BMR is the largest single component of total daily energy expenditure (TDEE) for almost every adult, typically 60-70% of the total. The other components — thermic effect of food, exercise activity, and non-exercise activity — sit on top of BMR.
BMR is technically a strictly defined laboratory measurement: subject in thermoneutral environment, after a 12-hour fast, having slept the prior night, immediately after waking, lying still. The clinical-grade test (indirect calorimetry under those conditions) is rare outside research settings. In practice, BMR is estimated from formulas:
- Mifflin-St Jeor (1990) — the most-cited modern formula, validated repeatedly against indirect calorimetry. Uses weight, height, age, and sex.
- Harris-Benedict (revised 1984) — older, slightly less accurate on average but still widely used.
- Katch-McArdle — uses lean body mass instead of total body weight; more accurate when body fat percentage is known.
The everyday confusion is BMR versus RMR (resting metabolic rate). RMR is measured under less strict conditions (rested, but not fully fasted, not immediately on waking) and runs about 5-10% higher than true BMR. For practical calorie target setting, the difference is small enough to ignore.
BMR drops with sustained weight loss for two reasons: less body mass to support (mechanical), and the body's adaptive response to chronic energy deficit (physiological). The mechanical part is the larger factor and is non-controversial. The adaptive part is real but smaller than dieting folklore suggests.