Basal metabolic rate

Basal Metabolic Rate

The Basal Metabolic Rate (BMR) is the rate of energy expenditure per unit time by endothermic animals at rest. It is reported in energy units per unit time ranging from watt (joule/second) to ml O₂/min or per unit mass of body weight, such as in g/hr or kg/day.

Definition and Measurement

BMR is measured under very restrictive conditions when a person is awake. An accurate BMR measurement requires that the person's sympathetic nervous system not be stimulated, a condition which requires complete rest. A more common measurement, which uses less strict criteria, is the Resting Metabolic Rate (RMR).

A person lying down in a relaxed state, with various monitoring devices attached to their body.

Factors Influencing BMR

BMR can vary greatly per individual depending on a number of factors. These factors include:

Age: BMR generally decreases with age. After 20 years, it drops about 2 percent per decade.

Body Size: Larger adults have a higher BMR than smaller adults.

Gender: Men generally have a higher BMR than women.

Genetics: Some people naturally have a higher BMR due to genetic factors.

Body Composition: Muscle cells require more energy than fat cells, so people with more muscle than fat will have a higher BMR.

Hormonal and Nervous Controls: Thyroid and stress hormones can increase the BMR.

Environmental Temperature: If temperature is very low or very high, the body has to work harder to maintain its normal body temperature, which increases the BMR.

Fasting/Starvation: Fasting or starvation can decrease the BMR.

Malnutrition: Malnutrition can lower the BMR.

Pregnancy: Pregnancy can increase the BMR.

Lactation: Lactation can increase the BMR.

BMR and Weight Loss

Understanding one's BMR is crucial for anyone trying to lose weight. This is because BMR represents the minimum amount of energy needed to keep your body functioning, including breathing and keeping your heart beating. Knowing your BMR can help you determine how many calories you need to either lose or maintain your weight. BMR can be used to calculate calorie needs using the Harris-Benedict principle. The Harris-Benedict equation can help you estimate your BMR.

BMR in Different Species

BMR is not only applicable to humans but also to animals. Different species have different metabolic rates. For example, birds have higher BMRs than mammals of the same size, while fish have lower metabolic rates than either group.