Understanding the mechanisms that govern sarcopenia (depletion of muscle mass with age) may suggest ways to preserve lean tissue and functional capacity, and to maintain quality of life in the elderly. We investigated the body-composition changes in normal aging in a cross-sectional study of 188 healthy volunteers aged 20-89 y, which examines the differences in body cell mass and fat as a function of age. In aging, the assumptions of indirect body-composition-measurement techniques, such as the "constant" hydration coefficient of lean body mass or the "constant" density of fat-free mass, may not hold. Therefore, we selected body-composition-measurement techniques that are not sensitive to assumptions about the composition of lean tissue. Cellular mass, lean body mass, and fat were assessed "directly" by total body potassium (TBK) measurements and neutron inelastic scattering. Our results show that TBK content declines at a rate of 7.20 +/- 1.00 mg K.kg body wt-1.y-1 for females (r = 0.601, P < or = 0.001) and 9.16 +/- 0.96 mg K.kg body wt-1.y-1 for males (r = 0.710, P < or = 0.001). Body fat measurements by neutron inelastic scattering have shown a significant increase of percentage body fat with age for female volunteers between the ages of 20 and 50 y and a continuous increase for male volunteers throughout adult life.