Metabolic rate: what actually determines it

Basal metabolic rate — the energy your body burns at complete rest just maintaining basic functions — accounts for the large majority of most people's total daily energy expenditure, generally more than exercise itself.

Muscle mass is the single largest modifiable driver of basal metabolic rate: muscle tissue burns meaningfully more calories at rest than fat tissue, which is a major reason resistance training is emphasized in metabolic health discussions beyond its strength benefits alone.

Age-related metabolic slowdown is real but smaller and later-onset than commonly assumed — research has found metabolic rate relatively stable through much of adulthood, with a more measurable decline generally appearing after age 60, and much of the earlier perceived slowdown tracking more closely with muscle loss and reduced activity than age itself.

Severe, prolonged calorie restriction can trigger metabolic adaptation — the body reducing energy expenditure beyond what body composition changes alone would predict — which is one argument among several for moderate, sustainable calorie deficits over aggressive short-term restriction.