EFFECT OF SEX AND AGE ON BONE MASS, BODY-COMPOSITION AND FUEL METABOLISM IN HUMANS

The mechanism(s) governing the gain of upper-body fat and its relation ship to the decrease in bone mass with age is still unclear. Therefore , four groups of subjects matched for weight, height, and body mass in dex (Ir = 119; 60 women, 59 men), but differing in age (above and belo w 50 y) and sex were investigated using dual-energy x-ray absorptiomet ry (DXA) to assess body composition (bone, lean, and fat mass as well as its distribution) and indirect calorimetry to determine resting fue l metabolism. Fat mass of trunk and arms (P < 0.01), but not legs, inc reased with advancing age in males, resulting in a continuous increase in the ratio of upper- to lower-body fat (r = 0.45, P < 0.001). in co ntrast, total fat mass remained stable in women, irrespective of menop ause, but a redistribution of fat occurred with advancing age (r = 0.4 3, P < 0.001), resulting in a higher upper- to lower-body fat ratio (P < 0.05) in older than in younger women. Total lean soft-tissue mass o f all segments of the body was greater in men than in women irrespecti ve of age (P < 0.001), and lower in the older groups than in the young er ones irrespective of sex. Ln males, but not females, lean soft-tiss ue mass in arms and legs decreased (r = 0.57, P < 0.001), whereas the ratio of total fat to lean soft-tissue mass increased (r = 0.53, P < 0 .001) with age. Bone mineral content correlated with total body fat in both groups of women and in young males (r > 0.5, P < 0.001), but not in older males. With advancing age, the proportion of lean soft-tissu e mass occupied by total skeleton declined in women in = 59, P < 0.001 ), but remained stable in males. Resting energy expenditure decreased with age in both sexes. Protein end carbohydrate oxidation were simila r in all four groups of subjects. Total fat oxidation and fat oxidatio n per kilogram of lean soft-tissue mass decreased with age (r > 0.36, P < 0.01) in males, but not in females, whereas it increased with incr easing fat mass in females (I > 0.32, P < 0.03), but not in males. In contrast, fat oxidation per kilogram of fat mass decreased with fat ma ss in males (r = 0.61, P < 0.001), but not in females. Our results sug gest that aging affects body composition and fuel metabolism different ly in each gender, leading to reduced fat oxidation and accumulation o f upper-body fat with loss of striated muscle in men, and to an increa sed ratio of upper- to lower-body fat and bone loss in women, the latt er depending on fat mass. (C)Elsevier Science Inc. 1997.