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7/31/2019 DESIDRATAO WELLNESS EXERCICIO
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ORIGINAL COMMUNICATION
Impact of mild dehydration on wellness and onexercise performance
RJ Maughan1*
1School of Sports and Exercise Sciences, Loughborough University, Loughborough, UK
Chronic mild dehydration is a common condition in some population groups, including especially the elderly and those whoparticipate in physical activity in warm environments. Hypohydration is recognised as a precipitating factor in a number of acutemedical conditions in the elderly, and there may be an association, although not necessarily a causal one, between a lowhabitual fluid intake and some cancers, cardiovascular disease and diabetes. There is some evidence of impairments of cognitive
function at moderate levels of hypohydration, but even short periods of fluid restriction, leading to a loss of body mass of 12%,lead to reductions in the subjective perception of alertness and ability to concentrate and to increases in self-reported tirednessand headache. In exercise lasting more than a few minutes, hypohydration clearly impairs performance capacity, but musclestrength appears to be relatively unaffected.European Journal of Clinical Nutrition (2003) 57, Suppl 2, S19S23. doi:10.1038/sj.ejcn.1601897
Keywords: dehydration; wellness; exercise; fatigue
IntroductionWater is the largest single component of the human body,
accounting for about 5060% of total body mass. For a
healthy lean young male with a body mass of 70 kg, totalbody water will be about 42 l. The turnover rate of water
exceeds that of most other body components. For the
sedentary individual living in a temperate climate, daily
water turnover is about 23l. In other words, about 510% of
the total body water content is renewed every day (Lentner,
1981). In spite of its abundance, however, there is a need to
maintain the body water content within narrow limits, and
the body is much less able to cope with restriction of water
intake than with restriction of food intake. A few days of
total fasting has little impact on health and functional
capacity, provided fluids are allowed, and even longer
periods of abstinence from food are well tolerated. In
contrast, except in exceptional circumstances cessation ofwater intake results in serious debilitation after times
ranging from only an hour or two to a few days at most.
The performance of prolonged exercise, particularly in
warm environments, can result in a substantial loss of body
water, with the potential for adverse effects on performance
capacity, and an increased risk of heat-related illness. An
athlete training hard in a spell of warm weather, or a person
with a heavy manual job working in the same conditions,
may lose several litres of sweat in a single day: in extreme
conditions of work in the heat, daily sweat losses may reach1012l or even more. This amounts to about one-quarter of
the total body water content for the average man, and about
one-third for the average woman. In spite of this high daily
rate of exchange, a water deficit of only a few percent will
impair physical performance: a slightly larger loss will bring
symptoms of tiredness, headache and general malaise. If the
loss of water reaches 1015% of body mass, about 2030% of
total body water, death is the likely outcome. It may not be
possible to prevent loss of substantial volumes of sweat in
either physically demanding occupational tasks or in sport-
ing competition, so there must be an emphasis on fluid
replacement strategies to protect health and to maintain
performance capacity.
Daily water turnoverWater is lost from the body in varying amounts via a number
of different routes: the main avenues of water loss are urine
(about 1400 ml), faeces (200ml), insensible losses from the
lungs (400ml) and loss via the skin (500 ml). The total daily
water loss is therefore about 2500 ml, but this varies greatly
between individuals and depends on the environmental
*Correspondence: RJ Maughan, School of Sports and Exercise Sciences,
Loughborough University, LE11 3TU, Loughborough, Leicestershire, UK
Guarantor: RJ Maughan
European Journal of Clinical Nutrition (2003) 57, Suppl 2, S19S23& 2003 Nature Publishing Group All rights reserved 0954-3007/03 $25.00
www.nature.com/ejcn
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conditions. When the air is dry, water loss from the skin and
lungs is increased because of the increased vapour pressure
gradient, and more water is also lost by these routes in hot
weather. The amount lost in the urine depends very much on
the volume of fluid consumed and on the total losses by
other routes. It also depends on the solute content of the
diet, and high intakes of salt (sodium chloride) or protein
will increase the daily fluid requirement because of the
limited capacity of the kidneys to concentrate the urine. If
water intake is restricted, the kidneys will conserve water by
producing a more concentrated urine: the concentrating
capacity varies between individuals, but the maximum urine
osmolality is typically about 9001200 mosm/kg (Lentner,
1981). Equally, the body cannot store excess water, so the
kidneys get rid of any temporary excess by producing a large
volume of dilute urine. Daily fluid intake in man is usually in
excess of perceived need and water balance is maintained by
urinary losses (Engell & Hirsch, 1991).
For individuals at rest in temperate environmental condi-
tions, body temperature is maintained at a comfortable levelprimarily by behavioural mechanisms: adjusting the envir-
onmental conditions or the amount of clothing worn are
effective at increasing or reducing heat loss. When the
environmental temperature is high, physical transfer of heat
from the body is not possible, and evaporation of sweat from
the skin is the bodys only way of getting rid of excess heat.
All the metabolic reactions in the body result in heat
production, but at rest the body produces heat only slowly:
about 60 W for an average 70 kg individual. In exercise, the
rate of heat production rises: for a 70 kg person running at
15 km/h the rate of metabolic heat production will increase
to about 1 kW, and body temperature would rise rapidly if
there was not a corresponding increase in the rate of heatloss. Sweating is a very effective way of preventing body
temperature from rising too far, but causes the loss of water
and electrolytes (salts) from the body. Our runner would
need to produce about 1.52 l of sweat per hour to balance
the rate of heat production, and trained athletes can sustain
this running speed for several hours with little rise in core
temperature. Even a loss of as little as 1 l of sweat will
increase the sense of fatigue and impair performance, so
replacement of fluid losses during such forms of exercise is
clearly a priority. A table of sweat losses in various sports
situations has been compiled by (Rehrer & Burke, 1996).
Maintenance of body temperature at rest and during
exercise
In all but the most extreme conditions, body temperature is
maintained within about 231C of the normal resting level of
371C. This, of course, applies to the temperature of deep
body structures, including especially the brain, rather than
to skin temperature. The temperature of bare skin follows the
environmental temperature and is not closely regulated.
There appears to be a critical body temperature above which
humans and other mammals will not continue to exercise
voluntarily. A consistent finding in the published literature
is that voluntary fatigue occurs at a core temperature of
about 401C (Gisolfi & Copping, 1974; Nielsen et al, 1993,
1997). An inability of the central temperature control
mechanism, or of the effector mechanisms that respond to
input from the hypothalamus, to compensate for orthostatic
simultaneously, metabolic and thermoregulatory demands
may result in heat syncope, characterised by extreme
peripheral vasodilation and a fall in arterial blood pressure
(Werner, 1993). This condition is not very harmful compared
to heat stroke during high heat stress, where the requirement
for thermoregulation is subordinated to cardiovascular and
metabolic demands (Werner, 1993). There are thermal limits
that the brain tolerates, and when these limits are reached a
host of physiological reactions occur that may be aimed at
reducing the rate of brain heating, but often result in heat
stroke. Important among these is the cessation of physical
activity that results in a marked reduction in the rate of
metabolic heat production.
Nielsen et al (1993, 1997) have speculated that at a criticalcore temperature (about 401C in humans) there may be a
negative effect on the brains motor control centres. Such
effects are consistent with the loss of motor coordination,
reduction in motor drive and increased perception of effort
that typically occur in the later stages of prolonged exercise
in the heat. Evidence for a direct effect of elevated core
(hypothalamic) temperature on impaired neuromuscular
function comes from studies that show increased exercise
time to fatigue when individuals exercise in cool conditions
(Galloway & Maughan, 1997; Parkin et al, 1999). Perfor-
mance of endurance exercise is also improved when subjects
are actively cooled during the period of exercise (MacDougall
et al, 1974) or have been cooled prior to starting exercise (Lee& Haymes, 1995).
Hydration status and wellnessDehydration is associated with a number of negative effects
on health and well-being, although the evidence that mild
dehydration is harmful is not supported by any strong
evidence. There is general agreement among clinicians that
chronic mild fluid restriction will compromise health, but
there is limited solid evidence to substantiate this clinical
impression. Severe dehydration is clearly detrimental to
health, and is associated with compromised cardiovascularfunction, renal impairment, weakness and lassitude, and a
number of diffuse symptoms, including headache, nausea
and general malaise. Specific health risks of a chronically
inadequate fluid intake are difficult to define, for a number
of reasons, including the diffuse nature of the symptoms, the
difficulty in obtaining reliable estimates of fluid intake, and
the absence of any agreed marker of hydration status. There
does seem to be some evidence of a link between habitual
fluid intake and cancers of the bladder (Michaud et al, 1999),
and colon (Shannon et al, 1996). Links with other disease
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states, including diabetes and cardiovascular disease have
also been postulated, but the evidence is somewhat tenuous
(Burge et al, 2001; Chan et al, 2002). It is also the case that
individuals with a high fluid intake may be chronically
hypohydrated if they also have a high water requirement, so
water turnover may not be a good index of tissue hydration
status (Shirreffs & Maughan, 1998).
There are undoubtedly some negative subjective symp-
toms associated with even modest levels of dehydration. Self-
ratings of alertness and ability to concentrate decline
progressively when fluid intake is restricted to induce body
mass deficits of even as little as 12%. At the same time,ratings of tiredness and headache increase (Figure 1). There
are also some indications in the published literature that
cognitive function, as assessed by decision making and
reaction time tests, is also impaired at relatively low levels of
dehydration (Gopinathan et al, 1988). This may be impor-
tant when decisions have to be made or where judgement
and skill are involved; driving a motor car is a good example
of such a task.
Populations at particular risk of dehydration and its
sequelae include the very young and the elderly. Limited
data are available on the prevalence of hypohydration, but
there is some evidence to suggest that this may be relatively
common among some sections of the elderly population(Leaf, 1984). Although the evidence that dehydration has a
significant negative effect on brain function in healthy
young individuals is limited, it is quite possible that mild to
moderate dehydration may exacerbate any pre-existing
impairment of cognitive function in the elderly, and
dehydration is recognised as one of the factors that may
precipitate acute confusion in the elderly (Mentes et al,
1998). Again, clinical experience suggests that the confused
elderly patient admitted to care is commonly in a state of
fluid deficit. This state may occur because of a reduced thirst
response to a fluid deficit in the elderly, a reduction in renal
function and an alteration in the secretion of hormones
involved in water and electrolyte homeostasis (Miller, 1998).
Elderly individuals suffering from chronic physical and/or
mental impairment are likely to have low levels of daily
water turnover and to be at increased risk of hypohydration
(Phillimore et al, 1998).
In spite of great improvements in sanitation and in the
availability of rehydration solutions, dehydration resulting
from infectious diarrhoeal disease remains one of the largest
single causes of death among young children, being
responsible for about 1.5 million deaths annually aroundthe world (WHO, 2002). Healthy children may also be at risk
of dehydration if there is a sudden increase in water loss for
any reason, and physically active children will be at
particular risk during periods of warm weather. The large
surface area:volume ratio of children relative to adults (the
average 6-year-old child has a surface:volume ratio about
50% greater than that of the average adult) means that they
gain more heat through the skin when the environmental
temperature exceeds skin temperature (Bar-Or, 1989). In this
situation, an increased rate of evaporative cooling is essential
to prevent a catastrophic rise in body temperature. This is
achieved at the expense of an increased loss of water from
the body, but children may be less aware of the need forincreased drinking and may have a relatively insensitive
thirst mechanism and may need encouragement to drink
when losses are high.
Effects of dehydration on exercise performanceAlthough the physiological consequences of dehydration
due to the sweat loss that occurs during exercise have been
the focus of much attention, there has been relatively little
20
30
40
50
60
70
80
0 13 24 370 13 24 37
0 13 24 370 13 24 37
Abilitytoconcentrate
0
5
10
15
20
25
30
35
40
45
Headache
30
35
40
45
50
55
60
65
70
75
Alertness
Fluid restriction
Control
30
35
40
45
50
55
60
65
70
75
Tiredness
Figure 1 Effects of mild hypohydration induced by fluid restriction on ratings of a variety of subjective symptoms. In all, 15 healthy adultsparticipated in two trials: in one trial, fluid intake was restricted for 37 h and normal drinking was allowed in the other trial. Body mass loss was2.68% in the fluid restriction trial and 0.58% in the control trial (Shirreffs, unpublished data).
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scientific interest in the effects of a fluid deficit incurred
prior to exercise. Both of these situations, however, are
common in sport. Individuals who begin exercise with a
fluid deficit will not perform as well as they will when fullyhydrated. This has been shown to be true whether the fluid
deficit is incurred by prolonged exercise in a warm environ-
ment, by passive heat exposure or by diuretic treatment
(Nielsen et al, 1981; Armstrong et al, 1985). An impaired
performance is observed whether the exercise lasts a few
minutes, or whether it is more prolonged, although
muscular strength appears to be relatively unaffected, and
tasks with a large aerobic component are affected to a greater
extent than those that rely primarily on anaerobic metabo-
lism (Sawka et al, 1990).
There seems to be relatively little effect of mild dehydra-
tion on muscle strength (Table 1). There are, however, some
difficulties in the interpretation of these studies as themethods used to induce dehydration usually include either
heat exposure of exercise, both of which will induce
elevations of muscle temperature which will in itself affect
the contractile properties of the muscles. Where dehydration
has been induced over a longer period by restriction of fluid
intake, there is also commonly a reduced food intake,
leading to changes in muscle glycogen content and in the
acidbase status of the muscle.
In exercise tests lasting more than a few minutes,
reductions in performance are apparent at modest levels of
body water loss amounting to 12% of the pre-exercise body
mass (Armstrong et al, 1985). For the average young male
(most of the subjects in these studies have been male, but theresponses of female subjects do not appear to be different),
body water accounts for about 60% of total body mass, so
these levels of hypohydration amount to about 23% of total
body water. It was reported by Adolph et al (1947) that
subjects do not report a sensation of thirst until they have
incurred a water deficit of about 2% of body mass. This
suggests that athletes living and training in the heat may not
be aware that they have become dehydrated to a level
sufficient to affect performance adversely. Athletes living
and training in warm environments, especially those used to
living in cool climates, will often fail to consume sufficient
fluid to maintain hydration status and may need encourage-
ment to do so (Shirreffs & Maughan, 1998).
Fluid losses due to sweating are distributed in varyingproportions among the various body compartments: plasma,
extracellular water and intracellular water. The decrease in
plasma volume that accompanies dehydration may be of
particular importance in influencing work capacity. Blood
flow to the muscles must be maintained at a high level to
supply oxygen and substrates, but a high blood flow to the
skin is also necessary to convect heat from the active muscles
and the body core to the body surface where it can be
dissipated (Nadel, 1989). When the ambient temperature is
high and blood volume has been decreased by sweat loss
during prolonged exercise, there may be difficulty in meet-
ing the requirement for a high blood flow to both these
tissues. In this situation, skin blood flow is likely to becompromised, allowing central venous pressure and muscle
blood flow to be maintained but reducing heat loss and
causing body temperature to rise (Rowell, 1986). More recent
data, however, suggest that dehydration may cause a
reduction in the blood flow to exercising muscles as well as
to the skin (Gonzalez-Alonso et al, 1998). It may be that a
falling cardiac output and an imminent failure to maintain
blood pressure is one of the signals responsible for exhaus-
tion when dehydration occurs during prolonged exercise.
These factors have been investigated by Coyle and his co-
workers in a series of elegant studies; their results clearly
demonstrate that rise in core temperature and heart rate and
the fall in cardiac stroke volume during prolonged exerciseare graded according to the level of hypohydration achieved
(Montain & Coyle, 1992a). They also showed that the
ingestion of fluid during exercise increases skin blood flow,
and therefore thermoregulatory capacity, independent of
increases in the circulating blood volume (Montain & Coyle,
1992b). Plasma volume expansion using dextran/saline
infusion was less effective in preventing a rise in core
temperature than was the ingestion of sufficient volumes of
a carbohydrate electrolyte drink to maintain plasma volume
at a similar level (Montain & Coyle, 1992b). This raises some
Table 1 Effects of acute dehydration induced by different means on muscle strength
Dehydration procedure n DBW(%) Results Reference
Sauna 10 1.5 6%k in strength Schoffstall et al (2001)Fluid restraiction 9 3 11%k in strength Bosco et al (1968)Exercise in heat 12 3 No change Greenleaf et al (1967)Sauna 7
4 No change Greiwe et al (1998)
Exercise/heat 10 4 No effect Montain et al (1998)Exercise/rubber suit 7 5 k in strengtha Webster et al (1990)Fluid restriction 4 8 11%k in strength Houston et al (1981)
aIn the study of Webster et al, a total of 16 strength measures involving both upper and lower limbs were used: a statistically significant reduction in strength was
seen in only two of the measures, but there was a strong trend towards a reduction in strength in all measures after dehydration.
These studies suggest that there is little effect of dehydration on muscle strength, but the methodology used to cause dehydration and the measures of strength
varied greatly between studies. This table includes only a small number of the pub-lished references in this field.
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questions about the mechanism of action of fluid replace-
ment during exercise, but these studies confirm the im-
portance of the ingestion of drink of a suitable composition
and in sufficient volume during prolonged exercise in a
warm environment.
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