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per kg of body weight) is highest during the first weeks of life and declines by some 40 percent during infancy. It declines further, but at a slower pace during childhood and adolescence. In a German study, mean water turnover at ages 1 to 3 months was 160 mL/kg/ day, compared with 97 mL/kg/day at ages 10 to 12 months, and 40 mL/kg/day at ages 13 to 15 years (Fusch et al., 1993). Daily fluid intake in bottle-fed infants was compared over a 15-day study period using two methods to determine intake (Vio et al., 1986). Water turnover as measured by deuterium tracer was compared with directly measured fluid intake. Daily fluid intakes of 0.71 L/day (153 mL/kg/day) and 0.70 L/day (151 mL/kg/day) were reported for the direct and water turnover methodology (r = 0.98), respectively. Other studies have found close agreement (Butte et al., 1988) or slightly higher (Butte et al., 1991) values for water turnover versus direct measurement of daily fluid intake in infants.

Table 4-6 provides studies examining daily water turnover for adults in a variety of conditions. These values are generally higher than in water balance studies because subjects are often more active and exposed to outside environments. Daily water turnover rates were approximately 3.2 L and 4.5 L for sedentary and active men, respectively. Several studies found daily water turnover rates greater than 5 L; presumably these were more active persons who may have encountered heat stress. Women generally had approximately 0.5 to 1.0 L/day lower daily water turnover rates than their male counterparts.

Water turnover was measured in 458 noninstitutionalized adults (ranging from 40 to 79 years of age) who lived in temperate climates (Raman et al., 2004). Daily turnover averaged 3.6 and 3.0 L in men and women, respectively. The water turnover values were corrected for metabolic water and water absorption from humidity to provide preformed water values. The preformed water values averaged 3.0 L/day (range 1.4 to 7.7 L/day) for men and 2.5 L/day (range 1.2 to 4.6 L/day) for women. The lower values in women were not accounted for by differences in body size.

METHODS FOR ESTIMATING HYDRATION STATUS

Total Body Water Changes

Total body water (TBW) is accurately determined by dilution of a variety of indicators. Repeated measurements are required to assess total body water changes. The technical requirements and cost for



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