Factors Affecting Basal Metabolic Rate (BMR)

The factors affecting Basal Metobolic Rate (BMR) are as follows:

1. Age: The BMR is greatest during first few years of life. Later it gradually falls throughout the remaining life.
2. Sex: Females have been found to have 2 to 12% less BMR than males. The difference is more marked in older age groups. The BMR of women undergoes change in relation to the menstrual cycle. It is more prior to and less after the menstruation.
3. Climate: The Western people on going to tropics show a fall in BMR.
4. Race: It was found that the Chinese living in the United States have less BMR than American of European origin. However, race is only a minor factor in determining BMR.
5. Body Temperature: With a rise in body temperature by each degree F, BMR increases by 7%. Thus a patient with temperature of 105oF would have about 50% more BMR than at 98oF on account of his fever.
6. Diet: Vegetarians have 11% less BMR than meat eaters. Eskimos who ingested a diet rich in proteins have higher BMR. BMR falls after prolonged starvation. Thus any wasting illness associated with a decreased food intake results in a lowered BMR.
7. Hormones: The thyroid hormone raises the BMR. The anterior pituitary gland can affect BMR through its thyroid stimulating hormone (TSH). Adrenaline and Noradrenaline also raises BMR.
8. Sleep: The greater relaxation of muscles during sleep than during walking time decreases BMR by about 10%.
9. Pregnancy: BMR is increased in pregnancy during the third trimester.
10. Habits: Individual who habitually indulge in muscular exercise e.g. athletes and laborers have higher BMR than sedentary workers.
11. Drugs: Dinitrophenol (DNP), caffeine, nicotine and Benzedrine increase the BMR. DNP was used for weight reduction but was found to be toxic and is no longer used.

   Variations of BMR in Diseases

1. Conditions associated with a raised BMR
2. Endocrinopathies, such as hyperthyroidism, Cushing’s Syndrome, Pheochromacytoma and diabetes insipidus: In diabetes insipidus, the raised BMR is due to shivering which is brought about by an excessive water intake having a temperature usually much below the body temperature.
3. Leukemias and polycythemia: In these disease, there are more blood cells undergoing metabolic activity.
4. Fevers: These act through raising body temperature.
5. Luft’s Syndrome: There is some derangement in mitochondrial function.
6. Conditions associated with a decreased BMR
7. Endocrinopathies Such as hypothyroidism, Addison’s disease and hypopituitarism.
8. Shock

   Clinical Interpretation of BMR

On account of the many factors which influence the BMR, this test is not very sensitive. A deviation of 15% (low or more) is considered to be within normal limits. Done correctly, it is of help in the diagnosis of thyroid disease. It is at present a common practice to measure BMR while the patient is under the influence of some tranquilizing drug which enables him to relax both mentally and physically.