Resting metabolic rate (RMR/BMR)

Resting metabolic rate (RMR) is the energy you burn at complete rest, just to keep your core systems running. (It is sometimes used interchangeably with basal metabolic rate, or BMR, though BMR needs stricter fasting and temperature conditions.) That energy powers ion pumps, protein turnover, your heart and breathing, and temperature control. RMR usually makes up 60 to 70% of your total daily energy use (TDEE). It is measured by indirect calorimetry. Prediction equations (Harris-Benedict, Mifflin-St Jeor) carry systematic errors of 10 to 15%. RMR falls with age, mainly because you lose muscle to sarcopenia. The Baltimore Longitudinal Study of Aging tracked RMR dropping over time, sped up by chronic disease (Zampino et al. 2020). A doubly-labelled-water study of 6,421 people, aged 8 days to 95 years, found something striking. Body-composition-adjusted TDEE holds steady from about age 20 to 60, then falls about 0.7% a year. So the age-related RMR drop is mostly from losing fat-free mass, not from organs intrinsically slowing down (Pontzer et al. 2021). During calorie restriction, RMR falls more than lean-mass loss alone predicts. That is a defended adaptation that fights your weight loss. Martin et al. (2022) found about 60% of post-diet RMR drop is tissue loss, and about 40% is true metabolic adaptation. And epidemiology consistently links low fat-free mass to higher all-cause death.

Last reviewed: June 22, 2026

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