water production is an average of approximately 250 to 350 L/day for sedentary persons—but which can increase to 500 to 600 mL/ day for physically active persons (Hoyt and Honig, 1996). Hence, respiratory water losses are roughly equivalent to, or offset by, metabolic water production (Table 4-2; Hoyt and Honig, 1996). Metabolic water, a by-product of metabolizing energy-yielding nutrients from foods into carbon dioxide and energy, does not include the water present in a foodstuff itself. This is considered compositional water, or moisture. It is often determined analytically as the difference in weight of a food item before and after drying to a constant weight.

Fluid is consumed in the form of food and beverages, and, regardless of form, is absorbed by the gastrointestinal tract and acts the same physiologically. In one survey of the adult U.S. population (1977–1978 Nationwide Food Consumption Survey), total water intake was approximately 28 percent from foods, 28 percent from drinking water, and 44 percent from other beverages (Ershow and Cantor, 1989). National survey data for adults (Appendix Tables D-1, D-3, and D-4) likewise suggest that approximately 20 percent of water comes from food, and the remaining 80 percent comes from fluids.

Drinking induced by water deprivation is homeostatic (Greenleaf and Morimoto, 1996). Other factors (e.g., social, psychological) that influence drinking behavior are nonregulatory (Rolls and Rolls, 1982). Over an extended period, fluid consumption will match body water needs (if adequate amounts are available). However, mismatches can occur over short periods (Johnson, 1964). The fluid intake for healthy adults can vary markedly depending on activity level, environmental exposure, diet, and social activities; nonetheless, for a given set of conditions, intake is reproducible within persons (Johnson, 1964). Therefore, it is reasonable to assume that for large population studies of apparently healthy individuals, the fluid volume consumed is equal to or greater than body water needs.

Water balance is regulated within ± 0.2 percent of body weight over a 24-hour period for healthy adults at rest (Adolph, 1943).