Effects of body composition on age- and sex-related differences in resting metabolic rate from a healthy aging cohort

Resting metabolic rate (RMR) accounts for the majority of the total energy expenditure. While RMR is known to decline with advancing age and is recognized to be lower in females compared to males, the mechanisms underlying these differences remain unclear. Changes in body composition are posited to account for age- and sex-related differences in RMR, but to what extent lacks consensus. We characterized 80 healthy adults aged 23-82 years from the San Diego Nathan Shock Center (SD-NSC) clinical cohort for body composition and RMR using dual-energy x-ray absorptiometry (DXA) and indirect calorimetry techniques, respectively. Body composition metrics-body surface area (BSA), lean tissue mass (LTM), and total body fat (TBF)-were modelled as predictive variables to assess their explanatory power against the age and sex effects on RMR. We found that the negative association between RMR and age persists even after adjusting for body composition and sex, with a predicted decrease in RMR per decade of 62.6 kcal/day (ꞵ = -62.6, P < 0.0001). While individual body composition metrics do not account for the observed sex differences in RMR, adjusting for all body composition metrics together explained the observation that females have lower RMR compared to males. Our results suggest that while differences in body composition can explain sex differences in RMR, additional factors independent of body composition contribute to age-related differences. These results provide new insights into RMR differences among healthy individuals across the human life-course, which may inform age-appropriate interventions for optimizing metabolism.