Influence of aerobic capacity, body composition, and thyroid hormones on the age-related decline in resting metabolic rate

Abstract

It has been suggested that changes in fat-free weight may not fully explain the decline of resting metabolic rate (RMR) that occurs with aging. We therefore examined the hypothesis that a reduction in maximal aerobic capacity (V o 2max) may partially explain the lower RMR in older men, after accounting for differences in fat-free weight and fat weight. We also considered differences in energy intake and plasma thyroid hormones as possible modulators of the age-related decline in RMR in men. Three-hundred healthy men (aged 17 to 78 years) were characterized for: (1) RMR (kcal/min) from indirect calorimetry; (2) body composition from underwater weighing; (3) maximal aerobic capacity from a test of V o 2max; (4) plasma thyroid hormones (total triiodothyronine [T 3], free T 3, total thyroxine [T 4], and free T 4); and (5) estimated energy intake (kcal/d) from a 3-day food diary. A curvilinear decline of RMR with age was found ( P < .01), in which no relationship was found in men less than 40 years of age ( r = .10, slope = 0.002 kcal/min/yr), whereas in men older than 40 years, RMR was negatively related to age ( r = −.52, slope = −0.008 kcal/min/yr). After statistical control for differences in fat-free weight and fat weight, a negative relationship between age and RMR persisted (partial r = −.30, P < .01). It was only after control for fat-free weight, fat weight, and V o 2max (partial r = −.10, P > .05) that no association between age and RMR was noted. When subgroups of younger and older individuals were paired for age and fat-free weight, but different V o 2max values, a higher RMR was noted in the trained younger (8%) and trained older men (11%) compared with untrained younger and older men, respectively. Thyroid hormones and daily energy intake were negatively related to age on a univariate basis, but no independent association was noted after control for fat-free weight. We conclude that, in addition to the loss of fat-free weight, the decline in V o 2max is an additional factor associated with the decline of RMR in aging men. Age-related changes in energy intake, fat weight, and thyroid hormones appear to be less important in explaining the reduction of RMR in aging men. Maintenance of fat-free weight and V o 2max by regular physical activity may attenuate the age-related RMR in healthy men.