Basal Metabolic Rate, or BMR, is the number of calories your body burns at complete rest to keep its essential processes running. Heart beating, lungs breathing, kidneys filtering, brain firing, cells dividing. BMR is what you would burn if you stayed in bed all day in a warm dark room.
The full picture
Your organs are surprisingly metabolic. The brain alone burns around 300 to 500 calories per day. The liver, heart, and kidneys together burn another 1000 calories per day. These organs do not stop just because you are not moving, which is why BMR is the largest single component of Total Daily Energy Expenditure.
BMR scales with body size, lean tissue, sex, and age. Bigger people burn more. Men burn more than women of the same weight because they carry more lean mass on average. Older adults burn less, partly from muscle loss and partly from cellular changes. By age 60, BMR is typically 10 to 15 percent lower than at age 30 in untrained populations.
Crash diets can drop BMR meaningfully. After 6 to 12 months of aggressive caloric restriction, BMR sits 10 to 15 percent below what equations predict for your new bodyweight. This is adaptive thermogenesis, and it is one of the main reasons aggressive cuts produce post-diet rebound weight gain.
How it is calculated
Four equations dominate practice. Mifflin-St Jeor uses weight, height, age, and sex and is the modern default for the general population. Harris-Benedict (revised 1984) uses the same inputs but slightly different coefficients and tends to overestimate in obese populations. Katch-McArdle bypasses sex adjustments by using lean body mass directly, so it needs a body fat percentage. Cunningham is similar to Katch-McArdle but uses different coefficients and is preferred for athletes.
Mifflin-St Jeor formula. Men: 10 × kg + 6.25 × cm − 5 × age + 5. Women: 10 × kg + 6.25 × cm − 5 × age − 161.
Common misconceptions
- BMR does not predict how easily you gain or lose weight. NEAT and TEF vary far more between people than BMR does at the same body size.
- You cannot meaningfully "boost BMR" with green tea, cayenne, or supplements. Effects are real but tiny, in the 30 to 80 calorie per day range at best.
- BMR does not collapse from skipping breakfast or eating less often. Meal timing has almost no measured effect on resting energy expenditure.
Citations
- Mifflin MD et al. (1990). A new predictive equation for resting energy expenditure in healthy individuals. AJCN, 51(2), 241-247.
- Cunningham JJ. (1991). Body composition as a determinant of energy expenditure. AJCN, 54, 963-969.
- Rosenbaum M, Leibel RL. (2010). Adaptive thermogenesis in humans. Int J Obes, 34, S47-S55.