water (tritiated water) in fat-free mass measured by dual energy X-ray absorptiometry (DEXA) in relation to age for men and women, respectively (Visser and Gallagher, 1998; Visser et al., 1997). Note that individual variation exists for the hydration of fat-free tissue and values remain relatively stable with increasing age. Neither ethnicity nor gender altered the hydration of fat-free mass. Similar values were reported for whites (men = 74 percent, women = 74 percent) and African Americans (men = 75 percent, women = 75 percent). Other investigators have supported the observation that age and gender do not markedly alter the hydration of fat-free mass in adults (Baumgartner et al., 1995; Goran et al., 1994; Mazariegos et al., 1994).

Total body water (TBW), comprising extracellular fluid (ECF) and intracellular fluid (ICF), averages approximately 60 percent of body weight, with a range from approximately 45 to 75 percent (Altman, 1961). Variability in TBW is primarily due to differences in body composition. TBW is usually measured by volume distribution of an appropriate indicator (e.g., antipyrine, deuterium oxide, tritium oxide). Table 4-1 provides TBW values for different age and gender groups based upon indicator dilution methods (Altman, 1961). Women and older persons have reduced TBW primarily because of having lower fat-free mass and increased body fat. Gender