Factors Affecting Body Composition

Body Composition and Age

Most investigators agree that body weight becomes progressively more difficult to maintain with age. As an example of this phenomenon, Williams (1997) recently argued that body weight and associated circumferences will increase with advancing age unless food intake is significantly reduced or physical activity levels are substantially increased (even in otherwise active individuals). Nevertheless, the Dietary Guideline Committee (1995) could find no rationale to liberalize the upper BMI range consistent with good health as individuals increase in age.

A large number of cross-sectional studies demonstrate an increase in fatness (percent body fat) with age, independent of body weight change, in women (Forbes, 1987; Gallagher et al., 1996b). In a study of non-exercising civilian women, the mean percent body fat of women with BMI of 25 increased from 30 between the ages of 17 and 20 to 34 for women 40 and older (Table 2-4) (Gallagher et al., 1996a). The implication of this is that lean body mass decreases with age.

Currently, the Army and the Air Force set their body fat limits according to age, while the Navy and Marine Corps standards are age-independent. The maximum body fat for Army women increases from 30 percent for women between the ages of 17 and 20 to 36 percent for women 40 and older. The Air Force sets an upper limit of 28 percent for women 29 years of less and 32 percent for women 30 and older. To date, no longitudinal studies have been performed with career military personnel to examine the effect of body weight standards on long-term weight and body fat stability in these individuals.

Genetic Influences on Body Composition

Adoption and twin studies that have assessed the relationship between body composition and genetics (Bouchard, 1996, 1997) have provided heritability estimates that are quite varied. In studies of monozygotic twins, BMI heritability estimates were reported in the range of 40 to 70

TABLE 2-4 Effects of Age on Relationship between Body Mass Index* and Percent Body fat

Age (years)

Body Mass Index

Percent Body fat

17–20

25

30

21–27

25

31

28–39

25

32

40+

25

34

* Weight in kilograms divided by the square of height in meters.

SOURCE: Adapted from Gallagher et al. (1996a).



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