What is BMR?

BMR or basal metabolic rate is defined as the amount of energy output per unit time by a person who is awake and lying at complete physical and mental rest in a comfortable environment and in post absorptive state (i.e., 12-14 hrs after the last meal).

What is basal metabolism?

Basal metabolism refers to the energy output of a person at basal condition. Basal condition means the perfect condition of complete physical and mental rest. Basal metabolism is expressed in terms of basal metabolic rate or BMR. Since heat is the final form of all energies, the energy output is expressed in terms of heat i.e., in kilocalorie (kcal) units. Moreover, since the value of BMR varies with the surface area of the person, the BMR is usually expressed as heat production per hour for each square meter of body surface area i.e., kcal/hr./sq.m.

What is normal values of BMR?

For normal healthy adult males, the value of BMR is about = 40 kcal/hr/sq.m.; while for normal healthy adult females it is about = 37 kcal/hr/ sq.m.

Implication of BMR

i) BMR indicates the minimum energy required for maintaining the essential activities of life like heart beats, breathing etc.
ii) It helps in determining the calorie requirement of an individual for dietary planning.
iii) BMR was earlier used for quantitative estimation of thyroid function.

Factors affecting BMR

Following are the affecting factors of BMR.

Physiological factors

The physical factors affecting the BMR are: -
i) Age : In children, BMR is higher than adults. This is due to the growth of children for which extra energy is required. In old age, BMR is reduced.
ii) Sex: BMR is less in females than in males. The higher BMR of males is attributed to the male sex hormones (androgens) which increase the BMR.
iii) Surface area of body : BMR is directly proportional to body surface area.
iv) Climate and races : Races and residents of cold climate have higher BMR than the tropical races dwelling at warm climate.
v) Nutritional status : Under nutrition or starvation ultimately lowers the BMR.

vi) Pregnancy and lactation : BMR is increased in pregnancy because of the additive effect of the foetal metabolism. Lactating mothers also have higher BMR due to the extra energy required for milk secretion.
vii) Habit: Athletes and manual laborers have higher BMR than persons leading a sedentary life.
viii) Hormones : Adrenal and thyroid hormones increase the BMR considerably. Androgens and posterior pituitary hormones also raise the BMR to a smaller extent.
ix) Body temperature : For every 1°C rise of body temperature, BMR rises by 10-12%.

Pathological factors

i) BMR is lowered in hypothyroidism, malnutrition, fasting, obesity etc.,
ii) BMR is raised in hyperthyroidism, fever, diabetes insipidus, leukemia, polycythemia etc.
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