basal metabolic rate The energy output of the body when fasting and completely at rest. All the constituent living cells are continuously metabolizing — releasing energy by the action of enzymes on chemical substrates derived ultimately from food. They use that energy for the synthesis or breakdown of vital substances, for maintaining the integrity of the cell by regulating influx and efflux across the membrane which encloses it, and for carrying out their own specialized functions; they release the rest as heat. Some energy is converted into the work of muscle contraction: at rest mainly the heart beat and the breathing muscles. Because the metabolism of the vast majority of body cells requires and consumes oxygen, the BMR can be estimated by measuring the rate at which oxygen is taken up and calculating the energy equivalent. A typical value for a man could be 350 litres of oxygen = 1700 kcal (7000 kJ) per day; or just under 0.25 litre, and just over 1 kcal per minute. Rigorous conditions for basal measurements are difficult to attain; more often the metabolic rate is measured when simply at rest; it varies with body weight and composition, but also between individuals who are similar in these respects.

Stuart Judge

basal metabolic rate (BMR) The energy cost of maintaining the metabolic integrity of the body, nerve and muscle tone, respiration, and circulation. It depends on the amount of metabolically active body tissue, and hence can be calculated from body weight, using different factors for males and females, and at different ages. For children the BMR also includes the energy cost of growth. Experimentally, BMR is measured as the heat output from the body, or the rate of oxygen consumption, under strictly standardized conditions, 12–14 hours after the last meal, completely at rest (but not asleep) and at an environmental temperature of 26–30 °C, to ensure thermal neutrality. Measurement of metabolic rate under less rigorously controlled conditions gives the resting metabolic rate (RMR).

For people with a sedentary lifestyle and relatively low physical activity, BMR accounts for about 70–80% of total energy expenditure. The energy costs of different activities are generally expressed as the physical activity ratio, the ratio of energy expenditure in the activity to BMR.

basal metabolic rate (BMR) The rate of energy metabolism required to maintain an animal at rest. BMR is measured in terms of heat production per unit time and is usually expressed in kilojoules of heat released per square metre of body surface per hour (kJ m^–2 h^–1). It indicates the energy consumed in order to sustain such vital functions as heartbeat, breathing, nervous activity, active transport, and secretion. Different tissues have different metabolic rates (e.g. the BMR of brain tissue is much greater than that of bone tissue) and therefore the tissue composition of an animal determines its overall BMR. For any comparable group of animals (such as mammals) BMR is proportional to body weight according to the allometric equation (see allometric growth); small animals tend to have a higher metabolic rate per unit weight than large ones.

basal metabolic rate (BMR) Minimum amount of energy required by the body to sustain basic life processes, including breathing, circulation, and tissue repair. It is calculated by measuring oxygen consumption. Metabolic rate increases above BMR during physical activity or fever, or under the influence of some drugs (including caffeine). It falls below BMR during sleep, general anaesthesia or starvation. BMR is highest in children and decreases with age. See also metabolism

ba·sal met·a·bol·ic rate • n. the rate at which the body uses energy while at rest to keep vital functions going, such as breathing and keeping warm.DERIVATIVES: ba·sal me·tab·o·lism n.