Lecture 3 Carbohydrates in Exercise and Recovery

NDD 30503

LECTURE 3Carbohydrates in Exercise and

RecoveryDR. SHARIFAH WAJIHAH WAFA BTE SST

WAFA & HH04THURSDAY (22 SEPTEMBER 2016), 2:30-

4:30PM

Introduction

• Carbohydrate as a Fuel for Exercise:– Well-documented that CHO is important for

athletic performance– High levels of stored glycogen before endurance

exercise (esp. > 1hr) can help increase performance & reduce time to fatigue

– High CHO post-exercise enhances recovery– Many athletes consume inadequate levels of CHO

to support their training

Dietary Carbohydrate

• Optimum dietary CHO levels depend on:– Total energy intake– Body size– Health status– Duration, intensity, frequency,

and type of exercise

Function, Classification, and Dietary Sources of Carbohydrate

Function of Carbohydrates

• CHO are:– Primary source of energy (1 of 3 macronutrients)– Provide the substrate necessary for glycogen replacement

(substrate: glucose)– When consumed during exercise, help maintain BG levels &

help prevent premature fatigue

• CHO recommendations for active individuals:– Moderate training: 5-7 g/kg of BW– Heavy training: up to 10 g/kg of BW (Burke, 2007)

Classification of Dietary CHO

• Different ways to classify CHO– Type of CHO found in the food– Level of commercial processing the food has

undergone– BG or glycemic response to the CHO within the

body

Structural Classification of CHO

• Complex carbohydrates: long complex chains of sugars linked together– Initially believed that all complex CHO were

digested more slowly than simple CHO– The term ‘complex carbohydrate’ only refers to

the structure of the CHO, not to any digestive properties

Food Examples of Complex CHO

• Nutritionists / dietitians generally consider the following foods “complex CHOs” because they are good sources of vitamins, minerals, and fibre– Vegetables & fruit– Whole grains (breads, cereals, pasta) – Legumes (beans, peas, lentils)– Primarily contain: starch and fibre



• Simple carbohydrates: primarily refer to processed foods or foods high in sugar– E.g. sweetener cereals, breakfast bars, candy,

regular pop, desserts– Are generally low in vitamins, minerals, and

fibre unless they are fortified– Primarily contain: mono-, di-, and oligo-

saccharides (glucose, sucrose, fructose, and high-fructose corn syrup)

Primary CHOs & Sugar in the Diet

• Monosaccharides: simplest form of sugar

– Glucose: main CHO in the bloodstream• Main energy source in the cell• Stored in the liver, muscles, and other organs as glycogen• Rapidly absorbed from the gut through sodium-dependent

glucose transporter– Fructose: simple sugar found in honey & fruit

• Tastes sweeter than table sugar (sucrose)• Absorbed from the gut through the glucose transporter 5

(GLUT5) and must be transported to the liver for conversion to glucose

– Galactose: simple sugar found in milk


• Disaccharides: made up of 2 simple sugars

– Sucrose: glucose + fructose• Common table sugar, extracted from sugar cane and beet sugar• Most common dietary disaccharide• Broken down into glucose and fructose in the gut prior to absorption

– Lactose: glucose + galactose• Sugar found in milk products• Lactose intolerant (lacking the lactase enzyme), common in Asians,

Native Americans, Hispanics, and blacks– Maltose: glucose + glucose

• Primarily formed from the breakdown of starch• Rapidly digested to glucose and absorbed quickly into the body


• Oligosaccharides: short chains of 3 to 10 monosaccharides linked together– Maltodextrin:

• Glucose polymer manufactured as long starch units are broken into smaller groups

• Sugar found in sports drinks and many processed foods• Rapidly digested to glucose and quickly absorbed

– Corn syrup:• Sweet syrup made up of glucose and short-chain

glucose polymers produced by enzymatic hydrolysisof corn starch

• Rapidly digested and absorbed


• Oligosaccharides: short chains of 3 to 10 monosaccharides linked together– High-fructose corn syrup:• Especially sweet corn syrup• 45% to 55% of the CHO is enzymatically hydrolyzed to

glucose and fructose (has nearly 2x the concentration of mono- and disaccharides found in regular corn syrup• Predominant sweetener found in commercially

sweetened foods

What Is High-Fructose Corn Syrup?

• Cornstarch is converted to corn syrup that is essentially 100% glucose

• Enzymes and processing techniques convert some of the glucose to fructose to achieve corn syrup that is 55% fructose (HFCS-55)

• HFCS-55 is the type of corn syrup used mainly in the beverage industry – Syrup is 55% fructose, 45%

glucose– Similar to sucrose (table sugar;

50% fructose, 50% glucose)

Primary CHOs & Sugar in the Diet• Polysaccharides: contain starch and fibre (“complex

carbohydrates”)– Starch: found in plants, seeds, and roots

• Made up of straight chains of glucose polymers called amylose and some branching chain polymers called amylopectin

• Starch is digested into glucose• Starches high in amylopectin are more rapidly digested and absorbed

than starches high in amylase– Dietary fibre: part of the plant that cannot be digested by

human gut enzymes• Goes from the small intestine into the colon, where it is expelled as

fecal material or fermented and used by gut bacteria as food• Soluble vs. insoluble fibre

Glycemic Response to Carbohydrates

Glycemic Response

• Glycemic response:– Classify foods as producing a high, moderate, or

low glycemic response– Glycemic response to both simple and complex

CHO foods can vary greatly– Some complex CHO (i.e. high in starch) can be

hydrolyzed and absorbed as quickly as simple sugars

Glycemic Response• High glycemic response: – Foods that produce a

large and rapid rise in blood glucose and insulin

– Can increase muscle glycogen more than foods that produce a low glycemic response

Glycemic Response

• Glycemic index (GI): scale that ranks CHO-rich foods by how much they raise blood glucose levels compared to a standard food– Determined by feeding 50 g of a particular food and

watching the blood glucose response over a 2 hr period– BG response is compared to a reference food (usually

white bread or glucose), with a GI = 100

GI = BG area of test food x 100 BG area of reference food

Glycemic Response

• Glycemic load (GL): accounts for both the amount and source of CHO in a meal– GL = (GI of a food or meal) x (g of available CHO

in the food or meal)– Example: Carrots (peeled, boiled) have a GI of 47

and 5 g CHO per serving

The GL of carrots is: (47 × 5) ÷ 100 = 2.4

Video: GI vs. GL

http://www.youtube.com/watch?v=Ea5vcpjOzxw

Carbohydrate Metabolism During Exercise

Carbohydrate as a Fuel Source

• Muscles use of CHO during exercise:– Amount of CHO required depends on the:

• frequency, intensity, duration, and type of exercise• environmental conditions

– CHO used during exercise comes from the following sources:• Endogenous production of glucose by the liver

(gluconeogenesis)• Blood glucose• Muscle and liver glycogen stores• CHO consumed during exercise (exogenous CHO)

Figure 2.1Crossover concept of fuel use

during exercise:– Low-to-moderate intensity:

CHO + lipids play major roles as energy substrates

– Higher intensity (relative aerobic power = 60-65%): CHO becomes increasingly important

– Lipids become important energy sources during recovery

Gluconeogenesis

• Gluconeogenesis: endogenous glucose production– Metabolic pathway that results in the generation of

glucose from non-carbohydrate carbon substrates – One of the main mechanisms humans use to keep BG

levels from dropping too low (hypoglycemia)– Main substrates during exercise: lactate, alanine,

glycerol, pyruvate• Primarily come from the muscle• Small amounts of glycerol come from adipose tissue• Are transported to the liver for glucose production

Figure 2.3 Gluconeogenesis Pathway

Gluconeogenesis

• Amount of gluconeogenesis that occurs during exercise is impacted by:– Available CHO reserves prior to exercise initiation– Amount of CHO provided during exercise– Type, duration, and intensity of the exercise bout– Exercise environment (e.g. temperature, altitude)– Level of endurance training

Gluconeogenesis Substrates• Lactate:– Primary source of lactate during exercise is from the

metabolism of glucose to lactate (through gylcolysis)– Lactate is transporated to the liver for glucose production

through the Cori cycle, or it may be used directly by adjoining cells as an energy source

– As glycogen is depleted in the working muscles, non-working muscles can give up some of their stored CHO by releasing lactate• Ahlborg & Felig (1982) showed that lactate released from the

arms increased both during and after 3-3.5 hr of leg exercise (cycling)

Figure 2.6 Cori Cycle

Gluconeogenesis Substrates

• Alanine:– Primary amino acid released by working muscles during

exercise– Alanine is synthesized as nitrogen (released from the

breakdown of aa in the muscles) and is combined with pyruvate

– Alanine is transported to the liver, where it is broken down into pyruvate and nitrogen• Pyruvate can be used as a gluconeogenic substrate• Nitrogen is converted into urea and eliminated through the

kidneys– This pathway is called the glucose-alanine cycle

Figure 2.7 Glucose-Alanine Cycle


• Glycerol:– Is the 3-carbon backbone

of a triglyceride– Adipose tissue or muscle

triglycerides can be brokendown to yield 3 FAs and glycerol

– FAs transported to the muscles for energy production

– Glycerol transported to the liver for gluconeogenesis


• Pyruvate:– Final substrate used for gluconeogenesis– 3-carbon compound– Can leak from working cells into the blood and is

transported to the liver to make glucose

Glycogenolysis

• Glycogenolysis: the chemical process by which glucose is freed from glycogen– Liver glycogenolysis: Another source of BG during exercise

is the breakdown of liver glycogen– Glucose from the liver can be released directly into the

bloodstream helping to maintain BG levels during exercise (unlike muscle glycogen)

– Liver glycogen can be depleted if exercise is strenuous and of long duration• Gluconeogenesis and consuming exogenous CHO (e.g. sports

drinks, gels) become increasingly important to maintain BG levels

Hormonal Control of Carbohydrate Metabolism During Exercise

Hormones & Exercise

• Hormonal changes:– Signal the body to break down stored energy for

fuel, which can then be used by the working muscles for energy

– Hormonal responses depend on 2 main factors:• Intensity and duration of the exercise• Individual’s level of physical fitness

Hormones & Exercise

• Norepinephrine & Epinephrine:– Blood levels rise dramatically within minutes of the

initiation of exercise– Stimulate the breakdown of stored fat (both adipose &

muscle tissue) and CHO (both liver & muscle glycogen), making these fuels available to the working muscles

• Insulin:– Levels decrease or are maintained at a low

concentration during exercise• Acute & chronic exercise increases the sensitivity of the

skeletal muscle to the action of insulin

Hormones & Exercise

• Glucagon:– Released from the pancreas in response to the low BG

levels that may occur with exercise– Potent stimulator of glycogenolysis and

gluconeogenesis– Helps to maintain BG levels by increasing the release of

glucose into the bloodstream• Cortisol:– Also stimulated gluconeogenesis and helps to mobilize

free FAs and amino acids

Carbohydrate Reserves and Dietary Intake

Carbohydrate Reserves• Primary fuel sources during exercise:– Carbohydrate (glucose) & fat (fatty acids)– Relative amounts used depend on the exercise intensity

and duration• CHO reserves:– Compared to fat & protein, the body’s CHO reserves are

severely limited– Total amount of energy stored as glycogen ranges from

800-2000 kcal• Depends on the diet, size of athlete, fitness level, and time of day• CHO consumed during exercise can supplement these reserves

Total Body Glycogen Reserves

• Total CHO storage:– Total glycogen found in the liver, muscle, and other

organs is not much greater than the amount of CHO consumed on average each day

– 2000 kcal/day 50% of kcal from CHO offers ~250 g of CHO

– After a typical meal, approximately 25-33% of CHO consumed is converted to liver glycogen; about 33-50% is converted to muscle glycogen; and the remainder is oxidized for energy in the hours after eating

Liver Glycogen

• CHO Reserves:– Primarily liver and muscle glycogen– Glycogen concentrations are highest in the liver

• Amount in a typical liver weighing 1.5 kg after an overnight fast is ~4% of the liver’s total weight, or 60 g

• After a meal, amount of glycogen can double to ~8% of the liver’s weight, or 120 g glycogen

• Liver glycogen plays a major role in maintaining BG levels throughout the night– morning meal containing CHO is important to replenish

glycogen stores

Muscle Glycogen

• Storage of glycogen in the muscle:– Lower than that in the liver– Deliberate CHO loading is required to increase the

amount to more than 2% of fresh weight of rested muscle (~400 g)

– Absolute amount of glycogen stored in the muscle can range from ~300 to 400 g (1200 - 1600 kcal) in a 70 kg athlete

Muscle Glycogen

• Use of muscle glycogen during exercise:– Depends on amount of glycogen available before

exercise begins– Exercise intensity and duration– Environmental conditions– Whether or not exogenous CHO is consumed

Dietary Carbohydrate Intakes of Active Individuals

• Dietary intake of CHO:– Active men & women usually report CHO intakes similar to

weight-matched inactive individuals– 45-55% of total energy from CHO or ~5-6 g/kg BW per day– Appropriate for recreational athletes who exercise for 1 hr

or less per day– May be too low for endurance athletes who engage in

daily intense training and whose glycogen stores need to be replenished rapidly• May require up to 10 g CHO/kg BW for men and

6-8 g CHO/kg BW for women

Carbohydrate Feeding Before Exercise

Pre-Exercise & Between-Competition Meals

• Goals of pre-exercise meal:– Promote additional glycogen synthesis– Supply the body with glucose for use during exercise– Minimize fatigue during exercise– Replenish liver glycogen, especially after an over-night

fast

• Timing: pre-exercise meal usually consumed 2-4 hours prior to the exercise event– Often can be safely eaten at late as 1 hour before exercise


• Pre-exercise meal should be:– Small, easy to digest– Familiar to the individual– Contain foods that do not cause gastrointestinal distress

(e.g. fibre, fat, carbonation)– Provide CHO to improve glycogen reserves and BG

• Glycemic index:– Low GI foods may offer better satiety and produce more

stable BG concentrations than high GI meals


• Nerves & appetite:– Nervousness before an exercise event can cause GI distress

and loss of appetite– Can use fruit juices, sport drinks, or glycogen replacement

products to provide the energy and CHO needed

• Multiple exercise bouts within a 24 hr period:– What to eat depends on athlete’s preferences, type of

event, and amount of time between exercise– If time is short, water, fruit juices, or sport drinks are most

appropriate (CHO can be rapidly absorbed)

Effects of Pre-Exercise Feeding on Performance and Fatigue

• A high-CHO pre-exercise meal 3-4 hours before exercise can improve performance– If this meal is then combined with CHO intake

during exercise (e.g. sport drink), the performance improvements are even greater (Wright, 1991)

– May be especially helpful for those who:• pay little attention to their diet • or who have had a poor diet during the 24 hour period

before an exercise event

Carbohydrate Consumption Immediately Before Exercise

• Controversy:– Does CHO eaten immediately before exercise cause

hypoglycemia during exercise?– Hypothesis: the high blood insulin levels resulting from

CHO consumption immediately before exercise (~30-60 min) may cause a decline in BG (hypoglycemia) at the onset of exercise, leading to premature fatigue

– Jeukendrup and colleagues have done a series of systemic studies in male cyclists to examine rebound hypoglycemia

SCHOOL OF NUTRITION AND DIETETICS . FACULTY OF HEALTH SCIENCES

Pre-Competition Meal (continued)

• Medium amount of energy – 300 to 500 kilocalories, more if there is time to digest before the event

• 2 to 3 hours before event (perhaps 1 hour with liquid meal) – Ingestion of carbohydrate 1 hour before exercise does not usually

impair performance– Depends on individual tolerance Liquid meals are popular for gastrointestinal comfort during the event

• GI of pre-event carbohydrates– No solid evidence for this, experiment in practice

• Include 1 to 2 cups of fluid• Avoid foods with a high fat content and/or excess fiber


Examples of Pre-Competition Meals

• Option 1, liquid meal (blend all ingredients)1

– 1 cup of vanilla yogurt– 4 to 6 peach halves, canned or fresh– 4 graham cracker squares– Dash nutmeg, optional Meal provides 450 kcal, 75% CHO, 15% protein, and 10% fat

• Option 2Food Kcal Carbohydrate, g Protein, g Fat, gOatmeal, instant 1 pkt 104 18 4 2

Skim milk, ½ cup 43 6 4 0

Toast, 2 slices 146 24 4 2

Banana, 1 medium 105 27 0 1

Orange juice, 6 oz 84 20 1 0

TOTAL 482 95 (80%) 13 (11%) 5 (9%)Abbreviations: CHO, carbohydrate; pkt, packet.

1. Clark N. Sports Nutrition Guidebook. Champaign, IL: Human Kinetics; 1989.

Carbohydrate Feeding During Exercise

Fatigue During Exercise

• Individuals fatigue during moderate exercise (60-80% of VO2 max) of long duration (>90 min) in part due to a decrease in BG and depletion of muscle and liver glycogen stores

• Exogenous CHO consumed during exercise may reduce fatigue and improve performance

Fatigue During Exercise

• Exogenous CHO feeding during exercise:– Spares muscle glycogen and oxidation of CHO– Spares synthesizing glycogen during low intensity

exercise– Provides CHO which has a direct effect on the

brain

Figure 2.9

CHO Feeding During Exercise Prevents Hypoglycemia

• For some individuals, exhaustive exercise (60-75% VO2 max for 2.5-3.5 hr) without exogenous CHO intake can result in hypoglycemia– Hypoglycemia = BG < 2.5 mmol/L– Symptoms: light-headedness, dizziness, inability to

concentrate, nausea, irritability, and fatigue– Hypoglycemia leads to a decline in total body

glucose oxidation and eventually to exhaustion• once BG dropped to 2.5-3.0 mmol/L, exhaustion occurred

and subjects could no longer exercise

CHO Feeding During Exercise Improves Performance & Reduces Fatigue

• Feeding CHO during prolonged exercise:– Improves performance– Lengthens the time an athlete can exercise before

becoming fatigued• Early research by Coyle et. al (1986)– Measured plasma glucose & muscle glycogen in 7

trained cyclists exercising at 70-75% VO2 max to fatigue

– 2 sessions: (1) with exogenous CHO, (2) one without


• CHO feeding during shorter (< 1 hr), more intense exercise sessions:– CHO feeding during exercise >75% of VO2 max (~1 hr) can

also improve performance– Below and Coyle, 1995: Male cyclists consumed a CHO drink

(with 78 g CHO) during 1 hr of high-intensity exercise (80-90% VO2 max) increased their mean exercise intensity by 6.3% compared to water only

– Davis et. al, 1997: no differences between genders– Unclear of the benefits of CHO feeding during exercise if

exercise is <1 hr and is of high intensity


• Recreational marathon running:– Real-world example of CHO feeding during exercise– Utter et. al, 2002: CHO feeding during the marathon

reduced marathon running times (~16 min shorter) and significantly decreased rate of perceived exertion during the last 10 km of the race compared to a placebo drink

Timing & Rate of Carbohydrate Feeding During Exercise

• CHO ingestion should generally begin early in an exercise event to ensure that adequate CHO is available during the later stages of exercise– Coggan and Coyle, 1987: the latest an individual

can consume CHO and still prevent fatigue is 30 min before the onset of fatigue

Timing & Rate of Carbohydrate Feeding During Exercise

• Current research studies typically provide CHO solutions (5-8%) at regular intervals, usually every 15-30 min– Most studies feed between 40 and 75 g of CHO

per hour and observe performance benefits– This provides ~ 1 g CHO per minute– Any sport drink containing at least 6% to 8% CHO

would provide 60 to 80 g of CHO per litre

Type of Carbohydrate

• What type of CHO should be consumed during exercise?

• Does one type of CHO absorb more quickly than another?


• CHO types:– All simple sugars (i.e. glucose, fructose, sucrose, and

maltodextrin) are absorbed rapidly from the gut– These CHO sources are equally effective in maintaining BG

levels during exercise– Glucose can be used to maintain BG levels immediately,

while fructose must 1st be converted to glucose in the liver– Using a combination of sugars increases the ability of

various transport mechanisms to be utilized in the gut, thus increasing absorption and subsequent oxidation of these sugars

Type of Carbohydrate in Sports Drinks

• Sport drinks– Use a combination of sugars– Absorbed, transported, and oxidized more quickly during

exercise than a single CHO source– Maltodextrins: frequently added to drinks/gels

because they are less sweet than glucose or sucrose, permitting a higher CHO concentration without making the product unbearably sweet

– Sport drinks on the market:• Combo of glucose, sucrose, fructose, maltodextrin • 6-8% CHO plus sodium are generally well-absorbed


• Fructose:– Fructose (usually in the form of HFCS) is absorbed

more slowly from the gut than glucose– Absorbed through facilitated diffusion vs. active

transport– Large doses of fructose can overload the absorption

capabilities of the gut and cause GI distress (i.e. cramping, diarrhea)

– Once absorbed, fructose is transported to the liver, where it is converted into glucose

Figure 2.13a


• Fructose:– Feeding of fructose in combination with other

sugars increases oxidation of CHO– Not great for the replacement of muscle glycogen

Solid vs. Liquid Carbohydrates

• Glycemic response of solid vs. liquid CHO:– Solid CHO (e.g. energy or sport bars, whole fruit)– Liquid CHO (e.g. sport drink, blended fruit)– Murdoch et. al, 1993; Jeukendrup, 2004: similar

amounts of CHO (solid vs. liquid)• Found no significant difference in BG levels during

exercise• Produced similar BG and insulin responses• Form of CHO consumed during exercise is a matter of

availability and personal preference

Practical Guidelines for Carbohydrate Intake During Exercise


• Test during training: Athletes should use the CHO supplement during training that they will use during competition.

• Ingest CHO early: Athletes should ingest CHO early in an exercise session to prevent the decrease in BG often seen during endurance events.

• Sports drinks: Should have a concentration of 6-8% CHO (60-80 g per 1 L) and contain Na.


• Drink enough fluid: Athletes should drink enough fluid to provide 40 and 75 g of CHO per hour. Long duration exercise events or extreme temperatures may require higher fluid and CHO intakes.

• CHO intake based on BW: Determine CHO intake during exercise based on 1-1.2 g CHO/kg/min.


Putting Together a Meal Plan

• Example: – 70-kg athlete requiring 4000 kcal/day and exercising 120 min/day 4

to 6 times/week• Macronutrient Target Recommendations

– Grams/kg (body weight)/day• Carbohydrate 7 to 10 g/kg/day (490 to 700 g/day)• Protein 1.5 to 2.0 g/kg/day (105 to 140 g/day)• Fat Typically use percentage of energy as method– Percentage of energy• Fat 20% to 35% of energy (88 to 156 g/day)– Target recommendations for this athlete• Carbohydrate 600 g (60% of energy)• Protein 130 g (13% of energy)• Fat 120 g (27% of energy)


A Potential Distribution of Macronutrients Over the Course of 6 Meals/Day

Meal Time Carbohydrate, g Protein, g Fat, gBreakfast 7:00 AM 90 15 15

Mid-AM snack 10:00 AM 25 10 5

Lunch Noon 75 20 20

Pre-Ex meal 1:30 - 2:00 PM 90 10 5

During Ex 3:00 - 5:00 PM 100 0 0

Post-Ex meal 5:00 PM 75 30 25

Dinner 6:30 PM 120 30 35

PM snack 9:00 PM 25 15 15

TOTALS 600 130 120

Carbohydrate Feeding Post-Exercise & During Training Periods

Carbohydrate Post-Exercise

• Post-exercise feeding:– Need to replenish muscle glycogen and refuel the

body for the next exercise event– Should provide the energy and nutrients to repair

and strengthen muscle tissue that may have been damaged during exercise

– Should provide fluids to rehydrate the body

Glycogen Synthesis Post-Exercise

• Glycogen depletion:– Can occur after 2-3 hours of continuous exercise

performed at 60-80% of VO2 max, or after high-intensity exercise (90-130% of VO2 max) that occurs intermittently over a shorter time (15-60 min)

– After exercise, the majority of glucose for glycogen synthesis comes from oral glucose ingestion

• Rate of muscle glycogen replacement:– Post-exercise ranges from 20-50 mmol/kg of dry muscle

per hour when a CHO supplement is provided post-exercise


• Factors that determine rate of glycogen synthesis:– Degree of muscle glycogen depletion– Degree of insulin activation of glycogen synthase– CHO content of the post-exercise diet

• Rapid vs. slow glycogen resynthesis:– Rapid phase: ~30-60 min post-exercise– Slow phase: lasts for several hours


• Protein & CHO:– Combining some protein

or aa’s with CHO post-exercise can lead to higher muscle glycogen synthesis versus the same amount of CHO without the additional protein

Figure 2.17a

High-CHO Diets During Training Improve Performance & Power Output

• Replacement of glycogen after exercise is important, esp. during periods of high training or endurance exercise– Do higher levels of glycogen always translate into increased

exercise performance?– Simonsen, 1991: high-CHO (10 g/kg BW, 70% of kcal) vs.

moderate-CHO (5 g/kg BW, 42% of kcal)• Male & female rowers randomly assigned 2 different diets• 4 weeks of intense twice-a-day rowing exercise• Mean power output in the TTs increased by 10.7% in the high-CHO

group, but only by 1.6% in the moderate-CHO group after 4 weeks of intense training

High-CHO Diets During Training Improve Performance & Power Output• Not all studies show improved exercise

performance with increased dietary CHO and improved glycogen stores– # of factors affect performance, and level of stored

glycogen is just one factor– When muscle glycogen levels are adequate,

increasing levels above normal may not increase exercise performance unless athletes are performing exercise that is strenuous enough to deplete muscle glycogen

Type & Amount of Carbohydrate

• Glucose, sucrose, maltodextrins, and starch all appear to replace muscle glycogen equally well– Fructose did not replace muscle glycogen nearly as well

as the other sugars

• 4 large meals (“gorging”) vs. 16 frequent small meals (“nibbling”) post-exercise– 10 g CHO / kg BW– No statistically significant difference between the groups

in muscle glycogen storage over a 24 hr period

Solid vs. Liquid CHO Post-Exercise

• Solid vs. liquid CHO post-exercise:– When solid and liquid CHOs are fed at the same

rate, muscle glycogen synthesis rates appear to be similar

– If quick glycogen replacement is needed, then the post-exercise CHO fed, regardless of the form, should have a high glycemic index and be adequate in amount (~ 1-1.2 g of CHO per kg BW / hour)

Glycogen Replacement Using High Glycemic Index Foods

• Hypothesis: Does feeding high GI foods post-exercise produce a greater increase in muscle glycogen storage than low GI foods, even if CHO content is held constant?– Tested by Burke (1993): 5 well-trained cyclists consumed

a low GI diet for the 1st TT and a high GI diet for the second TT

– Both diets provided 10 g CHO / kg BW, similar in kcal– Muscle glycogen content 24 hours after recovery was

significantly greater with the high GI diet

Timing & Rate of Post-Exercise Carbohydrate Feedings

• Timing and rate of CHO consumption after exercise can influence the amount of glycogen stores.– Glycogen synthesis rates are highest immediately after

exercise when the muscle is depleted and glycogen synthase activation is high

– Goal: get CHO into the system quickly (2 hr after exercise)– Athletes given a high-CHO replacement drink

immediately after exercise improved their time to exhaustion on the following day of exercise by 11% (Baker, 1994)

Timing & Rate of Post-Exercise Carbohydrate Feedings

• Timing and rate of CHO consumption after exercise can influence the amount of glycogen stores.– Katz, 1988: found that a 2 hour delay in feeding CHO

after exercise reduced the rate of glycogen synthesis by 47% compared with feeding CHO immediately after exercise

– When CHO is consumed frequently (every 15 min) compared to less frequently (every 1-2 hours), a higher insulin response is observed• Insulin stimulates the uptake of glucose by the cells for

glycogen storage and stimulates glycogen synthase

Determining Overall Carbohydrate Intake for Individuals

Carbohydrate Recommendations

• AMDR: 45-65% of kcal from CHO– Most athletes consume diets that contain 55-65%

of energy from CHO• CHO recommendations based on grams of

CHO per kg BW versus % of total energy– Works better in low-kcal situations– Recreational athletes: 5-7 g CHO / kg BW– Competitive Athletes: 10 g CHO / kg BW– Minimum of 3 g CHO / kg BW

Practical Guidelines for Feeding Carbohydrate Post-Exercise and During Training Periods

Practical Guidelines for Feeding Carbohydrate Post-Exercise

• Recommendations assume the athlete is in training or competition and thus requires maximum glycogen replacement.– Applicable to athletes frequently training 2x/day

for a total of 12 to 20 hours/week– Less stringent CHO recommendations are

appropriate for recreational athletes who exercise only 4 to 10 hours/week


• CHO per kg BW: If exercise is to occur again within less than 6 to 8 hours, feed approximately 1 to 1.2 g CHO / kg BW immediately after exercise and every 30-60 min for the first 5 hours after exercise. Combine with some dietary protein if possible.– Over a 2 hour period, feed ~ 5-7 g of CHO per kg BW

(moderate training); feed up to 10 g CHO per kg BW (heavy training)


• High GI Foods: Within 6 hours after exercise, high GI foods or simple CHO (glucose, sucrose, maltodextrin) provide the best glycogen replacement.

• Sport Drinks: Provide a CHO replacement beverage containing 40 to 80 g of CHO per serving immediately after exercise if athletes are eating self-selected diets, are unable to eat within 2 hours, or do not feel hungry after strenuous exercise.


• Individual preferences: Recommendations must be acceptable in relation to the athletes’ time and money constraints as well as their cooking abilities.

Muscle Glycogen Supercompensation

Carbohydrate Loading

• Muscle glycogen supercompensation– Aka glycogen loading– Aka carbohydrate loading

• Classical routine– Days 1-3: athletes ate a low-CHO diet (<10% of kcal from CHO),

performed a glycogen-depleting exercise– Days 3-6: high-CHO diet (>90% of kcal from CHO) with little or

no activity– Day 7: muscles were supercompensated with glycogen and

water for the exercise event on day 7

Carbohydrate Loading

• Modified routine– Days 1-3: athletes consumed a modified CHO diet (50%

of kcal from CHO, 353 g CHO per 3000 kcal), tapered exercise protocol

– Days 4-6: high-CHO intake (70% of kcal from CHO, 542 g CHO per 3000 kcal), little to no exercise

– Day 7: muscles were supercompensated with glycogen and water for the exercise event on day 7

– Produced similar amounts of muscle glycogen replacement, but was easier to follow than the classical routine

Thank you for listening…

Health & Medicine

Lecture 3 Carbohydrates in Exercise and Recovery