condition (Sawka and Montain, 2001). The sweating rates were predicted by using an equation that includes the effects of metabolic rate, climate, and clothing (Moran et al., 1995; Shapiro et al., 1982, 1995). Physical exercise and rest were varied (a 12-hour work period was used) to achieve a variety of total energy expenditure rates at different climatic conditions. Climatic heat stress was quantified by mean daily Wet Bulb Globe Temperature (WBGT), which combines the effects of ambient temperature, humidity, solar load, and wind (Sawka et al., 1996a). Figure 4-18 presents the range of daily fluid (water) requirements of persons performing light (1,800 total kcal/day) through hard (5,640 total kcal/day) work in climates with mean daytime WBGT ranging from 5° to 35°C (41° to 95°F). Note that daily fluid requirements increase with metabolic rate and heat stress. For sedentary to very active persons, daily fluid requirements range from 2 to 4 qt/day (1.9 to 3.8 L/day) in a cool climate and up to 8 to 16 qt/day (7.6 to 15.2 L/day) in very hot climates. For example, in Atlanta, Georgia, the mean daily WBGT temperature is approximately 30°C (86°F) during mid-summer, and persons living there will have daily fluid requirements of 4 to 14 qt/day (3.8 to 13.3 L/day), depending upon their activity levels and duration of exposure (e.g., sitting in air conditioning is not heat exposure). Generally, physical activity is curtailed in hot weather, so high levels of water intake, such as 14 qt/day (13.3 L/day), are rare.
The maximal hourly fluid replacement rate approximates the sweating rates often observed during intense physical exercise in the heat. This upper limit for fluid replacement rate during exercise-heat stress is determined by the gastric emptying rate, as maximal intestinal absorption is not limiting (Gisolfi and Ryan, 1996). The maximal gastric emptying rate approximates 1.0 to 1.5 L/hour for an average adult man (Mitchell and Voss, 1991; Murray, 1987) but has considerable individual variability and is influenced by gastric volume (the higher the volume, the greater the emptying rate). Gastric emptying rates are reduced somewhat during high- (greater than 75 percent VO2max) intensity exercise (Costill and Saltin, 1974; Neufer et al., 1989b), dehydration (Neufer et al., 1989a; Rehrer et al., 1990), and heat strain (Neufer et al., 1989a; Rehrer et al., 1990). Dehydration probably mediates reduced gastric emptying by increasing heat strain, as an inverse relationship (r = −0.76) between the fluid volume emptied and core temperature has been observed (Neufer et al., 1989a).
This is consistent with observations by Rehrer and colleagues (1990), who found that dehydration reduced gastric emptying rate during exercise when core temperature was elevated above