Impact of mild dehydration on wellness and on exercise performance

Impact of mild dehydration on wellness and on exercise performance

Top of pageAbstractChronic mild dehydration is a common condition in some population groups, including especially the elderly and those who participate in physical activity in warm environments. Hypohydration is recognised as a precipitating factor in a number of acute medical conditions in the elderly, and there may be an association, although not necessarily a causal one, between a low habitual 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 1 lead to reductions in the subjective perception of alertness and ability to concentrate and to increases in self reported tiredness and headache. For a healthy lean young male with a body mass of 70 kg, total body 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 2 l. In other words, about 5 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 of water 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 reach 10 l 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 10 of body mass, about 20 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 sporting competition, so there must be an emphasis on fluid replacement strategies to protect health and to maintain performance capacity.

Top of pageDaily 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 200 ml, insensible losses from the lungs 400 ml 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 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 900 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 conditions, body temperature is maintained at a comfortable level primarily by behavioural mechanisms: adjusting the environmental 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 heat loss. 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 1.5 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 exerciseIn all but the most extreme conditions, body temperature is maintained within about 2 of the normal resting level of 37 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 40 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 critical core temperature about 40 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. Performance 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.

Top of pageHydration 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 cardiovascular function, 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 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 symptoms 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 1 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 important 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. In all, 15 healthy adults participated 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 was 2.68% in the fluid restriction trial and 0.58% in the control trial Shirreffs, unpublished data.

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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 around the 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 for increased drinking and may have a relatively insensitive thirst mechanism and may need encouragement to drink when losses are high.

Top of pageEffects 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 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 fully hydrated. This has been shown to be true whether the fluid deficit is incurred by prolonged exercise in a warm environment, 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 metabolism Sawka et al, 1990.

There seems to be relatively little effect of mild dehydration on muscle strength Table 1. There are, however, some difficulties in the interpretation of these studies as the methods 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 acid 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 1 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 the responses 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 2 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 encouragement to do so Shirreffs Maughan, 1998.

Fluid losses due to sweating are distributed in varying proportions 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 meeting the requirement for a high blood flow to both these tissues. In this situation, skin blood flow is likely to be compromised, 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 exhaustion 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 exercise are 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 questions about the mechanism of action of fluid replacement during exercise, but these studies confirm the importance of the ingestion of drink of a suitable composition and in sufficient volume during prolonged exercise in a warm environment.