BMR (Basal Metabolic Rate) Explained

BMR Explained: Your Basal Metabolic Rate

Your Basal Metabolic Rate (BMR) is the energy your body burns just to keep you alive at complete rest — powering your heart, lungs, brain and the constant background work of your cells. It is the biggest single component of your total daily energy expenditure (TDEE), typically 60–70% of it, which makes it the foundation of understanding your calorie needs. This guide explains what sets your BMR, how to estimate it, and how to protect it after 40 — for anyone, of any sex.

BMR vs RMR

You will see two similar terms. BMR is measured under strict conditions (fully rested, fasted, lying down). RMR (Resting Metabolic Rate) is measured under less strict conditions and comes out slightly higher because it includes a little everyday activity. In practice the two are used almost interchangeably, and both describe the same idea: the calories you burn doing essentially nothing.

What Determines Your BMR

Lean body mass — the biggest modifiable factor. Muscle and organs are metabolically active tissue, so the more lean mass you carry, the higher your resting burn.
Body size — larger bodies (more total tissue to maintain) have higher BMRs.
Age — BMR tends to fall with age, largely because muscle mass declines.
Sex — on average men have higher BMRs than women, mostly because they typically carry more muscle; this is why prediction equations use sex as an input.
Genetics and hormones — thyroid function and inherited factors also nudge BMR up or down.

How to Estimate Your BMR

The most widely used equation is Mifflin-St Jeor, which uses weight (kg), height (cm) and age:

Men: BMR = (10 × weight) + (6.25 × height) − (5 × age) + 5
Women: BMR = (10 × weight) + (6.25 × height) − (5 × age) − 161

The result is your estimated resting calories per day. Multiply it by an activity factor to get your TDEE, as covered in the daily calorie needs guide. Remember it is an estimate: two people with identical stats can differ by a few hundred calories, so use the number as a starting point and let your real-world weight trend fine-tune it.

Can You Increase Your BMR?

You cannot change your age or height, but the largest modifiable lever — lean mass — is squarely in your control:

Build and keep muscle — resistance training is the most reliable way to add and preserve the metabolically active tissue that supports BMR. This is where creatine and progressive training earn their place.
Eat enough protein — protein supplies the raw material for muscle and has the highest thermic effect of the macros.
Avoid crash dieting — very aggressive, prolonged calorie deficits can lower resting expenditure beyond what weight loss alone predicts (adaptive thermogenesis) and tend to cost you muscle. A moderate deficit protects both.

Why BMR Falls After 40 — and What to Do

The decline in BMR with age is driven mostly by the gradual loss of muscle (sarcopenia), not by an inevitable “slowing metabolism” you are powerless against. The countermeasure is direct: resistance training and adequate protein preserve the muscle that keeps BMR up. Defending your lean mass is one of the most effective things you can do to keep your metabolism robust through the decades.

Frequently Asked Questions

What is BMR?
Basal Metabolic Rate — the calories your body burns at complete rest to keep its basic functions running. It is usually 60–70% of your total daily energy expenditure.

What is the difference between BMR and TDEE?
BMR is resting energy only. TDEE is your total daily burn, which adds the thermic effect of food, exercise and non-exercise movement (NEAT) on top of BMR.

How do I calculate my BMR?
The Mifflin-St Jeor equation using your weight, height, age and sex is the most widely used. Treat the output as an estimate and refine it against your actual weight trend.

Does BMR really slow with age?
It tends to fall, mainly because muscle mass declines. Resistance training and protein counter this by preserving lean tissue.

Can I speed up my metabolism?
The most reliable lever is building and keeping muscle. Extreme diets do the opposite — they can suppress resting expenditure and cost you muscle.

BMR (Basal Metabolic Rate) Explained — UK Guide