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Effects of dehydration

Studies into the effects of dehydration on physical performance have tended to focus on athletic ability. As a result, the study settings and environments may not reflect "average" conditions or normal levels of physical activity. This makes it difficult to determine how, or whether, the data from such studies would apply to "normal, sedentary individuals". Nevertheless, it is interesting to note the effects that dehydration can have on athletic performance. The research literature on this topic is extensive, but some of the main findings are discussed below.

The negative effects of dehydration on exercise performance were clearly demonstrated in the 1940s. In his 1999 review of these effects, Barr(i) noted that even at modest levels (less than 2% loss of body weight), dehydration reduced aerobic endurance and resulted in increased body temperatures, heart rate and perceived exertion. At 2-3% loss of body weight, the reduction in plasma volume places a significant strain on circulatory function. This ultimately impairs the capacity for both exercise and thermoregulation (body temperature control). At dehydration equivalent to 5% loss of body weight, rectal temperature and heart rate increase and sweating rate, exercise capacity and VO2max (maximal oxygen uptake - a measure of aerobic fitness) decrease. Performance decrements become substantial when fluid losses exceed 5% of body weight, and at 6-10% of body weight, heat stroke and heat exhaustion become life-threatening. Excessive sweating can cause circulatory failure, and the core body temperature may rise to lethal levels.(ii)

When the body experiences a fluid deficit, it distributes this between the various body compartments, such as plasma, extracellular water and intracellular water. In this way, existing water stocks are shared according to need. During exercise, blood flow to the muscles must be maintained at a high level in order to supply the oxygen and substrates needed. A high blood flow is also necessary to convect heat away from the muscles and maintain core body temperature (prevent overheating). If blood flow has been decreased by sweat loss during exercise, the body may not be able to meet both of these requirements. Under such circumstances, blood flow to the skin is likely to be most compromised (ie reduced below optimal levels), in order to maintain central venous pressure and muscle blood flow. This may impair cooling of the body and result in a rise in body temperature. If dehydration becomes more severe, the body may no longer be able to maintain blood pressure or achieve high muscle blood flow. Failing cardiac output and an imminent failure to maintain blood pressure may be one of the signals responsible for exhaustion when dehydration reaches severe levels in prolonged exercise.(iii)

Levels of performance decrement

Dehydration can compromise performance in high-intensity exercise and endurance activities. Subjective feelings of tiredness and alertness decline progressively at mild dehydration levels of as little as 1-2% loss of body weight.(iii) Even at this low level of dehydration, reductions in performance have been noted in exercise tests lasting for more than a few minutes. Exercise performance is impaired when an individual is dehydrated by as little as 2% loss of body weight, and when losses exceed 5%, the capacity for work can decrease by about 30%.(iv) Prolonged exercise, resulting in fluid losses corresponding to 2.5% of body weight has been shown to reduce physical performance capacity by 44%.(v) Dehydration equivalent to 4.3% of body weight reduces walking endurance by 48% and VO2max by 22%.(vi) Other methods of dehydration, such as by sauna exposure or diuretic administration have been shown to decrease work capacity by 35% and 18% respectively.(v) Mild dehydration appears to have relatively little effect on muscle strength. Aerobic tasks seem to be more adversely affected by dehydration than those where metabolism is predominantly anaerobic.(iii) This may be related to a reduction in blood flow to exercising muscles and hence adversely affect oxygen supply.

Sweating rates

Heat is produced during exercise and evaporation of sweat is the most effective method of cooling the body. Environmental conditions influence fluid losses (eg fluid loss is increased under conditions of excessive heat or cold, low humidity or high altitude). Even a loss of 1 litre of sweat will increase the sense of fatigue and impair performance.(iii) Sweating rates of 1-2 litres per hour are typical during moderately hard exercise, but may be as high as 2 litres per hour in high ambient temperatures.(vii) At such high sweat rates, an appreciable fraction drips from the skin without evaporating. For a 2hr 30min marathon runner, this would result in the loss of 5 litres of body water. As sweating rates increase, the fluid deficit reduces the body's ability to dissipate heat and increases the rate at which heat is stored within the body (overheating). This reduced heat tolerance severely compromises cardiovascular function and exercise capacity.(vi) As a result, exercise capacity has been shown to be greatly reduced under hot conditions (31°C) compared to the same exercise performed under cool conditions (11°C).(iv,viii)

Drinking requirements

In spite of these findings, there is no general agreement on the optimum formulation nor on the frequency or volume of drinking that is most appropriate. This will depend on a number of factors including duration and intensity of the exercise, the environmental conditions and the characteristics of the individual.(viii) Recommendations regarding hydration strategies for athletes consist of anticipating and preventing dehydration.(ix)

Concern has been expressed regarding guidelines for ingestion of fluids during exercise that advise athletes to “replace all the water lost through sweating, or consume the maximal amount that can be tolerated, or drink 600-1200ml per hour”.(x) Overconsumption of water can result in the serious condition of hyponatraemic encephalopathy. Noakes reported in 2003 that there had been more than 250 cases and seven fatalities from this condition described in the medical literature. Since this condition is preventable, he recommended the provision of rational, evidence based advice to protect all exercisers. In particular, exercisers should be advised not to over-consume fluids before, during, or after exercise. The best advice is to drink when you are thirsty and to aim to consume between 400ml and 800ml per hour in most forms of recreation and competitive exercise; less for lower intensity exercise in milder conditions and more for superior athletes competing at higher intensities in warmer environments.

Physical work

The main non-athletic situation in which these findings may be relevant is in relation to workers who undertake physically demanding jobs, especially when working in a hot environment. Sweating does not cool the body unless the moisture is removed from the skin by evaporation, and this may be prevented by some forms of protective clothing. Workers' performance declines and fatigue occurs as dehydration levels increase. Mental capacity may also be affected, which may compromise safety and those performing delicate or detailed work may find it difficult to concentrate. Workers performing strenuous physical labour should ensure that they are properly hydrated by drinking water before, during and after the job. Workers and their supervisors need to be vigilant about recognising and treating the signs of dehydration and heat stress.(xi)

Children

Children exercising in warm weather are at particular risk of the negative side effects of dehydration because they are inherently less efficient at thermoregulation than adults.(xii,xiii) 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. In this situation, the only way to prevent a catastrophic rise in body temperature is through an increased rate of evaporative cooling, resulting in an increased loss of water from the body. In addition, children may be susceptible to exercise dehydration because they may not understand the need for increased fluid consumption. Some evidence suggests that children also have a relatively insensitive thirst mechanism and consequently need encouragement to drink particularly when the risk of dehydration is high.(iii)

Swimming

Water immersion can alter the thirst mechanism(xiv), causing swimmers to have a reduced thirst response during immersion. This coupled with the blunted thirst response associated with exercise means swimmers are particularly susceptible to dehydration.(xv)

Water keeps athletes healthy

There is much evidence to show that exercise has a positive effect on health. Athletes are generaly very health-conscious individuals, however Stookey(xvi) proposes that one further reason for the beneficial effects of exercise on health. It may be that, in the process of burning large amounts of fuel to meet their increased energy requirements, athletes produce greater amounts of metabolic water than non-athletes. This additional metabolic water improves their hydration status and helps to protect against dehydration.

Last update: November 2004

(i) Barr SI. Effects of dehydration on exercise performance. Canadian Journal of Applied Physiology 1999;24:164-72
(ii) Naghii MR. The significance of water in sport and weight control. Nutrition and health 2000;14:127-32
(iii) Maughan RJ. Impact of mild dehydration on wellness and on exercise performance. European Journal of Clinical Nutrition 2003;57 (Suppl 2):S19-23
(iv) Clinical sports nutrition. Burke L and Deakin V (Eds). 2nd ed. McGraw-Hill. Australia 2000
(v) Nilsen B, Kubica R, Bonnesen A, Rasmussen IB, Stoklosa J, Wilk B. Physical work capacity after dehydration and hyperthermia. Scand J Sports Sci 1981;3:2-10
(vi) Exercise physiology: Energy, Nutrition and Human Performance. McArdle AD, Katch FI, Katch VL (Eds). 5th ed. Lippincott Williams and Wilkins. Maryland 2001
(vii) Kleiner SM. Water: An essential but overlooked nutrient. Journal of the American Dietetic Association 1999:99:201-7
(viii) Maughan RJ. Fluid balance and exercise. Inernational Journal of Sports Medicine 1992;13 (Suppl 1): S132-135
(ix) Burke LM. Nutritional needs for exercise in the heat. Comparative biochemistry and physiology. Part A, molecular and integrative physiology 2001;128:735-48
(x) Noakes TD. Overconsumption of fluids by athletes. BMJ 2003;327:113-114
(xi) Burke ER. Healthy hydration. Occupational Health & Safety May 2000;69(5):52-4
(xii) American Academy of Paediatrics Committee on Sports Medicine position paper: climatic heat stress and the exercising child. Pediatrics 1982;69:808-809
(xiii) Squire DL. Heat illness. Fluid and electrolyte issues for pediatric and adolescent athletes. Pediatric Clinics of North America 1990;37:1085-109
(xiv) Sagawa S, Miki K, Tajima F, Tanaka H, Choi JK, Keil LC, Shiralei K, Greenleaf JE. Effect of dehydration on thirst and drinking during immersion in men. J Appl Physiol. 1992;72:128-134
(xv) Convertino VA, Armstrong LE, Copyle EF, Mack GW, Swaka MN, Senay LC Jr, Sherman WM. American College of Sports Medicine, Exercise and fluid replacement. Med Sci Sports Exerc. 1996;28:i-vii
(xvi) Stookey JD. Another look at: fuel + O2 --> CO2 + H2O. Developing a water-oriented perspective. Medical Hypotheses 1999;52:285-290