body hydration status: one asymptote ascends steeply with hyperhydration, while the other descends gradually with dehydration (Lee, 1964). The apex of this hyperbolic relationship approximates a urine output of approximately 50 mL/hour. The extremes depicted in Figure 4-4 can be exceeded. For example, investigators have reported that urine output can transiently increase to approximately 600 to 1,000 mL/hour with water loading (Freund et al., 1995; Noakes et al., 2001; Speedy et al., 2001) and decrease to approximately 15 mL/hour with dehydration (Adolph, 1947b). Urine output can vary widely to maintain total body water; however, there are clearly limits to the amount of conservation and excretion.

Physical activity and climate also affect urine output. Exercise and heat strain will reduce urine output by 20 to 60 percent (Convertino, 1991; Mittleman, 1996; Zambraski, 1996), while cold and hypoxia will increase urine output (Freund and Young, 1996; Hoyt and Honig, 1996).

Gastrointestinal and thus fecal water loss in healthy adults is approximately 100 to 200 mL/day (Newburgh et al., 1930).

Water loss through the skin occurs via insensible diffusion and secreted sweat. For the average adult, loss of water by insensible diffusion is approximately 450 mL/day (Kuno, 1956). During heat stress, eccrine sweat glands secrete sweat onto the skin surface, which cools the body when water evaporates from the sweat. In hot weather, sweat evaporation provides the primary avenue of heat loss to defend the body’s core temperature. When a gram of sweat water is vaporized at 30°C, 2.43 kJ (0.58 kcal) of heat becomes kinetic energy (latent heat of evaporation) (Wenger, 1972). For a given hot weather condition, the required sweating rate for evaporative cooling is dependent upon the physical activity level (metabolic rate).

The following calculations provide the minimal sweat produced by persons performing moderately heavy (metabolic rate ≈ 600 W) exercise in the heat (Sawka et al., 1996a). If the activity is 20 percent efficient, the remaining 80 percent of metabolic energy produced is converted to heat in the body so that 480 W (0.48 kJ/second, or 28.8 kJ/minute or 6.88 kcal/minute) need to be dissipated to avoid heat storage. The specific heat of body tissue (amount of energy required for 1 kg of tissue to increase temperature by 1°C) approximates 3.5 kJ (0.84 kcal)/kg/°C. For example, a 70-kg man has a heat capacity of 245 kJ (59 kcal)/°C, and a 50-kg woman has a heat capacity of 173