Body composition changes with aging: the cause or the result of alterations in metabolic rate and macronutrient oxidation?

It has been well documented that as individuals age, body composition changes, even in the absence of changes in body weight. Studies have shown that fat mass increases and muscle mass decreases with age. However, it is unclear why such changes occur. Resting metabolic rate (RMR) and substrate oxidation rates have been examined with aging. It has been proposed that reductions in RMR and fat oxidation may lead to changes in body composition. Alternatively, changes in body composition with aging may lead to reductions in RMR. The purpose of this review is to provide an overview of the literature surrounding the impact of aging on RMR and substrate oxidation. Although long-term longitudinal studies are lacking, most cross-sectional studies or short-term longitudinal studies show a reduction in RMR with aging that cannot be explained by changes in body composition including loss in fat-free mass, where the latter includes atrophy or decreases in the mass of high metabolic rate organs. There is indirect evidence suggesting that the metabolic rate of individual organs is lower in older compared with younger individuals. With aging, we conclude that reductions in the mass of individual organs/tissues and in tissue-specific organ metabolic rate contribute to a reduction in RMR that in turn promotes changes in body composition favoring increased fat mass and reduced fat-free mass.