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Fat-Free Mass

Body water volume, as a percentage of fat-free mass, is highest in infants and declines in older children (Fomon, 1967; Van Loan and Boileau, 1996). High body water volume is particularly evident in newborns, whose body water content of fat-free mass may exceed 75 percent (Fomon, 1967). Infants also have a relatively higher water content in the extracellular compartment and a lower water content in the intracellular compartment compared with older children (Van Loan and Boileau, 1996). Figure 4-1 presents total body water as a percentage of fat-free mass and body mass in children through the teenage years. Total body water as percentage of fat-free mass decreases during childhood, albeit more slowly than in infancy.

For adults, fat-free mass is approximately 70 to 75 percent water, and adipose tissue is approximately 10 to 40 percent water. With increasing fatness, the water fraction of adipose tissue decreases (Martin et al., 1994). Figures 4-2 and 4-3 provide the percentage of

FIGURE 4-1 Total body water as a fraction of body mass (FW) and as a fraction of fat-free mass (FWFFM). Reprinted with permission, from Van Loan and Boileau (1996). Copyright 1996 by CRC Press.

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