water intake requirements increase to approximately 3.2 L/day (Greenleaf et al., 1977; Gunga et al., 1993). Cold exposure did not alter intake, but heat stress increased total daily water intake (Welch et al., 1958).

Limited data were available for women. Women are physically smaller, thus they probably have lower water requirements due to lower metabolic expenditures. A study of three Japanese women (likely smaller than average U.S. adult women) indicated a water intake requirement of approximately 1.6 L/day (Yokozawa et al., 1993).

Water turnover studies have been conducted to evaluate water needs and assume a balance between influx and efflux (Nagy and Costa, 1980). Rates of body water turnover can be determined by administering a drink with deuterium (D₂O) or tritium (³H₂O) labeled water and then following the decline (or disappearance) in hydrogen isotope activity over time. The isotope activity declines because of loss of the labeled water via excretion, evaporation, and dilution from intake of unlabeled water. If proper procedures are employed, these measurements will yield values within 10 percent or less of actual water flux (Nagy and Costa, 1980).

Figure 4-7 provides data on the daily water turnover for infants and children (Fusch et al., 1993). Water turnover (when expressed

FIGURE 4-7 Daily water turnover per kg of body weight in infants and children. Reprinted with permission, from Fusch et al. (1993). Copyright 1993 by Springer-Verlag.