Can you actually change your metabolism?

Metabolism refers to all the chemical processes keeping you alive, but in everyday use it means how many calories you burn. Total daily energy expenditure has three parts: basal metabolic rate, the energy used at complete rest, which accounts for roughly 60 to 70 percent of the total; the thermic effect of food, about 10 percent, spent digesting and processing what you eat; and physical activity, covering both deliberate exercise and everyday movement. Body size and lean mass are the dominant determinants of the resting portion, which is why larger people burn more calories at rest and why the same weight loss reduces energy needs.

A large study published in Science in 2021, analyzing data from thousands of people across a wide age range, found a pattern that contradicts the common narrative. After adjusting for body size and composition, metabolic rate rises steeply through infancy, declines gradually to about age 20, then stays essentially flat from 20 to 60 before declining slowly afterward. The widely believed idea that metabolism slows noticeably in your thirties was not supported. Weight gain in middle age is therefore more plausibly explained by reduced activity and increased intake than by a metabolic shift.

What genuinely moves the number is limited. Building muscle raises resting expenditure, though the effect per pound of muscle is smaller than commonly claimed. Non-exercise activity thermogenesis, meaning fidgeting, standing, and general movement, varies substantially between individuals and can account for meaningful differences. Severe caloric restriction produces adaptive thermogenesis, a downward adjustment that persists and is part of why weight regain is common. What does not work: metabolism-boosting supplements, eating many small meals rather than fewer larger ones, which studies find makes no difference to total expenditure, and green tea or spicy food, whose effects are real in laboratory measurement and trivial in practice.