Basal Metabolic Rate for Adults

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Your body is burning energy all the time, not just when you are physically working or exercising; even when you are resting or asleep. The Basal Metabolic Rate (or BMR) is this background rate of energy consumption, just to keep your breathing, circulation and metabolism ticking over. BMR is closely related to Resting Metabolic Rate (RMR) which is a similar measure but including body processes that are not essential, at least in the short term, such as the digestion of food. This distinction is probably more important in scientific studies than in estimating the amount of calories you need on a daily basis.

For most people the BMR accounts for the majority of the calories burned. As you get older, other things being equal, your BMR will decrease. This partly explains why there is a tendency to put on weight as you get older (although a much more important cause is that we tend to exercise less as we age). The body controls the rate of metabolic energy consumption mainly through the hypothalamus, which is located in the brain stem. This process is completely autonomous although it can be affected by our mood, stress or excitement, and also by the environment as the body maintains a constant temperature.

The formula for basal metabolic rate uses variables such as your gender, height, weight and age to predict the speed at which you burn calories when at rest. It does not take into account your body fat composition. In reality a person of heavy muscular build would have a higher BMR than a person of the same weight who was just carrying more fat. The body requires an additional 16 calories daily for each pound of lean muscle, or 35 calories per kilogram. The difference in the formulae for men and women is mainly due to the different amounts of fat tissue in the male and female anatomy.

To compute your daily energy needs you also need to take into account your activity level using the Harris-Benedict formula. This multiplies the BMR by a factor to take into account activity, for example 1.2 for a sedentary person, up to a factor of two or more for an athlete. As lifestyles have changed, a slightly more accurate formula proposed by Mifflin has replaced the Harris-Benedict formula for BMR.