BMR
BMR, or basal metabolic rate, is the energy your body spends at complete rest to keep organs, temperature and basic cell function going; it is the floor of your daily expenditure, not a calorie target.
BMR is what you would burn lying still in a dark room, fasted, doing nothing. For most adults it accounts for the majority of daily energy use, which surprises people who assume the gym is where the calories go.
What drives it
Total body mass is the single biggest predictor, and lean tissue costs more per kilo than fat does. Organs — liver, brain, kidneys, heart — are metabolically expensive and account for a large share despite their small weight. Age lowers it gradually, mostly through lost lean mass rather than anything mysterious about ageing itself.
Why it is not your calorie target
This is the mistake that sends people into trouble. A calculator returns a BMR of 1,500 and someone eats 1,500 kcal, having subtracted nothing for the fact that they also walked, worked and trained that day. The result is an unintentionally severe deficit, followed by the hunger, poor sleep and eventual abandonment that go with it.
Your intake target is built from total expenditure, which includes movement. BMR is only the first term in that sum.
How it responds to dieting
Losing weight lowers BMR, simply because there is less body to maintain. There is also a modest additional drop beyond what the size change predicts. It is real, it is measured in tens of calories rather than hundreds, and it is not the reason a diet stopped working.
Sources
- 1.Dietary Reference Values — European Food Safety Authority
- 2.Body Weight Planner — NIDDK, National Institutes of Health
Related reading
- Maintenance calories, and how to find yours
The intake that holds your weight steady, measured from three weeks of your own data rather than predicted. Why it is a band, and why it matters most.
- How to calculate your TDEE, and what it can't say
Four components, three equations, and one better method. Why every calculated TDEE is a hypothesis and how to replace it with your own measurement.
- How to calculate your calorie deficit properly
Calculators give you a starting guess. Your own two weeks of data give you the answer. The method, the maths, and why the 3,500-calorie rule overstates things.
- Daily weight lies. The trend does not.
Weigh every morning and read only the seven-day average. Why the daily number is mostly noise, and how to build a trend that tells you what to do next.