basal metabolic rate

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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.

BMR

The basal metabolic rate is the minimum rate at which the body uses energy at complete rest. It is the minimum amount of energy needed to keep the body alive and is the largest component of an average person's daily energy expenditure. The BMR is usually expressed simply as kilocalories per day or in units of energy per unit surface area (or per kilogram body mass) per unit time. It is very difficult to determine the absolute minimum metabolic rate, but estimates are usually standardized by being made when a person is resting quietly after at least 8 hours sleep and 12 hours since the last meal. BMR for the average, healthy adult is usually between 1200 and 1800 kilocalories per day. It may, however, be less than 700 kilocalories in some individuals. BMR usually remains relatively constant for each individual, but it varies widely from one person to another. Factors which affect BMR include:

• AGE: BMR tends to decrease as we get older because of increased percentage body fat; children have a higher BMR because of the energy cost of growth
• SIZE: BMR tends to be greater in tall, thin people
• BODY COMPOSITION: Those with a high percentage lean body mass (low percentage fat) tend to have high BMRs because muscle is metabolically more active than fat
• BREAST-FEEDING: BMR is higher in nursing mothers than other women, because of the energy cost of synthesizing milk
• DIETING: BMR decreases during a weight-loss diet, fasting, or starvation
• EXERCISE: BMR increases with regular, very strenuous exercise, but is probably little affected by moderate levels of activity
• MALNUTRITION: BMR decreases during malnutrition
• PREGNANCY: BMR increases during pregnancy
• SEX: BMR is higher in males than females even at the same body weight because of their relatively large bulk of muscle
• STRESS: BMR increases during periods of emotional stress
• THYROID ACTIVITY: BMR is regulated by a hormone, thyroxine, secreted by the thyroid gland. It increases if the thyroid is overactive and decreases if the thyroid is underactive
• WEATHER: BMR increases during both very cold and hot weather.

People trying to lose weight often want to increase their BMR as an easy way of burning excess fat. Many manufacturers entice dieters to buy slimming products that claim to increase BMR (see kelp); most of these claims are not supported by rigorous scientific evidence. Drugs that can increase basal metabolic rate should be taken only under medical supervision.

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

See energy balance; metabolism; thyroid gland.

BMR

The minimal rate of metabolism of an individual at complete rest, at normal body temperature who is not digesting or absorbing food. BMR represents the lowest rate of energy use that can sustain life. It is usually expressed in units of energy per unit surface area per unit time. BMR is estimated when the subject is resting quietly in a laboratory under optimal conditions, after at least 8 h sleep and 12 h since the last meal. BMR for adults is between 1200 and 1800 kcal day⁻¹. It is relatively constant for a particular individual (although it may be increased by regular intensive exercise), but varies widely among different people. Factors affecting BMR include age, body size, body composition (especially the relative amount of fatty and lean body mass), and sex.

abbr.: BMR; the rate of energy metabolism of an animal at rest but not asleep. It is measured with the animal in a 'comfortable' ambient temperature, and in the postabsorptive state, at least 12 hours after taking food. It may be determined from the oxygen consumption and carbon dioxide (and urinary nitrogen) excretion, or from the heat production of the animal.

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Basal metabolic rate.

(bā′səlmet′ə-bol′ik)
n
BMR

The basal rate, or energy exchange, determined by means of a clinical test of oxygen consumption in a subject who has had a good night’s rest, has fasted for 12 to 14 hours, and has been physically, mentally, and emotionally at rest for 30 minutes; usually indicated as a percentage of the normal calorie production per surface area, the normal values ranging between plus and minus 20%.

For a list of words related to basal metabolic rate, see:

• Physiology - basal metabolic rate: BMR; metabolic rate when subject is awake but at mental and physical rest, at a comfortable temperature, and without food for at least twelve hours

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