between the two experimental groups.

During recovery from graded exercise in the type 1 diabetic group, the absence of a post-

exercise increase in insulin levels (Fig. 2.2) probably explains, at least in part, the early

post-exercise rise in blood glucose level and sustained increased glycaemia as is the case

following sustained high intensity aerobic exercise in type 1 diabetic individuals (Purdon

et al., 1993; Sigal et al., 1994c; Marliss and Vranic, 2002). Here, however, not only

plasma insulin levels did not differ between treatment groups, there was no increase in

plasma insulin post-graded exercise test in the control group. This is consistent with the

absence of post-exercise increase in blood glucose level in the control group. However,

our findings with the control group differ from the transient increase in plasma insulin

levels that occurs in non-diabetic individuals recovering from intense aerobic exercise

and which have been proposed to explain their lesser post-exercise rise in glycaemia

compared to diabetic individuals (Sigal et al., 1994c; Marliss and Vranic, 2002). Our

findings thus suggest that other mechanisms are likely to explain the different patterns of

glucose response between the control and type 1 diabetic participants.

Glucagon and cortisol are unlikely to play a major role in increasing glycaemia during

recovery from graded exercise testing in individuals with type 1 diabetes. The levels of

plasma glucagon, a potent activator of hepatic gluconeogenesis (Marliss and Vranic,

2002; Camacho et al., 2005), did not increase significantly during early recovery in type

1 diabetes or control participants although they did rise later during recovery (Fig. 2.2).

This pattern of response in control participants differs from the increase in glucagon

observed in response to sustained high intensity aerobic exercise (Purdon et al., 1993;

Sigal et al., 1994c). Assuming that portal glucagon and insulin levels follow patterns of

change similar to those of peripheral blood glucose and insulin levels, this suggests that

both glucagon and glucagon/insulin ratio play little role in mediating the post-exercise

rise in blood glucose levels. Although there is some published evidence that a rise in

cortisol levels may play a role in stabilising glycaemia due to its potential acute inhibitory

effect on glucose utilisation in skeletal muscles (Shamoon et al., 1980), this hormone is

unlikely to play a major role here firstly because the effects of cortisol on hepatic glucose

production and blood glucose levels require several hours to take place (McMahon et al.,

58

1988), and secondly because the increase in glycaemia observed here in type 1 diabetic

participants preceded that of cortisol.

Growth hormone is also unlikely to play a major role in the increase in glycaemia during

recovery from graded exercise as suggested by the absence of significant changes in the

levels of this hormone in type 1 diabetic individuals (Fig. 2.2). It is important to stress,

however, that a close inspection of Fig. 3c suggests that significant changes in growth

hormone levels in response to graded exercise might have been undetected due to the

large inter-individual variability in growth hormone levels. A trend for not only an

increase in growth hormone levels in both experimental groups, but also a more

pronounced early rise in growth hormone levels in the diabetic compared to non-diabetic

group is consistent with the findings of others that graded exercise test results in a lesser

post-exercise increase in growth hormone levels in non-diabetic compared to diabetic

participants (Coiro et al., 2004). These responses of growth hormone to graded exercise

suggest that these hormones could mediate the early increase in glycaemia post-graded

exercise. In support of this interpretation, the administration of a physiological growth

hormone pulse in non-exercised non-diabetic individuals results in a rapid fall in muscle

glucose uptake (Moller et al., 1990). However, even if the increase in growth hormone

levels had been significant, it would have probably played a role of lesser importance in

increasing glycaemia post-exercise in the type 1 diabetic group. This is because the

administration of a growth hormone pulse has been shown not to affect peripheral glucose

uptake in insulin-treated individuals with type 1 diabetes (Møller et al., 1992a) and also

because the infusion of octreotide, an inhibitor of growth hormone release, has no acute

effect on the magnitude of the hyperglycaemic effect of high intensity exercise (Sigal et

al., 1996).

Finally, the elevated lactate level after graded exercise (Fig. 2.1) is another factor that

may have contributed to the stabilisation of glycaemia early in recovery by providing

gluconeogenic precursors for hepatic glucose production (Miller et al., 2002) and by

increasing peripheral insulin resistance (Vettor et al., 1997). However, the fact that

lactate reached similar levels post-exercise in the type 1 diabetic and control participants

suggests that this factor alone is unlikely to explain the post-exercise increase in

glycaemia in type 1 diabetes since no change in glycaemia took place in the control

participants despite both groups experiencing a similar increase in plasma lactate levels.

59

Overall, despite the potential role for catecholamines and maybe growth hormone and

lactate in mediating, at least in part, the increase in glycaemia during recovery in the type

1 diabetic group, the fact that the patterns of change in plasma insulin and counter-

regulatory hormones were similar between the type 1 diabetic and control groups suggest

that other factors explain both the different blood glucose responses between these groups

and the absence of increase in blood glucose levels in the control group during recovery

from graded exercise. The counterregulatory hormone(s) responsible for increasing

glycaemia post-exercise in type 1 diabetes thus remain to be identified.

In conclusion, on clinical grounds, our findings suggest that the risks of hypoglycaemia

associated with graded exercise testing are minimal when performed under basal/near

basal insulin levels, with no carbohydrate administration required just before or early after

testing to prevent hypoglycaemia in individuals with type 1 diabetes. This is because this

type of exercise protocol increases glycaemia during recovery if performed when insulin

levels are not elevated. However, the risk of late onset post-exercise hypoglycaemia

during recovery from graded exercise testing needs to be investigated in future studies.

In addition, it is our view that health professionals should still regularly monitor

glycaemia before, during and after testing since it remains to be established whether

similar findings would have been reported with different graded exercise protocols and

individuals of different ages, circulating insulin levels or severity of diabetic

complications.

2.6 Acknowledgements

This research was funded by a Juvenile Diabetes Research Foundation/ National Health

Medical Research Council of Australia program grant to T.W. Jones and P.A. Fournier.

L.D. Ferreira is supported by a Juvenile Diabetes Research Fund International

Fellowship.

60

Chapter 3

A 10-second Maximal Sprint Effort: A Novel

Approach to Counter an Exercise-Mediated

Fall in Glycaemia in Individuals with Type 1

Diabetes

An amended version of this chapter has been published in Diabetes Care:

Bussau, V.A., Ferreira, L.D., Jones, T.W. & Fournier, P.A. (2006). The 10-s Maximal

Sprint: A Novel Approach to Counter an Exercise-Mediated Fall in Glycemia in

Individuals with Type 1 Diabetes. Diabetes Care 29 (3): 601-606.

61

3.1 Abstract

Objective: Our aim is to investigate whether a short maximal sprint effort can provide

another means to counter the rapid fall in glycaemia associated with moderate intensity

exercise in individuals with type 1 diabetes, and therefore decrease the risk of early post-

exercise hypoglycaemia.

Research Design and Methods: Seven healthy male participants with type 1 diabetes

injected their normal insulin dose and ate their usual breakfast. When their postprandial

glycaemia fell to ~11 mM, they pedalled at 40% peak 2OV for 20 min on a cycle ergometer

then immediately engaged in a maximal 10-sec cycling sprint (sprint trial) or rested

(control trial). Both trials were administered in a counterbalanced order.

Results: Moderate intensity exercise resulted in a significant fall (p < 0.05) in glycaemia

in both trials (mean ± S.E.M.: 3.6 ± 0.5 vs 3.1 ± 0.5 mM for sprint and control,

respectively). The subsequent short sprint opposed a further fall in glycaemia for 120 min,

whereas in the absence of a sprint, glycaemia decreased further (3.6 ± 1.22 mM; p < 0.05)

after exercise. The stabilisation of glycaemia in the sprint trial was associated with

elevated levels of catecholamines, growth hormone, and cortisol. In contrast, these

hormones remained at stable or near stable levels in the control trial. Changes in insulin

and free fatty acid levels were similar in the sprint and control trials.

Conclusions: These results suggest that following moderate intensity exercise, it is

preferable for young individuals with insulin-treated, complication-free type 1 diabetes

to engage in a 10-sec maximal sprint effort to acutely oppose a further fall in glycaemia

than to only rest, thus providing another means to reduce risk of hypoglycaemia in active

individuals with type 1 diabetes.

62

3.2 Introduction

It is well established that exercise of moderate intensity increases the risk of

hypoglycaemia both during and after exercise in individuals with type 1 diabetes

(Campaigne et al., 1987; Riddell et al., 1999) due, in part, to a contraction-mediated

activation of glucose utilisation rate in skeletal muscle (Peirce, 1999) and an increase in

insulin sensitivity (Wasserman and Zinman, 1994). In contrast, 10-15 min of high

intensity exercise (>80% of maximal rate of oxygen consumption [ peak 2OV ]) causes a

post-exercise increase in blood glucose levels in insulin-treated individuals with type 1

diabetes, irrespective of their glycaemic control (Mitchell et al., 1988; Marliss et al.,

1992a; Purdon et al., 1993; Sigal et al., 1994c, 1999; Marliss and Vranic, 2002). This

hyperglycaemic effect of prolonged high intensity exercise raises the intriguing

possibility that this type of exercise might provide a means other than carbohydrate intake

to counter a fall in glycaemia post-exercise in complication-free individuals with type 1

diabetes, and thus acutely reduce their risk of hypoglycaemia. Unfortunately, 10-15 min

of exercise at high intensity is unlikely to be well tolerated by most individuals with type

1 diabetes due to the very intense nature of such exercise combined with its impractical

duration.

A more practical way of using intense exercise as a means to prevent glycaemia from

falling might be to engage in a much shorter bout of exercise performed at maximal

intensity. The main difficulty with this suggestion is that a maximal sprint effort that lasts

in excess of 30-sec is associated with unpleasant consequences such as nausea, vomiting

and dizziness (Inbar et al., 1996; Van Praagh, 1998). This raises the question of whether

a shorter sprint could provide a potential means other than carbohydrate intake to oppose

a post-exercise fall in glycaemia. Although individuals at rest are unlikely to engage in a

sprint to stabilise their glycaemia, doing so might prove to be an effective and convenient

way to counteract a rapid fall in blood glucose level in response to moderate intensity

exercise. Because this possibility has never been examined before, it was the primary goal

of this study to determine whether a 10-sec maximal sprint effort is preferable to only

resting as a means to counter a further fall in glycaemia during recovery from moderate

intensity exercise in individuals with type 1 diabetes.

63

3.3 Research Design and Methods

3.3.1 Participants

Seven young men with type 1 diabetes were recruited to this study (aged 21.0 ± 3.5 years;

BMI 26.9 ± 4.0 kg/m2; c 44.5 ± 4.2 ml/kg/min; duration of type 1 diabetes 9.1 ± 3.6 years;

HbA1c 7.4 ± 0.8% ranging between 6.6 to 9.0 %). All participants were free from any

diabetic complications, had undetectable levels of C-peptide, and were not taking any

prescribed medication other than insulin. They had all been treated with a stable insulin

regimen composed of a combination of slow or intermediary acting insulin (eg NPH), and

fast acting insulin analogues for at least three months prior to the study. All participants

were required to attend our laboratory on three occasions. The first visit was a

familiarisation session during which their informed consent was obtained together with

anthropometric measurements, and their maximal rate of oxygen consumption was

determined as previously described (Fairchild et al., 2002b). The next two visits were the

sprint and control rest trials administered in a random counterbalanced order. The

Institutional Ethics Committee approved all the procedures described in this study.

3.3.2 Experimental trials

The participants were not permitted to exercise for 24 h prior to the experimental trial,

and testing was rescheduled if they had experienced a hypoglycaemic episode over the

previous 48 h. Participants were also instructed to maintain a similar diet and avoid

alcohol for 24 h before each testing session. On the morning of testing, the participants

were required to monitor their blood glucose regularly. They arrived in the laboratory at

around 8 A.M. and were instructed to self-administer their usual dose of morning insulin

and eat their breakfast as per normal before a catheter was inserted for blood sampling.

Participants’ insulin dose and breakfast content (1833 ± 224 kJ total energy; 57 ± 3%

carbohydrate; 17 ± 1% protein; 26 ± 4% fat) were kept identical for both trials. Glycaemia

was then measured every 15 min, and once blood glucose levels fell after peaking

postprandially, no mid-morning snack was allowed so as not to counter the fall in

glycaemia. If participants’ blood glucose levels were not decreasing at the

commencement of exercise, the testing session was cancelled because the purpose of this

study was to determine whether a short sprint could counter an exercise-mediated fall in

glycaemia.

64

When falling blood glucose levels reached ~ 11 mM (approximately 121 ± 15 and 109 ±

10 min after insulin injection in the sprint and control trials), they engage in 20 min of

moderate intensity exercise (40% peak 2OV ) on an air-braked Repco front access cycle

ergometer (Repco, Sydney, Australia), with the resistance to cycling increasing with

cycling rate. An intensity of 40% peak 2OV was adopted because it more closely mimics

the intensity of most activity patterns performed under ‘real life’ conditions for the

general population. Also, because all participants were tested at a time close to peak

plasma insulin levels and insulin-treated individuals with type 1 diabetes are discouraged

from engaging in any intense exercise during that time (Zinman et al., 2004), the

information gathered from exercising our participants at higher intensity would have been

of little practical relevance to most individuals with type 1 diabetes. The 20-min duration

was adopted because some preliminary work in our laboratory revealed that had the

exercise duration been longer, a large proportion of our participants would have reached

hypoglycaemic levels because of the rapid fall in glycaemia when exercising at 40%

peak 2OV , and testing would have had to end prematurely to avoid a hypoglycaemia-

mediated counterregulatory response. On completion of this moderate intensity cycling,

participants were instructed to rest or perform a 10-sec sprint, depending on their

experimental trial. All participants were instructed to cycle as hard as possible and not to

pace themselves for the whole duration of the 10-sec sprint. Venous blood from the arm

and arterialised capillary blood from the earlobe were sampled before the moderate

intensity exercise, immediately before the cycling sprint (in the sprint trial) and then at 0,

5, 10, 15, 30, 45, 60, 75, 90, 105 and 120 min of recovery or until blood glucose levels

declined to 3.5 mM, in which case the trial was ended and the participant was immediately

given carbohydrates to prevent hypoglycaemia.

At each sampling point, 35 l of arterialised capillary blood was taken from the earlobe

and assayed immediately for glucose and lactate levels using an ABL 625 blood gas

system (Radiometer, Copenhagen). A 15 ml blood sample was also removed from the

catheter for measuring hormones. Some of the blood was combined with sodium

metabisulphite, poly-ethylene glycol or aprotinin (Trasylol) for the assay of

catecholamines, insulin, and glucagon, respectively. All samples were then centrifuged

at 720g for 5 min, and the plasma was stored at -80C for later analysis of catecholamines,

free fatty acids, insulin, glucagon, cortisol, growth hormone and C-peptide levels.

65

3.3.3 Hormones and metabolite assays

Glucose and lactate were analysed using an ABL 625 Blood Gas System (Radiometer,

Copenhagen, Denmark). Heparinized plasma treated with sodium metabisulphate was

used to determine catecholamine levels by reverse phase high-performance liquid

chromatography using a Waters Novapak C18 reverse phase column and a model 5200A

Coulochem detector (ESA Biosciences Inc, Chelmsford, MA, USA). Free fatty acids

levels were measured in EDTA-treated plasma using the Roche Half Micro Test Free

Fatty Acids Assay kit (Roche, Mannheim, Germany). Heparinized plasma treated with

polyethylene glycol was assayed for free insulin using the Coat-a-Count Insulin kit

(Diagnostic Products Corporation, Los Angeles, CA, USA). Glucagon levels in plasma

collected with aprotinin (Trasylol) were measured from EDTA-treated plasma by

radioimmunoassay using a Linco glucagon radioimmunoassay kit (Linco Research, St

Charles, Missouri, USA). Cortisol levels were assayed from venous serum by

competitive immunoassay on an Immulite 2000 Analyser using the Immulite Cortisol

Assay kit (Diagnostic Products Corporation, Los Angeles, CA, USA). Growth hormone

levels were determined from serum by immunometric assay on an Immulite 2000

Analyser using the Immulite Growth Hormone Assay kit (Diagnostic Products

Corporation, Los Angeles, CA, USA). Finally, C-peptide levels were determined by

solid-phase competitive chemiluminescent enzyme immunoassay on an Immulite 2000

Analyser using the Immulite C-peptide Assay kit (Diagnostic Products Corporation, Los

Angeles, CA, USA).

3.3.4 Statistical analyses

The results were analysed using a two-way (time trial) repeated-measures analysis of

variance (ANOVA) and Fisher least significant differences (LSD) test for a posteriori

analysis using SPSS 11.0 software. Statistical significance was accepted at p < 0.05.

Participants’ characteristics are expressed as means ± S.D, whereas all other results are

expressed as means ± S.E.M.

66

3.4 Results

3.4.1 Blood metabolite response

Before the bout of moderate intensity exercise, blood glucose levels in both experimental

trials fell significantly (p < 0.05; Fig. 1). When glycaemia reached ~11 mM (11.2 ± 0.4

vs 11.9 ± 0.4 mM in the sprint and control trial, respectively), 20 min of cycling at 40%

peak 2OV was initiated, with the total workload being identical between treatments for each

participant (total work of 1176 ± 105 and 1178 ± 104 kJ/kg for sprint and control trials,

respectively). This resulted in a further rapid significant decrease in glycaemia in both

experimental trials (sprint: 3.6 ± 0.5 mM, p < 0.05; control: 3.1 ± 0.5 mM, p < 0.05; Fig.

3.1). When a 10-sec maximal sprint effort was performed immediately after the moderate

intensity exercise, the sprint opposed a further fall in blood glucose levels for the

following 120 min. In contrast, blood glucose levels decreased further (3.6 ± 1.22 mM;

p < 0.05) in the control trial (Fig. 3.1).

The response of free fatty acid levels to the sprint and control trials was similar, with

stable levels observed during moderate intensity exercise followed by a marginal increase

later in recovery (Fig. 3.2). In response to moderate intensity exercise, lactate levels

increased moderately and rose to a greater extent in response to the sprint, reaching

maximal levels after 5 min (p < 0.05) before decreasing to basal levels within 45 min after

the sprint. In contrast, in the absence of a short sprint, lactate levels remained at stable

basal levels throughout the recovery period.

67

Figure 3.1: Effect of a 10-sec sprint on blood glucose levels after moderate intensity

exercise. The moderate intensity exercise commenced at time-point -20 min. The black

line represents the sprint whereas the shaded box represents moderate intensity exercise.

Blood glucose levels are expressed relative to those immediately after moderate intensity

exercise (time-point= 0). All data are means standard error. Black circles refer to the

sprint trial whereas white circles refer to the control trial. b represents a statistically

significant difference (p < 0.05) compared to the 0 min time point after moderate intensity

exercise, c represents a statistically significant difference (p < 0.05) compared to the 0 min

time point after moderate intensity exercise.

68

3.4.2 Hormonal response

In response to the 10-sec maximal sprint effort initiated immediately after moderate

intensity exercise, epinephrine and norepinephrine reached maximal levels at the onset of

recovery (p < 0.05 for each) and returned to pre-exercise levels within 5 min after the

sprint (Fig. 3.2). In contrast, catecholamine levels remained stable in the control trial

(Fig. 3.2). Likewise, the response of growth hormone differed between the sprint and

control trials, with growth hormone levels increasing progressively after the sprint to

reach maximal levels after 15 min of recovery (Fig. 3.2). The response of plasma cortisol

levels in the sprint trial also differed from that of the control trial, with cortisol levels

increasing significantly during recovery to reach maximal levels 30 min after the sprint

(p < 0.05), but remained at stable levels in the control trial (Fig. 3.2). Glucagon increased

early in the recovery period in the control trial (p < 0.05) but did not change significantly

in the sprint trial. Finally, the pattern of insulin response to exercise was similar in the

sprint and control trials, with insulin levels remaining relatively stable throughout

exercise and recovery (Fig. 3.2). It is important to note that both trials were performed at

a time of the day when insulin levels were elevated (121 ± 15 and 109 ± 10 min after

insulin injection in the sprint and control trials, respectively), and plasma insulin levels

were similar between trials (Fig. 3.2).

69

Figure 3.2: Effect of a 10-sec sprint on the levels of lactate (A), free fatty acids (B),

norepinephrine (C), epinephrine (D), growth hormone (E), cortisol (F), glucagon (G) and

free insulin (H) after moderate intensity exercise. The black line represents the sprint

whereas the shaded box represents moderate intensity exercise. Black circles refer to the

sprint group whereas white circles refer to the control group. All data are shown as mean

standard error. a represents a statistically significant difference (p < 0.05) between

control and sprint trials, b represents a statistically significant difference (p < 0.05)

compared to the 0 min time point after moderate intensity exercise in control, c represents

a statistically significant difference (p < 0.05) compared to the 0 min time point after

moderate intensity exercise in sprint trial.

70

3.5 Discussion

Current guidelines for minimising the risks of hypoglycaemia associated with exercise in

type 1 diabetes recommend a reduction in insulin dose or increased ingestion of

carbohydrates before exercise based on an individual’s previous glycaemic responses to

similar exercise (Zinman et al., 2004). This study investigated the intriguing possibility

of using a short bout of intense exercise as another means to counter an exercise-mediated

fall in glycaemia. In particular, it examined whether a short 10-sec cycling sprint could

acutely oppose an exercise-mediated fall in glycaemia during recovery from exercise in

individuals with insulin-treated, complication-free type 1 diabetes. Our results suggest

that to minimise the risk of a fall in glycaemia after a bout of moderate intensity exercise

in young individuals with complication-free type 1 diabetes, it is preferable to engage in

a 10-sec maximal sprint effort before resting than to only rest during recovery. Such a

sprint opposed a further fall in blood glucose levels for at least 120 min, whereas

glycaemia decreased significantly (p < 0.05) by ~ 3.5mM if no sprint was performed (Fig.

3.1). Sprinting is likely to counter the exercise-mediated decrease in blood glucose levels

through an increase in catecholamine, lactate, cortisol and growth hormone levels. The

ability of the sprint to oppose the fall in glycaemia was more remarkable considering the

sprint and control trials were performed when insulin levels were elevated, a time when

exercise is not usually recommended (Rabasa-Lhoret et al., 2001).

It is likely that the marked rise in catecholamine levels at the onset of recovery after the

sprint explain how such a sprint counters an exercise-mediated fall in glycaemia, as the

levels of the other counterregulatory hormones examined in this study did not change

significantly during this time (Fig. 3.2). It is generally acknowledged that high

catecholamine levels oppose an insulin-mediated fall in glycaemia (Marliss and Vranic,

2002) via their activation of hepatic glucose production and inhibition of insulin-mediated

glucose uptake in skeletal muscles (Nonogaki, 2000). Likewise, elevated lactate levels

(Fig. 3.2) may contribute to the stabilisation of glycaemia early in recovery by providing

gluconeogenic precursors for hepatic glucose production (Miller et al., 2002). The above

interpretation has to be taken with caution given recent findings from this laboratory that

a 10-sec sprint by itself has no effect on hepatic glucose production rate but decreases

peripheral glucose utilisation rate despite causing a rise in plasma catecholamines and

lactate levels comparable to those measured here (Fahey et al., 2012).

71

In this study, catecholamines returned rapidly to basal levels after the sprint, thus raising

the question of whether other hormones opposed the decrease in glycaemia as recovery

progressed in the sprint trial. Because insulin levels did not change significantly from pre-

exercise levels over the two hours of recovery (Fig. 3.2), they could not explain how

sprinting opposed the exercise-mediated fall in glycaemia. Likewise, the absence of an

increase in plasma glucagon levels in the sprint trial makes it unlikely that glucagon was

responsible for opposing the decrease in glycaemia (Fig. 3.2). Although there is some

evidence that the progressive rise in cortisol levels may play a role in stabilising

glycaemia due to cortisol’s potential acute inhibitory effect on glucose utilisation in

skeletal muscles (Shamoon et al., 1980), it is likely that this hormone played only a minor

role because the effects of cortisol on hepatic glucose production and blood glucose levels

have been shown to require several hours to take place (McMahon et al., 1988).

The elevated levels of growth hormone after exercise (Fig. 3.2) might play some role in

opposing the decrease in glycaemia later during recovery from sprinting. In support of

this view, the administration of a physiological growth hormone pulse in non-exercised

non-diabetic individuals has been reported to result in a rapid fall in muscle glucose

uptake (Moller et al., 1990, 1992b, 2003) and a 1-2 h delayed increase in lipolysis,

circulating free fatty acid levels, and fat oxidation rates (Moller et al., 1990, 1992b, 2003),

which could contribute further to lowering glucose utilisation rates (Møller et al., 1992b).

However, the aforementioned fall in muscle glucose uptake in response to a growth

hormone pulse does not occur in insulin-treated individuals with type 1 diabetes (Møller

et al., 1992a) and is not associated with a corresponding change in glycaemia and glucose

appearance (Ra) and disposal (Rd) rates (Møller et al., 2003), thereby making it unlikely

that growth hormone had a role in opposing the post-sprint fall in glycaemia. More

importantly, the administration of this hormone after a bout of moderate intensity exercise

in growth hormone-deficient individuals has no effect on glucose Ra and Rd (Kanaley et

al., 2004), and the administration of octreotide (an inhibitor of growth hormone secretion)

in non-diabetic individuals has also no acute effects on the magnitude of the

hyperglycaemic effect of high intensity exercise (Sigal et al., 1996). It is important to

stress, however, that no study so far has evaluated whether glucose metabolism in

response to a short sprint is affected by growth hormone levels. The identity of the

counterregulatory hormone(s) responsible for opposing the fall in glycaemia when a

sprint is performed following a bout of moderate intensity exercise remains to be

established.

72

In conclusion, this study provides the first evidence that a short maximal sprint effort

performed immediately after moderate intensity exercise is preferable to only resting as

a means to counter a further fall in glycaemia after exercise, thus decreasing the risk of

early post-exercise hypoglycaemia in individuals with type 1 diabetes. On this basis, one

might tentatively recommend that following exercise of moderate intensity, young

individuals with complication-free type 1 diabetes consider performing a short 10-sec

sprint before resting to counter a further fall in their blood glucose levels rather than only

resting, particularly if a source of dietary carbohydrate is not readily available. This

recommendation does not extend to intermittent high intensity exercise, as a study from

our laboratory has shown recently that blood glucose remains at stable levels for at least

one hour after this type of exercise (Guelfi et al., 2005a, 2005c). Although the long-term

health benefits of regular exercise are generally recognised, to the best of our knowledge

these findings provide the first example of a bout of exercise offering immediate short-

term benefits (stabilisation of glycaemia). It is important to stress, however, that different

results might have been obtained had sprinting been initiated after exercise of higher

intensity or longer duration, in younger or older individuals with reduced sprinting

capacity, or in individuals with impaired counterregulatory responses. For these reasons,

more studies of the kind described here will be required to identify the subpopulation of

type 1 diabetic individuals for whom a short maximal sprint effort can be recommended

as a safe approach for the short-term stabilisation of blood glucose levels.

3.6 Acknowledgements This research was funded jointly by a Juvenile Diabetes Research Foundation/National

Health Medical Research Council of Australia program grant to T. Jones and P.A.

Fournier. L. D. Ferreira is supported by a Juvenile Diabetes Research Fund International

Fellowship. The authors acknowledge the technical assistance of Leanne Youngs.

73

Chapter 4

A 10-second Sprint Performed Prior to

Moderate-Intensity Exercise Prevents Early

Post-Exercise Fall in Glycaemia in

Individuals with Type 1 Diabetes

An amended version of this chapter has been published in Diabetologia:

Bussau, V.A., Ferreira, L.D., Jones, T.W. & Fournier, P.A. (2007). A 10-s sprint

performed prior to moderate-intensity exercise prevents early post-exercise fall in

glycaemia in individuals with type 1 diabetes. Diabetologia 50 (9): 1815-1818.

74

4.1 Abstract

Objective: We investigated whether a 10-sec maximal sprint effort performed

immediately prior to moderate-intensity exercise provides another means to counter the

rapid fall in glycaemia associated with moderate-intensity exercise in individuals with

type 1 diabetes.

Research Design and Methods: Seven complication-free type 1 diabetic males (21.6 ±

3.6 years; mean ± SD) with HbA1c levels of 7.4 ± 0.7% injected their normal morning

insulin dose and ate their usual breakfast. When post-meal glycaemia fell to ~11 mM,

participants were asked to perform a 10 sec all-out sprint (sprint trial) or to rest (control

trial) immediately before cycling at 40% of peak rate of oxygen consumption for 20 min,

with both trials conducted in a random counterbalanced order.

Results: Sprinting did not affect the rapid fall in glycaemia during the subsequent bout

of moderate-intensity exercise (2.9 ± 0.4 mM fall in 20 min; p = 0.00; mean ± SE).

However, during the following 45 min of recovery, glycaemia in the control trial

decreased by 1.23 ± 0.60 mM (p = 0.04) while remaining stable during early recovery in

the sprint trial, subsequently decreasing in this latter trial at a rate similar to that in the

control trial. The large increase in norepinephrine (p = 0.005) and lactate levels (p =

0.0005) may have contributed to the early post-exercise stabilisation of glycaemia in the

sprint trial. During recovery, epinephrine and free fatty acid levels increased marginally

in the sprint trial, but other counterregulatory hormones did not change significantly (p <

0.05).

Conclusions: A 10-sec sprint performed immediately prior to moderate-intensity

exercise prevents glycaemia from falling during early recovery from moderate intensity

exercise in individuals with type 1 diabetes.

75

4.2 Introduction

Individuals with type 1 diabetes mellitus are faced with the daily challenge of maintaining

their blood glucose levels within a narrow physiological range in order to minimise the

risks of developing long-term diabetic complications associated with chronic

hyperglycaemia (DCCT, 1993). Unfortunately, the achievement of good glycaemic

control results in an increased risk of experiencing severe episodes of hypoglycaemia

(DCCT, 1993). This is further exacerbated in active individuals, since physical activity

of moderate intensity increases the risk of hypoglycaemia both during exercise

(Tuominen et al., 1995; Riddell et al., 1999; Rabasa-Lhoret et al., 2001) and for several

hours afterwards (MacDonald, 1987). For these reasons, current guidelines aimed at

minimising the risk of a fall in glycaemia associated with exercise recommend a reduction

in insulin dose or increased ingestion of carbohydrates prior to exercise based on the

individual’s previous glycaemic responses to similar exercise (Zinman et al., 2004)

It is well established that prolonged exercise of high intensity (>10 min at >80% peak rate

of oxygen consumption; O2peak) differs from moderate-intensity exercise in that it causes

an increase rather than a fall in glycaemia in insulin-treated individuals with type 1

diabetes (Marliss and Vranic, 2002). For this reason, our laboratory recently examined

whether the performance of one or several short maximal sprint efforts could offer an

alternative means to counter the fall in glycaemia associated with moderate-intensity

exercise or elevated plasma insulin levels in individuals with type 1 diabetes (Chapter 3;

Guelfi et al., 2005b, 2005c; Bussau et al., 2006). This research showed that the fall in

glycaemia during and after exercise of moderate intensity is not as marked if this type of

exercise is interspersed with several 4 sec sprints, despite more work being performed

(Guelfi et al., 2005b). Also, as described in Chapter 3 (Bussau et al., 2006), we found

that immediately after a bout of moderate-intensity exercise it is preferable to sprint for

10 sec than to only rest as a means of countering a decrease in glycaemia during recovery.

It is important to note that the sprints in these studies (Chapter 3; Guelfi et al., 2005b,

2005c; Bussau et al., 2006) were short enough (4–10 sec) to be well tolerated by all

participants.

Given the benefits of sprinting during or after moderate-intensity exercise (Chapter 3;

Guelfi et al., 2005b; Bussau et al., 2006), this raises the intriguing question of whether

performing a short sprint effort immediately prior to, rather than after, moderate-intensity

V

76

exercise may offer a novel way of preventing the rapid fall in glycaemia normally

observed during and after moderate-intensity exercise. In other words, this study

examines whether short-duration sprinting should be an integral part of the preparatory

routine of individuals with type 1 diabetes before they engage in sustained physical

activities of moderate intensity.

77

4.3 Methods

4.3.1 Participants

Seven males with type 1 diabetes, (age 21.6 ± 3.6 years; BMI 26.7 ± 4.3 kg/m2; O2peak

45.2 ± 5.0 ml kg–1 min –1; diabetes duration 9.5 ± 3.3 years; HbA1c 7.4 ± 0.7%; total daily

insulin dose 94 ± 38 units; all means ± SD), who were free from diabetic complications,

had undetectable levels of C-peptide and were hypoglycaemia-aware, gave informed

consent in accordance with both the University of Western Australia and Princess

Margaret Hospital Ethics Committees. Participants were not taking any prescribed

medication other than insulin and had not changed their insulin regimen for at least 3

months before the study. Following a familiarisation session, during which

anthropometric measurements were taken, O2peak was determined as described

previously (Fairchild et al., 2002b). The next two visits were the sprint and control rest

trials conducted in a random counterbalanced order.

4.3.2 Experimental trials and assays

In the 48 h prior to the experimental trial, participants were not allowed to exercise as

antecedent exercise could have affected the endocrine response to exercise (Galassetti et

al., 2001b). In addition, testing was rescheduled if participants had experienced a

hypoglycaemic episode during the 48 h pre-trial phase because prior hypoglycaemia can

also affect the counter-regulatory response during exercise (Galassetti et al., 2003).

Following their arrival in the laboratory at 08:00 hours, participants self-administered

their usual dose of morning insulin into their abdomen, with insulin dosage kept identical

between trials (mean dose 15 ± 2 units short-acting insulin, 16 ± 9 units intermediate-

acting insulin), and a catheter was inserted for blood sampling. Blood glucose levels were

similar (p = 0.843) before insulin injection, with levels of 10.1 ± 1.4 and 10.3 ± 1.5 mM

in the sprint and control trials, respectively. Participants then consumed breakfast to

increase glycaemia above 11 mM. Breakfast food choice and nutritional content reflected

that normally eaten by the participants, with breakfast kept the same (p > 0.05) for both

trials (1736 ± 193 kJ total energy, with relative energy content as follows; 56 ± 3%

carbohydrate; 17 ± 1% protein; 27 ± 4% fat). However, one of the control participants

consumed additional carbohydrate to achieve post-breakfast glycaemia above 11 mM.

After breakfast, no physical activity was allowed and glycaemia peaked at 13.8 ± 0.8 and

13.9 ± 0.50 mM in the sprint and control trials, respectively (p = 0.890). When glycaemia

V

V

78

post-breakfast fell back to ~ 11 mM (approximately 111 ± 10 and 106 ± 8 min after insulin

injection in the sprint and control trials, respectively; p = 0.564), participants either rested

or performed a 10-sec maximal sprint effort on an air-braked cycle ergometer (Repco,

Sydney, NSW, Australia). This was immediately followed by 20 min of moderate-

intensity exercise (40% O2peak), with the rationale underlying the intensity and duration

of this bout of moderate-intensity exercise similar to that discussed previously in Chapter

3 (Bussau et al., 2006). Blood sampling and assays of metabolites and hormones were

also performed as described in earlier studies from this laboratory (Chapter 3; Guelfi et

al., 2005b; Bussau et al., 2006).

4.3.3 Statistical analyses

Data were analysed using a two-way repeated-measures ANOVA and Fisher’s least

significant differences test for a posteriori analysis with SPSS 13.0 for Windows software

(SPSS, Chicago, IL, USA). Statistical significance was accepted at p < 0.05.

Participants’ characteristics are expressed as mean ± SD; all other results are mean ±

SEM.

V

79

4.4 Results

4.4.1 Blood metabolite response

Before the bout of moderate intensity exercise, blood glucose levels in both experimental

groups fell significantly (p = 0.00; Fig. 4.1). When glycaemia reached ~11 mmol/l (11.4

± 0.5 and 11.8 ± 0.5 mmol/l in the sprint and control trial, respectively; mean ± S.E., n=7,

p = 0.446), participants either rested or performed a 10-sec all-out sprint immediately

before cycling at 40% O2peak for 20 min. The total workload during the 20-min bout of

exercise did not differ between treatments (total work of 295 ± 18 and 281 ± 25 kcal/kg

for the sprint and control trials, respectively). Performing either a 10-s all-out sprint prior

to exercise or only resting did not affect the rapid fall in glycaemia during the subsequent

bout of moderate intensity exercise (2.9 ± 0.4 mmol/l in the sprint trial, p = 0.00; 3.2 ±

0.5 mmol/l in the control trial, p = 0.001; Fig. 4.1). During the initial 45 min of recovery,

glycaemia in the control trial decreased linearly by 1.23 ± 0.60 mmol/l (p = 0.04), and

fell by 3.80 ± 1.33 mmol/l after 120 min of recovery (p = 0.03; Fig. 4.1). In contrast,

glycaemia remained stable in the sprint trial during the initial 45 min of recovery before

eventually decreasing at a rate similar to that in the control trial, thus resulting in a delayed

fall in blood glucose levels post-exercise (Fig. 4.1).

In the sprint trial, blood lactate levels increased significantly immediately post-sprint (p

= 0.0005) and returned to pre-exercise levels within 30 min of recovery (Fig. 4.2). In

contrast, in the control trial, lactate levels only increased immediately post-exercise (p =

0.031) before returning to pre-exercise levels after 5 min of recovery (Fig. 4.2). The

responses of free fatty acid levels to the sprint and control trials were similar early in

recovery; however these levels increased significantly between 90 (p = 0.0005) and 120

min (p = 0.0005) post-exercise in the sprint trial (Fig. 4.2).

4.4.2 Hormonal response

In response to the 10-s maximal sprint effort initiated immediately before moderate

intensity exercise, norepinephrine (p = 0.005) reached maximal levels immediately after

the sprint before gradually decreasing during the bout of moderate intensity exercise (Fig.

4.2). During recovery, epinephrine and norepinephrine returned to basal levels within 5

and 30 min, respectively. In contrast, epinephrine levels in the control trial remained

relatively stable, whereas norepinephrine levels were only elevated immediately

V

80

following moderate intensity exercise (p = 0.013) and returned to basal levels within 5

min post-exercise (Fig. 4.2). The responses of growth hormone and cortisol in the sprint

differed markedly from those of the catecholamines in that growth hormone levels did

not change significantly and cortisol levels only increased marginally immediately after

the sprint (p = 0.026; Fig. 4.2). Glucagon levels also did not change significantly in

response to the sprint, but increased transiently (P < 0.05) at the onset of recovery from

exercise in the control trial (Fig. 4.2). Finally, insulin levels before and after exercise did

not change significantly in both trials (Fig. 4.2). It is noteworthy that both trials were

performed at a time when plasma insulin levels were elevated (111 ± 10 and 106 ± 8 min

in the sprint and control trials, respectively, post-insulin administration).

81

Fig. 4.1 Effect of a 10 sec sprint on blood glucose levels during and after moderate-

intensity exercise. Black box, sprint; hatched box, moderate-intensity exercise; closed

circles, sprint trial; open circles, control trial. The moderate-intensity exercise was

commenced at the -20 min time-point. Blood glucose levels are expressed relative to

those immediately after moderate-intensity exercise (time-point = zero). All values are

shown as meanSEM. b p < 0.05 compared to the 0 min time point after moderate-

intensity exercise in the control trial; c p < 0.05 compared to the 0 min time point after

moderate-intensity exercise in the sprint trial.

c c

c

b b

b Cha

nge

in B

lood

Glu

cose

(mM

)

120 100 80

Time post-exercise (min)

60 40 20 0 -20 -40

-4

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82

120 100 80 60 40 20 0 -20

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a,c a,c a,c a,c a

c a c c c c 0.05

0.10

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0.25

0.30

0.35 A B

20

40

60

80

100 G

0.05

0.10

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1

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7 a,c

b

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Lact

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(pg/

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wth

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100

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Free fatty

acids (m

M)

Epinephrine (nmol/l)

Cortisol (nm

ol/l) Free Insulin (pm

ol/l)

H

b b b b b b b

b

b

c

a

c

Time post-exercise (min)

100

400

300

200

500

120 100 80 60 20 40 0 -20

83

Fig. 4.2 Effect of a 10-sec sprint on the levels of lactate (A), free fatty acids (B),

norepinephrine (C), epinephrine (D), growth hormone (E), cortisol (F), glucagon (G) and

free insulin (H) during and after moderate-intensity exercise. Black boxes, sprint; hatched

boxes, moderate-intensity exercise; closed circles, sprint trial; open circles, control trial.

All values are shown as mean SEM. a p < 0.05 compared with control; b p < 0.05

compared to the 0 min time point after moderate-intensity exercise in the control trial; c p

< 0.05 compared to the 0 min time point after moderate-intensity exercise in the sprint

trial.

84

4.5 Discussion It is well established that exercise of moderate intensity increases the risk of

hypoglycaemia both during and after exercise in type 1 diabetes (MacDonald, 1987;

Tuominen et al., 1995; Riddell et al., 1999; Rabasa-Lhoret et al., 2001). However, in

Chapter 3 we reported that a single all-out 10-s sprint performed immediately after

moderate intensity exercise provides a simple means to oppose a decrease in glycaemia

during recovery (Bussau et al., 2006). Here we show that although a 10-sec sprint

initiated immediately before moderate-intensity exercise did not affect the rapid decline

in glycaemia during a subsequent bout of moderate-intensity exercise performed when

plasma insulin levels were elevated, it did prevent the fall in glycaemia for at least the

first 45 min of recovery (Fig. 4.1). This suggests that including a short sprint as part of

the preparatory routine of individuals with type 1 diabetes before they engage in sustained

moderate-intensity exercise might provide another means of temporarily stabilising

glycaemia during early recovery.

The marked rise in norepinephrine and lactate levels immediately after exercise in the

sprint trial might explain, in part, how sprinting before moderate-intensity exercise delays

the fall in glycaemia early into recovery. Norepinephrine levels during early recovery

were elevated in the sprint trial while blood glucose levels were stable, but soon after the

return of norepinephrine to basal pre-exercise levels, blood glucose levels in the sprint

and control trials fell at comparable rates (Fig. 4.2). Although high catecholamine levels

have been generally acknowledged to counter insulin-mediated falls in glycaemia

(Marliss and Vranic, 2002) via activation of hepatic glucose production and inhibition of

insulin-mediated glucose uptake in skeletal muscles (Nonogaki, 2000), their role as

counter-regulatory hormones during exercise in type 1 diabetic individuals has been a

controversial issue (Coker and Kjaer, 2005). Elevated lactate levels (Fig. 4.2) are another

factor that may have contributed to the stabilisation of glycaemia early in recovery by

providing gluconeogenic precursors for hepatic glucose production and by increasing

peripheral insulin resistance (Vettor et al., 1997). The above interpretation is challenged,

at least in part, by recent findings from this laboratory that sprinting for 10 sec has no

effect on hepatic glucose production rate but decreases peripheral glucose utilisation rate

despite causing a rise in plasma catecholamines and lactate levels comparable to those

measured here (Fahey et al., 2012).

85

The patterns of change in the levels of insulin, cortisol, glucagon and growth hormones

in the sprint and control trials suggest that these hormones are unlikely to explain the

delayed fall in glycaemia during recovery in the sprint trial. Insulin levels in both trials

did not change significantly during recovery (Fig. 4.2). Also, the levels of glucagon did

not increase significantly during and after exercise in the sprint trial. The question of

whether portal glucagon levels responded in a similar manner remains to be established

(Fig. 4.2). Although there is some published evidence that a rise in cortisol levels may

play a role in stabilising glycaemia due to its potential acute inhibitory effect on glucose

utilisation in skeletal muscles (Shamoon et al., 1980), this hormone is likely to play only

a minor role here not only because of the absence of changes in the levels of this hormone

during recovery in both trials, but also because the effects of cortisol on hepatic glucose

production and blood glucose levels require several hours to take place (McMahon et al.,

1988). Finally, growth hormones are also unlikely to play a role as no significant changes

in growth hormone levels were observed in either of the groups (Fig. 4.2). However, a

close inspection of Fig. 4.2e suggests that significant changes in growth hormone levels

might have been masked by the large inter-individual variability in growth hormone

levels. Nevertheless, even if growth hormone levels had increased significantly, this

would probably have been of lesser importance because it has been reported that (1)

administration of a growth hormone pulse does not affect peripheral glucose uptake in

insulin-treated individuals with type 1 diabetes (Møller et al., 1992a); and (2) the infusion

of octreotide, an inhibitor of growth hormone release, has no acute effects on the

magnitude of the hyperglycaemic effect of high-intensity exercise (Sigal et al., 1996).

There is evidence that the aforementioned factors proposed to explain the stabilising

effects of a short sprint on post-exercise glycaemia might be similar, irrespective of

whether sprinting is performed before, during or after moderate-intensity exercise.

Indeed, elevated lactate and catecholamines levels have also been proposed to stabilise

glycaemia at the onset of recovery from moderate-intensity exercise interspersed either

with several short sprints (Guelfi et al., 2005b) or followed by a 10-sec sprint (Chapter 3;

Bussau et al., 2006). It is less clear, however, whether GH is also involved under these

conditions because of a lack of consistencies across studies. Clearly, more research is

required to identify the factors responsible for the sprint-mediated stabilisation of

glycaemia after exercise.

86

On practical and clinical grounds, our findings suggest that before engaging in moderate-

intensity exercise, individuals with type 1 diabetes might consider incorporating into their

warm-up routine a short 10-sec maximal sprint effort, especially as a 10-sec sprint is well

tolerated (Chapter 3; Bussau et al., 2006). However, before recommending the use of

sprinting to prevent an early post-exercise fall in glycaemia, a number of issues must be

addressed such as the extent to which the efficacy of prior sprinting is affected by the

intensity and duration of the subsequent bout of exercise and the identification of any

subpopulations of type 1 diabetic individuals unlikely to benefit from sprinting, such as

very young children and older individuals who have limited capacity to engage in a

maximal sprint effort..

In conclusion, our findings show that sprinting before a bout of exercise of moderate

intensity could be a novel tool of clinical importance for preventing the fall in glycaemia

during early recovery in young healthy individuals with type 1 diabetes. These findings

provide one of the very few examples (Chapter 3; Guelfi et al., 2005b, 2005c; Bussau et

al., 2006) of exercise offering an immediate short-term clinical benefit (delayed fall in

glycaemia).

4.6 Acknowledgements This research was funded jointly by a Juvenile Diabetes Research Foundation

(JDRF)/National Health Medical Research Council of Australia program grant to T. W.

Jones and P. A. Fournier. L. D. Ferreira is supported by a JDRF Fellowship. The authors

acknowledge the technical assistance of L. Youngs and A. Thompson.

87

Chapter 5

Counterregulatory Response to a 10-second

Sprint in Individuals with Type 1 Diabetes

Mellitus

88

5.1 Abstract

Objective: A 10-sec sprint performed before or after moderate intensity exercise reduces

the risk of post-exercise hypoglycaemia in hyperinsulinaemic fed individuals with type 1

diabetes mellitus. Since the glucoregulatory response to such a short sprint in these

individuals has never been examined, this study investigates the response of the

counterregulatory hormones to a 10-sec sprint in individuals with type 1 diabetes under

hyperinsulinaemic fed conditions.

Research Design and Methods: Seven complication-free male individuals with type 1

diabetes (HbA1c = 7.0 ± 0.3%) were recruited to the study. On the morning of testing, the

participants followed their normal insulin regimen and ate their usual breakfast. Blood

glucose levels were determined regularly and allowed to fall to approximately 6.0 mM.

Then, participants pedalled for 10 sec at maximal intensity on a cycle ergometer.

Results: Immediately after sprinting, plasma norepinephrine and epinephrine levels rose

significantly (p < 0.05) to maximal levels (7.3 ± 1.0 and 0.66 ± 0.09 nmol/L, respectively,

p < 0.05). Growth hormone increased to maximal levels (33.1 ± 6.7 mIU/L) 30 min after

sprinting (p < 0.05), and remained elevated for the whole duration of the recovery period.

Glucagon levels remained at stable levels. Similarly, plasma insulin and cortisol levels

did not change significantly. Before sprinting, glycaemia fell significantly and remained

stable during recovery.

Conclusions: The response of the counterregulatory hormones to a 10-sec sprint in type

1 diabetic individuals in a moderate hyperinsulinaemic and fed state is characterised

mainly by marked changes in catecholamines and growth hormone levels.

89

5.2 Introduction

Individuals with type 1 diabetes mellitus are encouraged to participate in regular physical

activity due to the numerous physiological and psychological health benefits associated

with an active lifestyle (Norris et al., 1990; Moy et al., 1993; Laaksonen et al., 2000;

Zinman et al., 2004; Riddell and Iscoe, 2006; Chimen et al., 2012, Tonoli et al., 2012).

Unfortunately, the risk of hypoglycaemia is increased both during exercise (Riddell et al.,

1999; Tuominen et al., 1995; Rabasa-Lhoret et al., 2001) and for several hours during

recovery (MacDonald, 1987a; Tsalikian et al., 2005; McMahon et al., 2007; Maran et al.,

2010; Iscoe and Riddell, 2011; Davey et al., 2013a). Due to this increased risk of

hypoglycaemia, it is not surprising that fear of hypoglycaemia is a major barrier to regular

physical activity in individuals with type 1 diabetes (Ludvigsson et al., 1980; Guelfi et

al., 2007c; Brazeau et al., 2008).

As mentioned in Chapter 1, not all types of exercise increase the risk of hypoglycaemia.

Several studies have reported that 10-15 min of high-intensity aerobic exercise (>80% of

peak 2OV ) under basal insulinaemic conditions is accompanied by an increase in blood

glucose levels during and after exercise (Mitchell et al., 1988; Marliss et al., 1992a;

Purdon et al., 1993; Sigal et al., 1994c, 1999; Marliss and Vranic, 2002). This is also the

case in response to a 10-sec maximal sprint effort (Fahey et al., 2012). This glycaemia-

rising effect of aerobic short sprint is such that, in theory, this type of exercise might be

beneficial if adopted to prevent or delay hypoglycaemia when no carbohydrate is readily

available to exercising individuals with type 1 diabetes.

As indicated in Chapter 3 and 4, when blood glucose level in individuals with type 1

diabetes is decreasing rapidly during moderate intensity exercise, engaging in a maximal

10-sec sprint immediately before or after exercise causes blood glucose levels after

exercise to stabilise for 0.5-2 hours, thus decreasing the early risk of hypoglycaemia post-

exercise (Chapters 3 and 4; Bussau et al., 2006, 2007). It is noteworthy that sprinting in

these studies was beneficial despite elevated plasma insulin levels, a time when physical

activity is generally not recommended due to the increased risk of experiencing

hypoglycaemia (Rabasa-Lhoret et al., 2001).

In an attempt to identify the endocrine mechanisms likely to mediate the glucoregulatory

benefits of sprinting, the levels of several counterregulatory hormones were measured in

90

the aforementioned studies (Chapters 3 and 4; Bussau et al., 2006, 2007), with our

findings suggesting that a post-exercise increase in catecholamines and growth hormone

levels might contribute to the protective effect of sprinting in type 1 diabetic individuals

(Chapters 3 and 4; Bussau et al., 2006, 2007). However, one difficulty with comparing

these findings with the literature has been the lack of information on the effect of short-

duration sprinting per se on the levels of counterregulatory hormones in

hyperinsulinaemic diabetic individuals in a fed state. Indeed, only one study has examined

the effect of a 10-sec sprint on the levels of these hormones in individuals with type 1

diabetes (Fahey et al., 2012). However, this study was performed in overnight fasted

individuals and under basal insulinaemic conditions, thus leaving unanswered the

question of the effect of sprinting on the counterregulatory hormones responses to short-

duration sprinting under combined hyperinsulinaemic and fed conditions. Given the

clinical benefits of a 10-sec sprint for the prevention of hypoglycaemia in

hyperinsulinaemic fed individuals with type 1 diabetes, and the lack of information about

the counterregulatory response to this type of exercise, the purpose of this study was to

investigate the effect of a single 10-sec sprint on the levels of the counterregulatory

hormones under dietary and insulinaemic conditions mimicking those where sprinting has

been shown to be beneficial to individuals with type 1 diabetes. Given that our studies on

the benefits of sprinting were performed in hyperinsulinaemic fed individuals (Chapters

3 and 4; Bussau et al., 2006, 2007), this was also the physiological state examined here.

The information thus obtained should provide the basis for future studies aimed at

elucidating the mechanisms underlying the clinical benefits of sprinting in the prevention

of hypoglycaemia in type 1 diabetic individuals.

91

5.3 Methods

5.3.1 Participants

Seven young males with type 1 diabetes (aged 17.7 0.5 years; BMI 25.9 4.1kg/m2;

peak 2OV 43.5 ± 4.8 ml/kg/min; duration of diabetes 5.8 ± 1.8 years) were recruited from

Princess Margaret Hospital and the University of Western Australia, respectively. All

participants were in moderate glycaemic control (Hb A1c = 7.0 ± 0.3%), free from

diabetic complications, were hypoglycaemia aware, had undetectable levels of c-peptide,

and were not taking any prescribed medication other than insulin. Moreover, these

participants were on a multiple-injection insulin regime that had not changed for at least

three months prior to the study. All participants were subjected to a familiarisation session

during which their informed consent was obtained as well as their height, body mass and

maximal rate of oxygen consumption as described previously (Fairchild et al., 2002a).

Then, each participant was required to attend our laboratory on two occasions. Both the

University of Western Australia and the Princess Margaret Hospital Ethics Committees

approved the procedures described in this study.

5.3.2 Experimental trials

Participants were not allowed to exercise for 48 h prior to the experimental trial since

antecedent exercise has the potential to affect the endocrine response to exercise

(Galassetti et al., 2001b). Also, testing was rescheduled if they had experienced a

hypoglycaemic episode over the previous 48 h because prior hypoglycaemia can also

affect the counterregulatory response to exercise (Davis et al., 2000d; Galassetti et al.,

2003). Each participant was also required to maintain their normal diet and to avoid

alcohol for 24 h prior to testing. On the morning of testing, all participants were required

to self-monitor their blood glucose levels regularly before attending the laboratory at ~

07.30 h where they were instructed to self-administer their usual morning dose of insulin

into the abdomen (mean dose 34.9 ± 10.2 units). Then, they ingested their breakfast as

described in Chapters 3 and 4 (1933 ± 287 kJ total energy; 62 ± 3% carbohydrate; 16 ±

1% protein; 22 ± 4% fat), and a catheter was inserted for blood sampling. Following

breakfast and when glycaemia started falling, blood glucose levels were measured every

15 min. Then, a 10-sec maximal sprint effort was initiated on a Repco front access cycle

ergometer (Repco, Sydney, Australia) when blood glucose levels reached approximately

6.0 mM. The participants were then instructed to cycle as hard as possible for 10 sec and

not to pace themselves for the whole duration of the sprint. Blood was sampled prior to

92

exercise and then at 0, 5, 10, 15, 30, 45 and 60 min post-sprint. At each sampling point,

0.2 ml of blood was first removed for the measurement of blood glucose, pO2, pH and

lactate. A second sample of 15 ml was also removed for the measurement of hormones.

Some of the blood was combined with sodium metabisulphite, poly-ethylene glycol or

trasylol for the assays of catecholamines, insulin and glucagon, respectively. All samples

were then centrifuged at 720g for 5 min and the plasma stored at -80C for later analysis.

5.3.3 Hormones and metabolite assays

Glucose, lactate, pH and pO2 were analysed using an ABL 625 blood gas system

(Radiometer, Copenhagen, Denmark). Free fatty acids levels were measured using the

Roche half micro test free fatty acids assay kit (Roche Diagnostic, Germany). All

hormones were assayed as described previously (Chapters 3 and 4; Bussau et al., 2006,

2007). Plasma catecholamine levels were determined by reverse phase HPLC using a

Waters Novapak C18 reverse phase column and a model 5200A Coulochem detector

(ESA Inc, USA). Free insulin levels were measured by radioimmunoassay using the

Phadeseph insulin RIA kit (Pharmacia, Uppsala, Sweden). Glucagon levels were

measured by radioimmunoassay using a Linco glucagon RIA kit (Linco Research, St

Charles, Missouri, USA). Cortisol levels were assayed by competitive immunoassay on

an Immulite 2000 analyser using the Immulite cortisol assay kit (Diagnostic Products

Corporation, Los Angeles, CA, USA). Growth hormone (GH) levels were determined by

immunometric assay on an Immulite 2000 analyser using the Immulite growth hormone

assay kit (Diagnostic Products Corporation, Los Angeles, CA, USA). Finally, C-peptide

levels were determined by solid-phase competitive chemiluminescent enzyme

immunoassay on an Immulite 2000 analyser using the Immulite C-peptide assay kit

(Diagnostic Products Corporation, Los Angeles, CA, USA).

5.3.4 Statistical analyses

The results were analysed using a one-way repeated measure analysis of variance and

Fisher LSD test for a posteriori comparisons. Statistical significance was accepted at p

< 0.05 and all analyses were carried out using SPSS 17.0 software. Participants’

characteristics are expressed as means ± S.D whereas all other results are expressed as

means ± S.E.M.

93

5.4 Results

5.4.1 Hormonal response to a 10-sec sprint

In response to sprinting, plasma norepinephrine levels rose significantly (p < 0.05) and

reached maximal levels at the onset of recovery (7.3 ± 1.0 nM) before returning to pre-

exercise levels within 10 min after exercise (Fig. 5.1). Similarly, plasma epinephrine

reached maximal levels at the onset of recovery (0.66 ± 0.09 nM, p < 0.05) and remained

elevated above pre-sprint levels during the first 10 min of recovery before returning to

pre-sprint levels (Fig. 5.1). Plasma growth hormone levels increased progressively during

recovery from sprinting to reach maximal levels of 33.1 ± 6.7 mIU/L after 30 min of

recovery (p < 0.05; Fig 5.1), with these levels still remaining above pre-sprint levels 60

min after exercise. In response to sprinting, plasma glucagon, cortisol and insulin levels

did not change significantly (p > 0.05; Fig. 5.1). However, it must be noted that there

was a non-significant trend for cortisol levels to increase and reach maximal levels after

30 min of recovery.

94

Figure 5.1: Effect of a single 10-sec sprint on the levels of norepinephrine (A),

epinephrine (B), growth hormone (C), cortisol (D), free insulin (E), and glucagon (F) in

participants with type 1 diabetes. All graphs are shown as mean standard error. The

sprint is represented by the black line. a represents a statistically significant difference (p

< 0.05) compared to the rest time point in the type 1 diabetic participants.

95

5.4.2 Blood metabolite response to a 10-sec sprint

In response to sprinting, blood lactate levels reached maximal levels after 5 min of

recovery (p < 0.05; Fig. 5.2) before decreasing to pre-sprint levels within 60 min after

exercise. Blood pH fell to minimal levels at 5 min after sprinting (p < 0.05; Fig. 5.2)

before returning to pre-sprint levels within 60 min of recovery. Free fatty acid levels

increased progressively after exercise to reach maximal levels after 60 min of recovery

(p < 0.05; Fig. 5.2). Before sprinting, blood glucose levels fell significantly (p < 0.05;

Fig. 5.2) and remained stable for 60 min after sprinting.

5.4.3 Work load and peak power associated with a 10-sec sprint

The total work performed during the 10-sec sprint was 82.8 8.4 kJ/kg and the peak

power achieved was 10.7 0.9 W/kg.

96

Figure 5.2: Effect of a single 10-sec sprint on blood glucose (A), free fatty acids (B), pH

(C) and blood lactate (D) levels in participants with type 1 diabetes. The sprint is

represented by the black line. All results are shown as mean standard error. Black

circles refer to the type 1 diabetic participants. a represents a statistically significant

difference (p < 0.05) compared to the rest time point in the type 1 diabetic participants.

97

5.5 Discussion

In chapters 3 and 4, we showed that even when plasma insulin levels are elevated, a 10-

sec sprint performed either immediately before or after moderate intensity exercise can

prevent blood glucose levels from falling after exercise (Bussau et al., 2006, 2007), thus

providing a novel clinical tool to acutely decrease the risk of post-exercise

hypoglycaemia. Although, the combined effects of sprinting and moderate intensity

exercise on counterregulatory hormones levels were examined in these studies (Chapters

3 and 4; Bussau et al., 2006, 2007), surprisingly the responses of these hormones to

sprinting per se in type 1 diabetic individuals who are fed and under hyperinsulinaemic

conditions has never been investigated before, thus making it impossible to compare our

earlier findings with those of others. Here, for the first time, the effect of short-duration

sprinting per se on the levels of several counterregulatory hormones was examined in

hyperinsulinaemic fed individuals with type 1 diabetes. We found that a 10-sec maximal

sprint in those individuals resulted in marked changes in plasma catecholamines and

growth hormone levels, with no significant effect on plasma cortisol, insulin and glucagon

levels. In addition, the responses of growth hormones and cortisol levels differed from

those observed when a sprint is performed immediately before or after a bout of moderate

intensity exercise in individuals with type 1 diabetes (Chapters 3 and 4; Bussau et al.,

2006, 2007).

Our findings show that a 10-sec sprint results in an early rise in plasma epinephrine and

norepinephrine levels in hyperinsulinaemic fed individuals with type 1 diabetes. The

magnitude of this rise, however, was submaximal and comparable to that observed when

a 10-sec sprint is performed immediately before or after a bout of moderate intensity

exercise under hyperinsulinaemic fed conditions (Chapters 3 and 4; Bussau et al., 2006,

2007). Only one other study, published from this laboratory, has investigated the effect

of a 10-sec sprint per se on plasma catecholamines levels in type 1 diabetic individuals

and also reported an increase in the levels of these hormones, but all participants were in

an overnight fasted state and under basal insulinaemic conditions (Fahey et al., 2012).

Interestingly, the rise in catecholamines levels reported here was more pronounced than

those reported by others in non-diabetic individuals subjected to a 6-sec sprint (Moussa

et al., 2003; Botcazou et al., 2007; Bracken et al., 2009), but generally of a lesser

magnitude compared to longer duration sprinting (Brooks et al., 1988; Nevill et al., 1989;

98

Langfort et al., 1997; Zouhal et al., 1998, 2001; Jacob et al., 2002; Moussa et al., 2003;

Vincent et al., 2003; Jacob et al., 2004; Vincent et al., 2004; Zouhal et al., 2009).

The pattern of change in growth hormone levels in response to sprinting in

hyperinsulinaemic type 1 diabetic individuals in the fed state differed in some respects

from that when sprinting is performed immediately before or after a bout of moderate

intensity exercise (Chapters 3 and 4; Bussau et al., 2006, 2007). Indeed, growth

hormones levels remained elevated for longer in response to sprinting alone. Only one

other study has investigated the effect of a 10-sec sprint per se on plasma growth

hormones levels in type 1 diabetic individuals, and showed that the levels of this hormone

increased post-sprinting (Fahey et al., 2012). However, as mentioned earlier, the

participants involved in that study were in an overnight fasted state and under basal

insulinaemic conditions (Fahey et al., 2012) rather than being in a hyperinsulinaemic fed

state. It is noteworthy that sprinting for as little as 6 sec has been found to result in a small

significant increase in growth hormone levels in non-diabetic individuals (Stokes et al.,

2002), with the magnitude of growth hormone response increasing with sprint duration

(Stokes et al., 2002). Other factors are also likely to affect the sprint-mediated increase

in growth hormone levels. For instance, GH secretion increases in response to both high

catecholamines levels and acidosis (Stokes, 2003), but is opposed by elevated plasma

insulin (Lanzi et al., 1997; Frystyk, 2004) and fatty acids levels (Godfrey et al., 2003).

Although sprinting for 10 sec did not change significantly plasma cortisol levels, there

was a trend towards a small increase after exercise. This trend is consistent with earlier

work from our laboratory where sprinting performed immediately before or after a bout

of moderate intensity exercise has been shown to result in a significant but small increase

in cortisol levels in hyperinsulinaemic fed individuals with type 1 diabetes (Chapters 3

and 4; Bussau et al., 2006, 2007). Only one other study has investigated the effect of a

10-sec sprint per se on plasma cortisol levels in type 1 diabetic individuals and shown

that sprinting results in a small increase in plasma cortisol levels after exercise (Fahey et

al., 2012). However, as mentioned earlier, the participants in that study were in an

overnight fasted state and under basal insulinaemic conditions (Fahey et al., 2012) rather

than being in a hyperinsulinaemic fed state.

The absence of any effect of a 10-sec sprint on plasma glucagon levels in

hyperinsulinaemic fed participants with type 1 diabetes is consistent with earlier

99

observations from our laboratory that a 10-sec sprint performed immediately before or

after moderate intensity exercise has no effect on plasma glucagon levels (Chapters 3 and

4; Bussau et al., 2006, 2007). In the absence of any change in plasma insulin levels, our

findings also suggests that the plasma glucagon/insulin ratio is not affected by a sprint of

short duration. The main limitation with this interpretation, however, is that it assumes

that portal glucagon and insulin levels follow patterns of change similar to those of

peripheral blood glucose and insulin levels. The only other study that has examined the

effect of a 10-sec sprint on plasma glucagon levels has also reported no changes in plasma

glucagon levels, but all participants were in an overnight fasted state and under basal

insulinaemic conditions (Fahey et al., 2012). Since catecholamines and insulin stimulate

and inhibit glucagon secretion (Ahren, 2000; Gromada et al., 2007) respectively, maybe

the rise in epinephrine levels was not sufficient to stimulate glucagon release.

The post-exercise rises in plasma lactate and free fatty acids levels in response to sprinting

in hyperinsulinaemic type 1 diabetic individuals in the fed state were similar to those

observed when a 10-sec sprint is performed immediately before or after a bout of

moderate intensity exercise (Chapters 3 and 4; Bussau et al., 2006, 2007). These findings

are also comparable to those obtained in a study examining the effect of a 10-sec sprint

on plasma lactate and free fatty acid levels, but all participants were in an overnight fasted

state and under basal insulinaemic conditions in that study (Fahey et al., 2012). Finally,

others have reported similar plasma lactate levels in response to a 6- or 10-sec sprint in

non-diabetic individuals (Zajac et al., 1999; Moussa et al., 2003). Plasma pH, another

potential glucoregulatory variable with the capacity to affect hepatic glucose production

and muscle glucose transport (Kashiwagura et al., 1985; Kristiansen et al., 1994) fell in

response to the 10-sec sprint, consistent with the decrease in blood pH that occur in non-

diabetic individuals after a sprint lasting 6 sec or more (Allsop et al., 1990; Bogdanis,

1995; Bogdanis and Nevill, 1996; Stokes et al., 2002, 2005).

It is important to note that the comparison made here between the findings of this study

and those obtained in participants were sprinting was performed before or after moderate

intensity exercise (Chapters 3 and 4; Bussau et al., 2006, 2007) should be performed with

caution. Although the work load was well matched between these studies, as indicated by

their similar total work and peak power, and blood glucose levels were falling, it is

important to note that pre-sprint plasma insulin levels were not fully matched at the time

of the sprint. Indeed, the hyperinsulinaemic state of our participants in this study was not

100

as marked as in those other studies. Since the release of growth hormone and glucagon

are opposed by elevated plasma insulin levels (Lanzi et al., 1997; Ahren, 2000; Frystyk,

2004; Gromada et al., 2007), it is possible that a less marked increase in plasma growth

hormone level would have occurred under more pronounced hyperinsulinaemic

conditions.

In conclusion, this study is the first one to examine the counterregulatory responses to a

maximal sprint effort of short duration in fed type 1 diabetic individuals under

hyperinsulinaemic conditions. What remains to be established in future studies is the

extent to which the changes in the levels of each of the counterregulatory hormones

examined here affects blood glucose levels, hepatic glucose production and peripheral

glucose utilisation in response to sprinting. Overall, this study provides the basis for more

research to understand better the mechanisms underlying the clinical benefits of sprinting

in hypoglycaemia prevention.

5.6 Acknowledgements This research was funded by a Juvenile Diabetes Research Foundation/National Health

Medical Research Council of Australia program grant to T. Jones and P.A. Fournier. L.

D. Ferreira was supported by a Juvenile Diabetes Research Foundation International

Fellowship.

101

Chapter 6

General Discussion

102

6.1 General Discussion

Maintaining blood glucose levels within physiological range is a constant challenge for

individuals with type 1 diabetes. As a result of the lack of an acute response of plasma

insulin levels to changes in glycaemia, blood glucose levels are usually managed by

combining daily exogenous insulin administration, carbohydrate intake, and regular

glucose monitoring. Unfortunately, despite the numerous physiological and

psychological health benefits of exercise (Norris et al., 1990; Moy et al., 1993; Laaksonen

et al., 2000; Zinman et al., 2004; Riddell and Iscoe, 2006; Chimen et al.,2012), physical

activity can make the acute management of blood glucose levels even more difficult for

individuals with type 1 diabetes (Riddell and Perkins, 2006; Guelfi et al., 2007c; Younk

and Davis, 2012). In fact, the risk of hypoglycaemia is increased both during (Tuominen

et al., 1995; Riddell et al., 1999; Rabasa-Lhoret et al., 2001) and after exercise

(MacDonald, 1987a; Tsalikian et al., 2005; McMahon et al., 2007; Maran et al., 2010;

Iscoe and Riddell, 2011; Davey et al., 2013). Consequently, it is not surprising that the

biggest barrier to regular physical activity in individuals with type 1 diabetes is fear of

hypoglycaemia (Brazeau et al., 2008). As a result, many people with type 1 diabetes

avoid physical activity (Ludvigsson et al., 1980; Fremion et al., 1987; Guelfi et al., 2007c;

Brazeau et al., 2008) with ~60-65% of individuals with type 1 diabetes found to be

inactive (Thomas et al., 2004; Plotnikoff et al., 2006). It is also concerning that young

individuals are discouraged from performing vigorous physical activity by physicians,

school staff or parents due to their fear of exercise-induced hypoglycaemia (Fremion et

al., 1987). As discussed at length in Chapter 1 of this thesis, not all types of exercise result in an

elevated risk of hypoglycaemia. Aerobic exercise performed at high intensity (>80% of

V O2max) as well as sprinting or intermittent high intensity exercise are all associated with

an increase in glycaemia during and after exercise. This raises the intriguing possibility

that these types of exercise might be beneficial if adopted to counter a fall in glycaemia

in complication-free individuals with type 1 diabetes, and thus might help to prevent or

delay hypoglycaemia if no carbohydrate is readily available. The problem here is that

adopting such a strategy to prevent hypoglycaemia is unlikely to be well tolerated by most

individuals with type 1 diabetes due to the very intense nature and impractical duration

of these types of exercise. This raises the primary aim at the core of this thesis which was

to determine whether a much shorter bout of exercise performed at maximal intensity

103

could be adopted to prevent glycaemia from falling. Since a single maximal intensity

sprint effort lasting 30-sec or more is associated with undesirable physiological

consequences such as nausea, vomiting and dizziness (Inbar et al., 1996; 1998; Laurent

et al., 2007; Stickley et al., 2008; Little et al., 2010), we undertook to explore the

glucoregulatory benefits of a maximal sprint effort lasting only 10 sec. For this reason,

the primary goal of this thesis was to determine whether a 10-sec maximal sprint effort

performed after (Chapter 3) or before (Chapter 4) moderate intensity exercise provides a

possible means other than carbohydrate intake to prevent glycaemia from falling when

exercise is performed under hyperinsulinaemic conditions by complication-free

individuals with type 1 diabetes, thus decreasing acutely their risk of hypoglycaemia.

Also, given that for this type of study, it is common practice to subject participants to a

graded exercise test to determine their V O2peak not only to determine their aerobic fitness,

but also to set exercise intensity relative to V O2peak, a secondary objective of this thesis

was to determine whether the risk of hypoglycaemia is increased early during recovery

from this exercise protocol (Chapter 2). Finally, given that the counterregulatory response

to sprinting has not been examined in hyperinsulinaemic fed individuals with type 1

diabetes, thus making it difficult to compare the findings of Chapters 3 and 4 with the

literature, our last aim was to examine the counterregulatory responses to sprinting in type

1 diabetic individuals under hyperinsulinaemic conditions (Chapter 5).

Given that most laboratory-based studies on exercise in diabetes involve the performance

of graded exercise for V O2peak and lactate threshold determination, the primary aim of the

study described in Chapter 2 was to determine whether the risk of hypoglycaemia

increases in response to graded exercise in individuals with type 1 diabetes. For this study,

eight non-diabetic control male participants and seven complication-free type 1 diabetic

male individuals in good glycaemic control were recruited. On the morning of testing,

the type 1 diabetic participants followed their normal insulin regimen, and both groups

ate their usual breakfast. Then, participants were subjected to graded exercise testing

approximately four hours later. We found that this type of exercise result in a rapid post-

exercise increase in blood glucose levels (> 2 mM), which remain elevated for the first

two hours of recovery. On clinical grounds, these findings suggest for the first time that

the early post-exercise risks of hypoglycaemia associated with graded exercise testing are

minimal when performed under near basal plasma insulin levels, with no carbohydrate

administration required after testing to prevent hypoglycaemia. However, it is important

104

to stress that the risk of late onset post-exercise hypoglycaemia associated with this type

of exercise was not examined and should be investigated in future studies. In addition, it

is our view that health professionals should still regularly monitor glycaemia before,

during and after testing since it remains to be established whether similar findings would

have been obtained with different graded exercise protocols and individuals with different

circulating insulin levels.

The primary goal of the study described in Chapter 3 was to determine whether a short

10-sec maximal sprint effort is preferable to only resting as a means to counter a further

fall in glycaemia during recovery from moderate intensity exercise in hyperinsulinaemic

individuals with type 1 diabetes, thus providing another clinical tool for the prevention of

early post-exercise hypoglycaemia. To meet our objective, seven healthy complication-

free male participants with type 1 diabetes injected their normal insulin dose and ate their

usual breakfast. Then, when their postprandial glycaemia fell to ~11 mM they pedalled

at 40% peak 2OV for 20 min on a cycle ergometer followed immediately by either a

maximal 10-sec sprint or a rest. Our results show that during exercise blood glucose levels

fell rapidly. However, sprinting immediately after exercise opposes a further fall in blood

glucose levels for at least 120 min while glycaemia decreases significantly (p < 0.05) by

~ 3.5mM when no sprint was performed. Our findings also suggest that sprinting is likely

to counter the exercise-mediated decrease in blood glucose levels through an increase in

catecholamine, lactate, and growth hormone levels. These glucoregulatory benefits of

sprinting are remarkable considering the sprint trial was performed when insulin levels

were elevated, a time when exercise is not usually recommended. On the basis of these

findings, one might tentatively recommend that in order to minimise the risk of early

hypoglycaemia post-moderate intensity exercise, it is preferable for complication-free

young individuals with type 1 diabetes to engage in a 10-sec maximal sprint effort before

resting than to only rest during recovery, particularly if a source of dietary carbohydrate

is not readily available. It is important to stress, however, that our findings do not exclude

the possibility that different results might have been obtained if sprinting had been

initiated after a bout of exercise of higher intensity or longer duration, in younger or older

individuals with reduced sprinting capacity, or in individuals with impaired

counterregulatory responses. For these reasons, more studies of the kind described here

are required in order to identify the conditions and subpopulation of individuals with type

1 diabetes for whom a short maximal sprint effort can be recommended as a safe approach

for the short-term stabilisation of blood glucose levels post-exercise.

105

Given the glycaemia stabilising effect of sprinting performed after moderate-intensity

exercise (Chapter 3), this raises the issue examined in the study described in Chapter 4 of

whether performing a short sprint effort immediately prior to, rather than after, moderate-

intensity exercise may offer a novel way of preventing the rapid fall in glycaemia

normally observed both during and after moderate-intensity exercise performed under

hyperinsulinaemic conditions. In other words, this study examined whether short-

duration sprinting should be an integral part of the preparatory routine of individuals with

type 1 diabetes before they engage in sustained physical activities of moderate intensity.

To this end, seven complication-free type 1 diabetic males injected their normal morning

insulin dose and ate their usual breakfast. When post-meal glycaemia fell to ~11 mM,

they were asked to perform a 10 sec all-out sprint (sprint trial) or to rest (control trial)

immediately before cycling at 40% of peak rate of oxygen consumption for 20 min. We

found, against expectations, that sprinting for 10 sec immediately before moderate-

intensity exercise performed under hyperinsulinaemic conditions does not affect the rapid

decline in glycaemia during exercise. However, sprinting rather than resting before

moderate intensity exercise did prevent glycaemia from falling for at least the first 45 min

of recovery in individuals with type 1 diabetes. This suggests that including a short sprint

as part of the warm-up routine of individuals with type 1 diabetes before they engage in

sustained moderate-intensity exercise might provide another means of temporarily

stabilising glycaemia during early recovery. However, before recommending such a use

of sprinting, a number of issues must be addressed such as the extent to which the efficacy

of prior sprinting is affected by the intensity and duration of the subsequent bout of

exercise and the identification of any subpopulations of type 1 diabetic individuals

unlikely to benefit from sprinting.

In our attempt to identify the endocrine mechanisms mediating the glucoregulatory

benefits of sprinting, the levels of several counterregulatory hormones were measured in

the studies described in Chapters 3 and 4. Unfortunately, one difficulty with comparing

these findings with the literature has been the lack of information on the effect of short-

duration sprinting per se on the responses of counterregulatory hormones in diabetic

individuals in a hyperinsulinaemic fed state. For this reason, the purpose of the study

described in Chapter 5 was to investigate the effect of a single 10-sec sprint on the levels

of the counterregulatory hormones in type 1 diabetic individuals under hyperinsulinaemic

fed conditions approaching those reported in Studies 3 and 4. In this final study, we found

106

that performing a 10-sec maximal sprint resulted in patterns of change in plasma

catecholamines, growth hormone, cortisol and glucagon levels comparable to those

observed when a sprint is performed immediately after a bout of moderate intensity

exercise in individuals with type 1 diabetes (Study 2) and also comparable to those

observed in response to a sprint performed after an overnight fast (Fahey et al., 2012).

What remains to be established in future studies is the extent to which the changes in the

levels of each of the counterregulatory hormones examined here affects blood glucose

levels, hepatic glucose production and peripheral glucose utilisation in response to

sprinting. Overall, this final study provides the basis for more research to understand

better the mechanisms underlying the clinical benefits of sprinting in hypoglycaemia

prevention.

6.2 Clinical Implications, Limitations with our Findings and Direction for Future

Studies

The counter-intuitive findings described in this thesis that the risk of hypoglycaemia

associated with exercise can be decreased by sprinting before or after moderate intensity

exercise have the potential to cause a significant shift in the way blood glucose levels are

managed in active individuals with type 1 diabetes. Some authors have mentioned the

potential of incorporating sprints to help oppose an exercise-mediated decline in

glycaemia in young healthy adults with type 1 diabetes (Tonoli et al., 2012; Canadian

Diabetes Association Clinical Practice Guidelines Expert Committee, 2013; Robertson et

al., 2014; Yardley & Sigal, 2015). It is important to stress, however, that before

recommending the general adoption of short-duration sprinting as a safe and reliable

clinical tool for the short-term stabilisation of blood glucose levels in exercising type 1

diabetic individuals, a number of issues must be addressed as in our view it would be

premature and irresponsible at this stage to advocate its widespread adoption. Indeed, it

is of paramount importance to determine whether there are conditions likely to impair the

efficacy of sprinting in hypoglycaemia prevention. One must remember that the findings

arising from this thesis are based on data obtained from a very specific population of

healthy complication-free young adults with type 1 diabetes, and for this reason may not

apply to the wider population of individuals with type 1 diabetes. For instance, it would

be ill advised to recommend the use of a short sprint as a means to reduce the risk of

exercise-mediated hypoglycaemia for individuals with an impaired capacity to engage in

a maximal sprint effort, such as very young children or old sedentary individuals with or

107

without pre-existing medical conditions, or for diabetic individuals with advanced

neuropathy, or for individuals with diabetic complications for whom intense exercise is

contra-indicated. But what about the large population of complication-free type 1 diabetic

individuals who have the capacity to engage in an all-out sprint effort?

Even with the population of type 1 diabetic individuals who would be responsive to the

benefits of sprinting, a number of important issues must also be addressed before

recommending the adoption of short-duration sprint as a safe and reliable clinical tool for

the short-term management of blood glucose levels. For instance, it remains to be

determined whether our findings obtained on a cycle ergometer in a well-controlled

laboratory environment would extend to all modes of all-out sprinting including running

and swimming. It is also unclear whether performing an ‘all out’ sprint and reaching

maximal power output early in the sprint is important as opposed to ‘pacing’ for the

duration of the sprint. The effect of the fitness and training status of the participants on

the glucoregulatory response to sprinting also remains to be investigated. Since the rate

of fall in glycaemia during moderate intensity exercise increases with both exercise

intensity and plasma insulin levels, it is possible that the efficacy of sprinting at

preventing hypoglycaemia post-moderate intensity exercise might be reduced in response

to more intense aerobic exercise or more severe hyperinsulinaemic conditions than those

tested in this thesis. Given our findings that catecholamines and growth hormones might

mediate the effect of sprinting on glycaemia (Chapters 3, 4), this raises the possibility that

a defect in the secretion of these hormones could impair the ability of a sprint to counter

glycaemia in exercising individuals. Moreover, it remains to be established whether the

glucoregulatory benefits of sprinting increase with one’s maximal power output since the

catecholamine response to exercise increases with exercise intensity. Finally, it would be

interesting to investigate the effect of gender and phase of menstrual cycle on the

glucoregulatory responses to a single 10-sec sprint.

Several questions related to the counterregulatory status of diabetic individuals prior to

exercise must also be answered before recommending the adoption of short duration

sprinting as a safe and reliable tool for the short-term management of blood glucose

levels. For instance, the depletion of hepatic glycogen stores by a prolonged fast, and

associated changes in counterregulatory hormone levels prior to exercise might impair

the glucoregulatory benefit of sprinting. However, this prediction is not supported by the

finding that intense exercise results in comparable hyperglycaemic effect in fed and

108

fasted non-diabetic individuals (Lavoie et al., 1987). Given that the counterregulatory

response to exercise is reduced following antecedent hypoglycaemia (Galassetti et al.,

2003, 2004) or antecedent exercise performed hours earlier (Galassetti et al., 2001a), this

raises the intriguing possibility that the efficacy of sprinting as a means to prevent

glycaemia from falling post-exercise might be impaired if preceded hours earlier by

either an episode of hypoglycaemia or exercise. Such a potential detrimental effect of

antecedent hypoglycaemia on the glycaemia-stabilising effect of sprinting might also be

more pronounced in men than in women given that antecedent hypoglycaemia has been

reported to result in a relatively greater fall in the responses of counterregulatory

hormones (catecholamines, growth hormone, glucagon) and glucose production in men

compared with women in response to a subsequent prolonged bout of moderate intensity

exercise (Galassetti et al., 2004). However, the possibility that antecedent hypoglycaemia

impairs the benefit of sprinting is not supported by a study recently published from this

laboratory where a one-hour hypoglycaemia period was reported to have no effect on the

glycaemia-rising effect and counterregulatory response of a sprint performed hours later

(Davey et al., 2014).

Finally, another important clinical issue to address is whether sprinting impairs the

counterregulatory responses to a subsequent hypoglycaemia episode, thus increasing the

risk of late onset hypoglycaemia. That this might be an important issue is suggested by

the work of Galassetti and colleagues who showed that the rise in catecholamines, growth

hormone and cortisol levels in response to hypoglycaemia is impaired if preceded several

hours earlier by a bout of moderate intensity exercise, with more glucose being required

to prevent hypoglycaemia (Galassetti et al., 2001b; Sandoval et al., 2004). In addition,

this impairment is more pronounced in male compared to female participants. However,

given the short duration of the sprint investigated in this thesis and the small amount of

carbohydrate expected to be mobilised by during such a sprint, we predict that the effect

of sprinting on the counterregulatory responses to a subsequent episode of

hypoglycaemia should be minimal. This prediction is supported by recent findings from

this laboratory that a 10-sec sprint does not affect the glucose demands to maintain blood

glucose at stable levels for several hours post-exercise (Davey et al., 2013b). Overall, it is important to answer the aforementioned questions before recommending

the adoption of short duration sprinting as a safe and reliable tool for the short-term

management of blood glucose levels in individuals with type 1 diabetes as this clinical

109

tool has the potential to be integral to future glucose management guidelines aimed at

helping diabetic individuals exercise more safely and take advantage of the many

physiological and psychological health benefits of a physically active lifestyle.

110

Chapter 7

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