What is Basal Metabolic Rate?

Knowledge is power. So here's a quick guide to what your Basal Metabolic Rate is, and why it matters.

Your Basal Metabolic Rate (BMR) is a measurement of the energy your body uses while at rest.

More precisely, your BMR is a measurement of your body’s energy needs when it is completely at rest, not digesting food, and at an ambient temperature.

When we think of the energy our body needs, we are typically thinking of energy for physical activity, but only about 20-30% of the Calories we consume provide energy for any form of physical exertion. Even when you’re not climbing Kilimanjaro, your body does a million and one other things. Your heart beats, your brain computes, your lungs expand and contract, your liver and kidneys do their thing, your nose hairs grow. All of this stuff requires energy, and this “necessary for life” energy requirement is what we call the Basal Metabolic Rate (BMR).

For your body to function at this base or basal level requires little more than a Calorie a minute, but with 1,440 minutes in a day those Calories do add up. An average man (if such a thing exists) is said to need 2,500 Calories a day, more than half of which goes to simply maintaining life. And if you’re not particularly active, your BMR can account for as much as 70% to 80% of your total daily energy use.

How to measure your Basal Metabolic Rate

BMR is properly measured under strict conditions, but a rough-and-ready way to work out your BMR is to use either of the following formulas:

BMR (Calories per day) = 24 x your body weight in kg

or BMR (Calories per day) = 10.9 x your body weight in lb

So if you weigh 70kg (154lb) your BMR would be 1,680 Calories a day or thereabouts.

This is only a rough estimate of your true energy needs while at rest, however, because weight alone cannot determine your true BMR. Fat mass is less metabolically active than muscle mass so the more muscle you have the greater your BMR. The more total mass you have, the higher your BMR is too. And as we get older, our BMR falls independently of the loss of fat-free mass that accompanies ageing - even if we remain as active as our younger selves. Nobody knows exactly why this happens, but it does. Genetics also play a part, with some people naturally having a higher or lower BMR than others for reasons science also doesn’t yet fully understand.

Some people mistakenly believe that consuming fewer Calories than required to satisfy your BMR will have adverse health consequences, but that’s not the case, or at least not the case in the short to medium term, assuming you’re in good health. If your BMR is 1,680 Calories and you consume 1,500 Calories, your body will get those missing Calories from its ample energy reserves (that fat about your person is good for something) or it will adjust your BMR so it requires less energy to function at a basal level.

You will also hear people talk about having a slow metabolism. What they really mean is they have a low BMR (and so require less energy to power their day). Similarly, talk of "speeding up your metabolism" more accurately refers to increasing your Basal Metabolic Rate.

REFERENCES

Basal Metabolic Rate - an overview (Science Direct)
How Can I Speed Up My Metabolism? (NHS)

Article last updated: 17th January 2021