This is an excerpt from Physical Dimensions of Aging-2nd Edition by Waneen Spirduso,Karen Francis & Priscilla MacRae.
Determining age-related developmental changes in body size (i.e., height and weight) or weight relative to height (BMI) is inadequate for understanding the actual changes that occur, because the body is composed of many different tissues. Adults may have the same weight and height but have very different body compositions. One person may be very muscular and lean with a substantial percentage of the weight coming from metabolically active muscle tissue, whereas another may be very sedentary and pudgy with a large percentage of the weight accounted for by inert fat tissue. Even within individuals, changes in body composition occur over time with changes in nutrition, physical activity, and aging. A 40-year-old former athlete may actually weigh less than she did when she competed as a youth but, because of inactivity and 20 years of aging, may wear a skirt size larger than the one she wore in earlier years. She has traded muscle, which weighs more but is more compact, for fat, which weighs less but takes up more space.
Body weight is a general descriptor of an individual's total body mass, whereas body composition includes information on various components. The body, most often, is partitioned into two distinct compartments: fat mass and fat-free mass (McArdle et al., 2001). Fat mass (FM) plus fat-free mass (FFM), which is made up of water, protein, and bone mineral, equals total body mass. Many techniques are available to estimate FM and FFM, but all involve indirectly measuring either FM or FFM and deriving the other by subtraction. More specific descriptions of body fat, such as isolating internal abdominal or visceral fat from subcutaneous, appendicular, or truncal fat, can be made by more sophisticated and expensive laboratory instrumentation. Women and men differ significantly in relative quantities of specific body composition components in that the reference man is taller and heavier, his skeleton weight is greater, and he possesses a larger muscle mass and lower body fat content than the reference woman (table 3.4; McArdle et al., 2001).
Changes in body composition with aging have been increasingly recognized as a potentially modifiable factor in the quest for optimal health, function, and longevity. Although body composition, as well as the age-related changes in it, has a strong genetic component, it is also influenced by environmental factors. The primary influences are diet, disease, and physical activity. For example, failure to consume sufficient calories and protein in the diet can limit the development of muscle tissue or negatively influence the maintenance of muscle tissue, and failure to have adequate calcium in the diet has a major negative impact on bone formation and remodeling. Diseases such as osteoporosis drastically affect bone, and it has been well established that consistent, daily physical activity of a moderate intensity plays an important role in promoting bone health, maintaining muscle mass, and reducing the accumulation of body fat.
Age-related changes in body composition have important implications for successful aging. Such changes alter the pharmacokinetic and pharmacodynamic properties of drugs, so that dosages and schedules that are appropriate for young individuals are not appropriate for older adults. Because changes in body composition are related to disease and function, it is useful to monitor changes. For a variety of reasons, older adults eat less as they age, and it is easy for them to become undernourished. Careful monitoring of BMI and body composition can prevent malnutrition and simultaneously provide information about the effectiveness of maintenance, reduction, and weight gain programs.