been known to be the primary factors modifying the core temperature and thermoregulatory responses to muscular exercise. Aerobically fit people who are heat acclimated and fully hydrated optimize their ability to limit body heat storage and maintain performance during exercise-heat stress. Hydration level is particularly important because a body fluid deficit incurred before or during exercise in the heat neutralizes the thermoregulatory advantages conferred by high aerobic fitness (Cadarette et al., 1984) and heat acclimatization (Buskirk et al., 1958; Sawka et al., 1983).
In hot environments, body fluid is lost primarily through eccrine sweat gland secretion, which results in evaporative cooling of the body. For a given person, the sweating rate is dependent on environmental conditions (ambient temperature, dew point temperature, radiant load, and air velocity), clothing (insulation and moisture permeability), and physical activity level (Adolph and Associates, 1947; Shapiro et al., 1982). Adolph and Associates (1947) reported that for 91 men studied during diverse military activities in the desert, the average sweating rate was 4.1 liters every 24 h, but values ranged from 1 to 11 liters every 24 h. During more intense physical exercise, much higher sweating rates can occur, and sweating rates of 1 liter/h are very common (Shapiro et al., 1982).
During physical exercise in the heat, the principal problem is that of precisely matching the volume of fluid intake to the volume of sweat output. This is a difficult problem to solve since thirst does not provide a good index of body water requirements (Adolph and Associates, 1947; Engell et al., 1987). Numerous investigators (Adolph and Associates, 1947; Bar-Or et al., 1980; Phillips et al., 1984) report that ad libitum water intake results in incomplete water replacement or voluntary dehydration during exercise and heat exposure. It is not uncommon for individuals to voluntarily dehydrate 2%-8% of their body weight during exercise-heat stress, despite the availability of adequate amounts of fluid for rehydration (Adolph and Associates, 1947; Buskirk and Beetham, 1960; Greenleaf et al., 1983).
Thirst is probably not perceived until an individual has incurred a water deficit of approximately 2% of body weight (Adolph and Associates, 1947). As a result, it is likely that unless forced hydration is practiced, some level of dehydration will occur during exercise in the heat. Neufer et al. (1988) recently found that hypohydration reduces the gastric emptying rate of ingested fluids during exercise in the heat. They found an approximate 20% reduction in gastric emptying rate during three successive 15-min bouts of exercise in a warm environment (35°C, 20% relative humidity) when