To determine whether the age-related reduction in basal metabolic rate (BMR) is explained by a quantitative and/or qualitative change in the components 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 dual-energy X-ray absorptiometry (DEXA), which permitted four compartments to be quantified [bone mineral mass, fat mass (FM), appendicular lean tissue mass (ALTM), and nonappendicular lean tissue mass (NALTM)]. Absolute BMR and ALTM were lower, whereas FM was significantly higher in the older, compared with young, subjects. BMR, adjusted for differences in FM, ALTM, and NALTM, 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 ALTM and NALTM, compared with young subjects. Hence, the age-related decline in BMR is partly explained by a reduction in the quantity, as well as the metabolic activity, of DEXA-derived lean tissue components.

Objectives: To assess the accuracy of the Scho®eld, Scho®eld & James (1985) equations and those of Hayter & Henry (1994) for the prediction of the basal metabolic rate (BMR), of young Australians. Design: BMR was measured by indirect calorimetry, while fat free mass (FFM) and fat mass (FM) were measured by bioelectric impendence analysis (BIA) in 128 volunteers (39 men and 89 women), aged between 18 and 30 y. Setting: Deakin Institute of Human Nutrition, Deakin University, Melbourne, Australia. Results: The measured BMR of Australian men and women were signi®cantly lower (P 0.001) than the predicted BMR using the Scho®eld et al (1985) equation, with a mean (s.d.) bias (bias measured 7 predicted BMR) of 7406 (513) kJ/d in men and 7124 (348) kJ/d in women. The measured BMR of Australian men and women were similar to the predicted BMR using the equations of Hayter & Henry (1994) and bias was unrelated to body weight. BMR adjusted for FFM and FM was signi®cantly higher by three percent in women on oral contraceptive agents (OCA) as compared to those not on OCA. Conclusions: The Scho®eld et al (1985) equations are not valid for the prediction of BMR of young Australian men and women. The equations of Hayter & Henry (1994) for North Europeans and Americans, provide an accurate estimate of the BMR of Australian men and women at the group level. However, in young women not using OCA a correction factor of 0.97 applied to the predicted BMR provides a better estimate.