Review
Hydration effects on thermoregulation and performance in the heat

https://doi.org/10.1016/S1095-6433(01)00274-4Get rights and content

Abstract

During exercise, sweat output often exceeds water intake, producing a water deficit or hypohydration. The water deficit lowers both intracellular and extracellular fluid volumes, and causes a hypotonic–hypovolemia of the blood. Aerobic exercise tasks are likely to be adversely effected by hypohydration (even in the absence of heat strain), with the potential affect being greater in hot environments. Hypohydration increases heat storage by reducing sweating rate and skin blood flow responses for a given core temperature. Hypertonicity and hypovolemia both contribute to reduced heat loss and increased heat storage. In addition, hypovolemia and the displacement of blood to the skin make it difficult to maintain central venous pressure and thus cardiac output to simultaneously support metabolism and thermoregulation. Hyperhydration provides no advantages over euhydration regarding thermoregulation and exercise performance in the heat.

Introduction

Total body water can have a critical influence on thermoregulation and exercise performance in the heat. Total body water normally remains relatively constant (Sawka and Coyle, 1999), however, physical exercise and heat exposure will increase water flux to support thermoregulation (Sawka et al., 1996b). The more strenuous the exercise intensity and hotter the climate, the greater the dependence on evaporative cooling and a substantial volume of body water may be lost via sweating. If this fluid loss is not replaced the person will dehydrate. Persons dehydrate during exercise because of fluid non-availability or a mismatch between thirst and body water requirements. In these instances, the person starts the exercise task with normal total body water and dehydrates over a prolonged period. This scenario is common for most athletic and occupational settings, however, in some situations the person might start to exercise with a body water deficit. For example, in several sports (e.g. boxing, power lifting, wrestling) athletes will purposely dehydrate to compete in lower weight classes. Also, persons medicated with diuretics may be dehydrated prior to initiating exercise.

This paper reviews fluid and electrolyte balance in the heat and the effects of their imbalances on temperature regulation and physical exercise performance. Recent research has clarified several mechanism(s) related to reduced exercise performance and increased cardiovascular strain associated with hypohydration as well as extending our understanding of hyperhydration and its possible thermoregulatory actions. Throughout the paper, euhydration refers to normal body water content, hypohydration refers to body water deficit, and hyperhydration refers to increased body water content. Dehydration refers to the process of reducing total body water.

Section snippets

Fluid and electrolyte needs

A person's sweating rate is dependent upon the climatic conditions, clothing worn and exercise intensity (Shapiro et al., 1982). Persons in desert and tropical climates can have sweating rates that vary from 0.3 to 1.5 l/h while performing occupational activities (Eichna et al., 1945a, Eichna et al., 1945b). Fig. 1 provides an approximation of hourly sweating rates for runners (Sawka and Pandolf, 1990). Runners and athletes performing vigorous exercise in the heat often have sweating rates of

Temperature regulation

Hypohydration increases core temperature responses during exercise in temperate (Grande et al., 1959, Sawka et al., 1979) and hot (Sawka et al., 1983, Sawka et al., 1985) climates, where thermoregulation is possible (compensable heat stress). A deficit of only 1% of body weight elevates core temperature during exercise (Ekblom et al., 1970). As the magnitude of water deficit increases, there is a concomitant graded elevation of core temperature when exercising in the heat (Montain and Coyle,

Exercise performance

Body water losses can adversely influence exercise performance and the severity of effect depends upon the environmental conditions (Sawka and Pandolf, 1990). In brief, hypohydration does not alter isometric strength and endurance (Greiwe et al., 1998) or anaerobic performance (Jacobs, 1980), but decreases muscular endurance (Montain et al., 1998b) and maximal aerobic power (Sawka and Pandolf, 1990). Numerous studies have examined the influence of hypohydration on maximal aerobic power (Vo2max)

Hyperhydration

If a water deficit has adverse effects, perhaps greater than normal body water might have thermoregulatory advantages. Studies have examined hyperhydration effects on thermoregulation in the heat; however, most studies suffered serious design problems that confounded their results. Some investigators reported lower core temperatures during exercise after hyperhydration (Moroff and Bass, 1965, Nielsen et al., 1971, Nielsen, 1974, Gisolfi and Copping, 1974, Grucza et al., 1987), while other

Summary

Water and electrolyte balances are critical for a person's ability to thermoregulate and perform exercise in the heat. During exercise in the heat, water and electrolyte losses primarily occur from sweating. A person's sweating rate is dependent upon the climatic conditions, clothing worn and exercise intensity. Daily fluid requirements range (for sedentary to very active persons) from 2 to 4 l/day in a cool climate up to 8–16 l/day in very hot climates. Over a 24-h period, fluid and

Acknowledgements

The view, opinions, and/or findings contained in this report are those of the authors and should not be construed as an official Department of the Army position, or decision, unless so designated by other official documentation. Approved for public release; distribution unlimited.

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    This paper was presented at the International Conference on Physiological and Cognitive Performance in Extreme Environments, Canberra, Australia, March 2000.

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