Introduction
Depending on the climatic conditions, the relative contributions of evaporative and dry (radiative and conductive) heat exchange to the total heat loss will vary. The hotter the climate, the greater the dependence on evaporative heat loss and, thus, on sweating. Therefore, a substantial volume of body water may be lost via sweating to enable evaporative cooling in hot climates. Generally, the individual dehydrates during exercise because of fluid non availability or a mismatch between thirst and body water requirements. In these instances, the individual starts the exercise task as euhydrated but incurs an exercise-heat mediated dehydration over a prolonged period of time. 

Fluid and Electrolyte Needs
A person's sweating rate is dependent on the climatic conditions, clothing worn, and exercise intensity. Persons in desert climates often have sweating rates of 0.3-1.2 L . h-1 while performing occupational activities. Persons wearing protective clothing often have sweating rates of 1.2 L . h-1 while performing light-intensity exercise. Likewise, athletes performing high-intensity exercise commonly have sweating rates of 1.0-2.5 L . h-1 while in the heat. Fluid requirements will vary in relation to climatic heat stress, clothing worn, acclimation state, and physical activity levels. Daily fluid requirements might range (for sedentary to very active persons) from 2-4 L . day-1 in temperate climates and from 4-10 L . day-1 in hot climates. Electrolytes, primarily sodium chloride and to a lesser extent potassium, are lost in sweat. Sweat sodium concentration averages approximately 40 mEq . L-1 (range = lO-100 mEq L-1) and varies depending on diet, sweating rate, hydration, and heat acclimation level. Heat-acclimated persons have relatively low sodium concentrations (greater than 50 percent reduction) in sweat. 

During exercise-heat stress, a principal problem is to avoid dehydration by matching fluid consumption to sweat loss. This is a difficult problem because thirst does not provide a good index of body water requirements. Thirst is probably not perceived until an individual has incurred a water deficit of approximately 2 percent of body weight. Numerous investigators report that ad libitum water intake results in incomplete water replacement or voluntary dehydration during exercise and/or heat exposure. The flavoring and cooling of  ingested fluid increase its palatability and can help to minimize voluntary dehydration. Heat acclimation status may also influence the voluntary dehydration incurred during exercise in the heat. Although heat acclimation improves the relationship between thirst and body water needs, voluntary dehydration still occurs. Since thirst provides a poor index of body water needs, persons will dehydrate by 2-8 percent of their body weight during situations of prolonged sweat loss. 

Hypohydration and Temperature Regulation
Hypohydration (less than normal total body water) increases core temperature responses during exercise in temperate and hot climates. A critical deficit of 1 percent of body weight elevates core temperature during exercise. As the magnitude of water deficit increases, there is a concomitant graded elevation of core temperature during exercise heat stress. The magnitude of core temperature elevation ranges from 0.10 to 0.23ƒC for every percent body weight lost, and this elevation is greater during exercise in hot than in temperate climates. Hypohydration not only elevates core temperature response, but it also negates the core temperature advantages conferred by high-aerobic fitness and heat acclimation. Therefore, heat-acclimated persons (who have increased sweating rates) who do not drink adequately may more rapidly experience the adverse effects of hypohydration than their nonacclimated counterparts. Recent studies at our laboratory indicate that the core temperature elevation is greater with increased exercise intensity at low (3 percent body weight loss) but not higher (5 percent body weight loss) hypohydration levels. 

Hypohydration impairs both dry and evaporative heat loss (or, if the air is warmer than the skin, dehydration aggravates dry heat gain). Hypohydration delays sweating onset and skin vasodilitation. It also reduces sweating sensitivity. Hypohydration may be associated with either reduced or unchanged sweating rates at a given metabolic rate in the heat. The physiological mechanisms mediating the reduced dry and evaporative heat loss from hypohydration include both the separate and combined effects of plasma hyperosmolality and reduced blood volume. 

Hypohydration and Fatigue
A common complaint of hypohydrated persons is skeletal muscle fatigue; however, little research had been conducted to address whether hypohydration reduces skeletal muscle performance (in absence of heat stress). Recent research at our laboratory demonstrated that, in temperate conditions, hypohydration (4 percent body weight loss) reduced single-leg knee endurance time by 18 percent compared with euhydration. The mechanism(s) responsible for this are unclear, as hypohydration does not seem to markedly alter skeletal muscle glycogen utilization. To study possible mechanism(s), subjects are repeating these exercise experiments inside of a nuclear magnetic resonance (NMR) magnet and 3'P spectra are being collected. It is hypothesized that hypohydration might accelerate depletion of adenosine triphosphate (ATP) and PCr or impair the ability to buffer hydrogen and Pj ions produced during exercise. 

Hyperhydration
Hyperhydration, or greater than normal body water, has been suggested to improve, above euhydration levels, thermoregulation and exercise-heat performance. The concept that hyperhydration might be beneficial for exercise performance arose from the adverse consequences of hypohydration. It was theorized that body water expansion might reduce the cardiovascular and thermal strain of exercise by expanding blood volume and reducing blood tonicity, thereby improving exercise performance. Studies that have directly expanded blood volume (e.g., infusion) have usually reported decreased cardiovascular strain during exercise, but have reported disparate results on heat dissipation and exercise-heat performance. Studies that have attenuated plasma hyperosmolality during exerciseheat stress generally report improved heat dissipation, but have not addressed exercise performance. 

Ten studies have been published that evaluated hyperhydration effects on thermoregulation in the heat. Briefly, 6 of 10 studies observed smaller core temperature increases during exercise with hyperhydration. Together, these studies support the notion that hyperhydration can provide a thermoregulatory benefit; however, most of these studies suffer from serious experimental design flaws. Examination of their data indicates that "control" conditions generally do not represent euhydration, and that there may have been order problems so that subjects were more heat acclimated during hyperhydration trials. Recent studies at our laboratory have controlled for these confounding factors, and observed no thermal advantage with either water hyperhydration or glycerol hyperhydration during exercise-heat stress. 
 

References 

1. Adolph EF, et al. Physiology of man in the desert. New York: Interscience, 1947. 

2. Committee on Military Nutrition. In: Marriott BM, ed. Nutritional needs in hot environments. Institute of Medicine, Washington DC:
National Academy Press, 1993. 

3. Committee on Military Nutrition. In: Marriott BM, ed. Fluid replacement and heat stress. Institute of Medicine, Washington DC:
National Academy Press, 1994. 

4. Convertino VA, Armstrong LE, Coyle EF, Mack GW, Sawka MN, Senay LC, Sherman WM. Exercise and fluid replacement:
ACSM position stand. Med Sci Sports Exerc 1996;28:(1)I-vii. 

5. Greenleaf JE. Problem: thirst, drinking behavior, and involuntary dehydration. Med Sci Sports Exerc 1992;24:645-56. 

6. Sawka MN. Physiological consequences of hypohydration: body water redistribution, exercise performance and temperature
regulation. Med Sci Sports Exerc 1992;24:657-70. 

7. Sawka MN, Montain SJ, Latzka WA. Body fluid balance during exercise - heat exposure. In: Buskirk ER, Puhl SM, eds. Body
fluid balance in exercise and sport. Boca Raton, FL: CRC Press, 1996;143 61. 

8. Sawka MN, Wenger CB, Pandolf KB. Thermoregulatory responses to acute exercise - heat stress and heat acclimation. In: Blatteis
CM, Fregley MJ, eds. Handbook of physiology, section 4: environmental physiology. New York: Oxford University Press for the
American Physiological Society, 1996;157-86, Chapter 9.