Basal metabolic rate

Basal metabolic rate (BMR) is the lowest rate of energy use compatible with life: the energy spent on breathing, circulation, keeping ion gradients across cell membranes, making and repairing molecules, and running the brain. Because so many things change energy use, it is measured under strict conditions, in a person who is awake but lying still, has fasted for about 12 hours, is mentally relaxed and is in a room warm enough to need no shivering and cool enough to need no sweating. It is usually expressed in kilocalories or kilojoules per day.

For most people BMR is the largest part of daily energy expenditure, somewhere between about 45 and 70 percent, with the rest going on physical activity and on processing food. It depends mainly on body size and composition, because lean tissue such as muscle and organs burns far more energy than fat. It is therefore generally higher in men, falls with age as muscle is lost, and rises with thyroid hormone, fever, growth and pregnancy, while starvation lowers it.

BMR can be measured by indirect calorimetry, which works out heat production from oxygen use, but it is usually estimated from equations based on weight, height, age and sex. Nutritionists multiply it by an activity factor to estimate daily energy needs.