Resting metabolic rate (RMR/BMR)

Resting metabolic rate (RMR) — sometimes used interchangeably with basal metabolic rate (BMR), though BMR requires stricter fasting and thermoneutral conditions — is the energy expended at complete rest to maintain core physiology: ion-pump activity, protein turnover, cardiac and respiratory work, and thermoregulation. RMR typically accounts for 60–70% of total daily energy expenditure (TDEE) and is quantified by indirect calorimetry; predictive equations (Harris–Benedict, Mifflin–St Jeor) carry systematic errors of ±10–15%. RMR declines with age primarily because skeletal muscle is lost through sarcopenia; the Baltimore Longitudinal Study of Aging documented longitudinal RMR decreases accelerated by chronic disease (Zampino et al. 2020). A doubly-labelled-water study across 6,421 participants aged 8 days to 95 years found that body-composition-adjusted TDEE is stable from roughly age 20 to 60, then falls ~0.7% per year, attributing the age-related RMR drop largely to fat-free mass loss rather than an intrinsic organ-metabolism slowdown (Pontzer et al. 2021). During caloric restriction, RMR falls beyond what lean-mass loss predicts — a defended adaptation that impedes weight loss; Martin et al. (2022) quantified that ~60% of post-diet RMR suppression reflects tissue loss and ~40% true metabolic adaptation. Epidemiological data consistently link low fat-free mass with higher all-cause mortality risk.