BMR and resting metabolism: what your body burns at rest

Your BMR, or resting metabolism, is the number of calories your body burns when you do nothing. For an 80 kilo man, that is about 1,780 kcal per day, just to breathe, pump blood and stay warm. In most people, that is 60 to 70 percent of their daily burn.

I think BMR is the most underrated number in nutrition. Most of your burn happens while you lie on the couch, not in the gym.

What is your BMR or resting metabolism?

Your BMR is the energy your body needs at complete rest. Think of your heart, your brain, your liver and your kidneys working around the clock. That base burn accounts for 60 to 70 percent of all the calories most people use in a day. Movement comes on top of it.

Your basal metabolism is driven mostly by your fat-free mass, so your muscles and organs. More muscle mass means a slightly higher resting metabolism.

That is why a muscular athlete burns more at rest than someone of the same weight with less muscle.

How do you calculate your BMR?

The most used formula is Mifflin-St Jeor. For men: 10 times your weight in kilos, plus 6.25 times your height in cm, minus 5 times your age, plus 5. For women, the sum ends on minus 161 instead of plus 5. A woman of 65 kilo, 168 cm and 30 years lands around 1,390 kcal.

The formula by Mifflin and St Jeor (1990) predicts the resting metabolism of healthy adults most accurately. The table below gives a few examples.

If you would rather not calculate it, use our calorie needs calculator. It works out your BMR and your daily burn in one go.

What is the difference between BMR and TDEE?

Your BMR is your burn at rest. Your TDEE is your total burn across the whole day, so it includes movement, sport and the energy your food costs to digest. Your TDEE is always higher than your BMR. You calculate it by multiplying your BMR by an activity factor between 1.2 and 1.9.

For that 80 kilo man with a BMR of 1,780 kcal, the TDEE at three to five sessions a week sits around 2,760 kcal. How you choose that factor is in calculating your calorie needs.

In short: BMR is the engine idling, TDEE is the engine on the road.

Why does your metabolism drop when you diet?

When you lose weight, your metabolism drops for two reasons. You are lighter, so you burn less, and your body starts running more efficiently to soften the deficit. That is called metabolic adaptation. In athletes who diet for a long time, the burn can drop more than weight loss alone would predict.

A review by Trexler and colleagues (2014) describes how hormones and energy expenditure adapt under energy restriction, which slows weight loss and drives regain.

Take two men of 80 kilo, both with a calculated BMR of 1,780 kcal. One has dieted for three months, the other eats at maintenance. The dieter burns noticeably less at rest than those 1,780 kcal, purely through adaptation.

That is why maintenance breaks and enough protein work so well during a cut. They keep your metabolism and your muscle mass up. More on that in thyroid and weight.

Can you measure your metabolism in your blood?

Your blood does not measure your BMR in calories, but it does show whether your metabolism runs normally. Your thyroid is the main dial here. Values like TSH and free T4 show whether your thyroid slows your burn or speeds it up. A deviating value is one to discuss with your GP.

If your metabolism feels slow and you struggle to lose fat, your thyroid is worth a check. Read how it affects your performance in thyroid and your metabolism.

If you want your base in view, our 360 health blood test measures your thyroid values among others. That way you know your engine runs normally before you tinker with calories.

My advice: use your BMR as a fixed anchor, not a number to adjust daily. Measure your weight over weeks and adjust your food to the trend. For the full approach to nutrition around your training, read our plan for muscle growth and recovery.

References

1. Mifflin MD, St Jeor ST, Hill LA, et al. A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition. 1990. PMID: 2305711.
2. Trexler ET, Smith-Ryan AE, Norton LE. Metabolic adaptation to weight loss: implications for the athlete. Journal of the International Society of Sports Nutrition. 2014. PMID: 24571926.
3. Gezondheidsraad. Dietary reference values: energy. 2021.

Disclaimer

Every blood test result includes a professional assessment by a BIG-registered doctor. This article gives general information and is not a substitute for medical advice. A blood test is a tool to walk into the conversation with your GP better informed, not a diagnosis in itself. For treatment decisions, discuss your results with your GP.