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 Dietary Supplements: Fluid and Electrolyte Supplementation for Exercise-Heat Stress  
 
Michael Sawka N. PhD, Scott Montain , William Latzka A. ©
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 0.23°C 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.

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