Greater skeletal muscle oxidative capacity is associated with higher resting metabolic rate: results from the Baltimore Longitudinal Study of Aging.

BACKGROUND Resting metabolic rate (RMR) tends to decline with aging. The age-trajectory of decline in RMR is similar to changes that occur in muscle mass, muscle strength and fitness but while the decline in these phenotypes have been related to changes of mitochondrial function and oxidative capacity, whether lower RMR is associated with poorer mitochondrial oxidative capacity is unknown. METHODS In 619 participants of the Baltimore Longitudinal Study of Aging, we analyzed the cross-sectional association between RMR (kcal/day), assessed by indirect calorimetry, and skeletal muscle maximal oxidative phosphorylation capacity, assessed as post-exercise phosphocreatine recovery time constant (τPCr), by phosphorous magnetic resonance spectroscopy. Linear regression models were used to evaluate the relationship between τPCr and RMR, adjusting for potential confounders. RESULTS Independent of age, sex, lean body mass, muscle density and fat mass, higher RMR was significantly associated with shorter τPCr, indicating greater mitochondrial oxidative capacity. CONCLUSION Higher RMR is associated with a higher mitochondrial oxidative capacity in skeletal muscle. This association may reflect a relationship between better muscle quality and greater mitochondrial health.