What is BMR?

What exactly does BMR describe?

The basal metabolic rate is the energy cost of vital functions such as breathing, circulation, maintaining body temperature and renewing cells. By definition it applies under conditions of complete rest: awake but still, on an empty stomach, in a calm environment. In most adults this baseline makes up roughly 60–70 per cent of total daily energy expenditure. In other words, most of the energy you spend during the day comes not from exercise but from the body's own workings.

The Mifflin-St Jeor equation

One of the most widely used tools for estimating BMR outside a laboratory is the Mifflin-St Jeor equation, published in 1990:

• Men: (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5
• Women: (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161

An example: for a woman of 70 kg and 170 cm who is 40 years old, the estimate is (700 + 1062.5 − 200 − 161), or around 1400 kcal a day. The figure shows that even a day spent without moving carries a noticeable energy cost.

Why do we call it an estimate?

The equation is based on averages drawn from large groups. Two people of the same weight, height and age can have different real values because of factors such as muscle mass, genetic make-up and hormonal status. The real figure may come out noticeably below or above what the formula gives. It is therefore soundest to treat a calculated BMR not as an exact truth but as a starting point to which you add your own observation. The guide to why calorie counts are never exact gives more detail on the nature of these calculations.

The main factors that affect BMR

• Muscle mass: muscle tissue uses more energy than fat tissue even at rest, so people with a higher proportion of muscle generally have a higher BMR.
• Age: BMR tends to decline gradually with age, largely because of the loss of muscle mass.
• Body size: a larger body runs more tissue and therefore spends more baseline energy.
• Hormonal status: factors such as thyroid function can affect metabolic rate; assessing conditions of this kind is a matter for a doctor.

Common misunderstandings about BMR

A few claims that circulate around BMR need putting in their place. Claims about foods and drinks that “speed up the metabolism”, such as chilli, green tea and coffee, are not entirely baseless, but the measured effects are so small within the daily total as to be negligible, and they do not last; strategies leaning on such products disappoint. A second common belief is that frequent small meals raise the metabolism noticeably; research has not shown a meaningful effect of meal frequency on energy expenditure at the same total intake. A third is the idea that going without food briefly “stops” the metabolism; the body's adaptive responses are real, but they work on the scale of prolonged and substantial restriction rather than of hours. The variable with a genuinely lasting effect on BMR is muscle mass: tissue maintained and built through regular strength training raises the floor of resting expenditure. That is a matter of long-term habits rather than short cuts.

What is BMR useful for on its own?

BMR is not a number used on its own in daily life; its real function is to form the basis for calculating total daily energy expenditure (TDEE). TDEE adds the cost of movement, exercise and digestion on top of BMR. You will find the whole picture in the TDEE guide and the energy balance guide. Where pregnancy, breastfeeding or a chronic illness is involved, it is important to make decisions about energy needs together with a doctor or dietitian.

Frequently asked questions

This guide draws on the Mifflin-St Jeor equation published in 1990 and on the general framework of basal metabolism set out in clinical nutrition sources.