DESIDRATAÇÃO 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

Dehydration on wellness and on exercise performanceRJ Maughan

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European Journal of Clinical Nutrition


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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

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Fluid restriction

Control

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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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DESIDRATAÇÃO WELLNESS EXERCICIO