The basal metabolic rate, or BMR, is a measurement of the rate at which a person's body uses energy at a moderate temperature, in the absence of strenuous physical activity, and without digesting food. It is related to a similar measure, the resting metabolic rate (RMR), which is an estimate of the amount of calories burned at rest. Essentially, the basal metabolic rate is a measure of the baseline caloric requirement of the body simply in order to maintain the regular functioning of the heart, the lungs, the brain, other internal organs, and the muscles and skin.
The BMR is important because it is a useful estimate of the amount of energy which the body absolutely must take in to maintain basic functions, either through food or through some combination of food and internal energy reserves (such as fat and muscle tissue, which can be converted to energy). That said, the RMR is more commonly calculated in many fitness settings, since it provides a similar estimate but the requirements for measuring it are less strict. In any case, one can add the basal rate with the amount of calories used to perform physical activity to derive the estimated total calories consumed in a day.
There are a number of factors which can cause the metabolic rate to fluctuate. People's energy consumption can be altered by disease, by the temperature of the environment, and stress. In addition, the basal metabolic rate generally falls with age, and also tends to fall with body mass (the loss of lean body mass, such as muscle, means the body requires less calories to maintain basic function; the same is not so true for fat mass). The process of measuring the caloric expenditure and the metabolic rate is known as calorimetry.
Overall, about two-thirds of the total caloric expenditure of the human body is accounted for by the basal metabolic rate. (The precise number will vary based on the level of additional physical activity.) About a further one-tenth goes to the digestive system as it processes food, and the remaining one-fifth, more or less, is used in physical activity. Internally, the brain and the liver are the most energy-hungry organs.
Because personal metabolisms do vary, personal calorimetry is the only way to accurately gauge an individual person's basal metabolic rate. However, several formulas do exist which can produce useful estimates. The most historically influential formulas were a pair of equations (one for men, and another for women) produced by Harris and Benedict just after the First World War. Twenty years ago this was succeeded by the Mifflin-St. Jeor equation. The Mifflin-St. Jeor equation states that the basal metabolic rate, in terms of calories per day, is equal to 10 times a person's mass (in kilograms) + 6.25 times height (in centimetres) - 5 times age, and then either +5 (for men) or -161 (for women). For instance, the equation predicts that a 30-year-old woman who weighs 120 pounds (54.5 kilograms) and is 5'2" (157.5 cm) would have a basal metabolic rate of (545 + 984 - 30 - 121), or 1378 calories.