Is there evidence for an age-related reduction in metabolic rate?

To determine whether the age-related reduction in basal metabolic rate (BMR ) is explained by a quantitative and/or qualitative change in the component s of lean tissue, we conducted a cross-sectional study in groups of young(n = 38, 18-35 yr) and older (n = 24, 50-77 yr) healthy individuals. BMR was measured by indirect calorimetry. Body composition was obtained by using du al-energy X-ray absorptiometry (DEXA), which permitted four compartments to be quantified [bone mineral mass, fat mass (FM), appendicular lean tissue mass (A(LTM)), and nonappendicular lean tissue mass (NA(LTM))]. Absolute BM R and A(LTM) were lower, whereas FM was significantly higher in the older, compared with young, subjects. BMR, adjusted for differences in FM, A(LTM), and NA(LTM), was significantly lower in the older subjects by 644 kJ/day. In separate regression analyses of BMR on body compartments, older subjects had significantly lower regression coefficients for A(LTM) and NA(LTM), co mpared with young subjects. Hence, the age-related decline in BMR is partly explained by a reduction in the quantity, as well as the metabolic activit y, of DEXA-derived lean tissue components.