In this article, we examine the different factors that influence basal metabolic rate (BMR). Some of the factors include body temperature, age, sex, race, emotional state, climate and circulating levels of hormones such as catecholamine’s (epinephrine and norepinephrine) and thyroid hormones.
How do you define Basal Metabolic Rate?
Basal metabolic rate (BMR) refers to the amount of energy liberated by catabolism of food per unit time.
You can also define it as the energy released when the subject is at rest in a room with comfortable temperature and humidity, awake and sitting in a reclining position, 10-12 hours after the last meal. BMR basically, refers to the minimum energy required to maintain the respiration, heart rate, gastrointestinal function, muscle tone, kidney function etc.
Basal metabolic rate is measured under standard conditions 12 hours after a meal, at rest, in the absence of physical or emotional stressors, at a comfortable temperature (20 – 25 °C).
The BMR for a 70kg male is 70kcal/hr or 40kcal/m2/hr. In terms of oxygen consumption, it would amount to about 15 litre per hour.
Normal (physiologic) variations exist in BMR. For example, heavily built persons have higher BMRs, but the BMR per unit body weight is higher in the smaller built individuals.
Factors that Influence/Affect BMR:
There are numerous factors that affect the BMR. Some of these factors, which we mention below include body temperature, age, sex, race, emotional state, climate and circulating levels of hormones such as catecholamine’s (epinephrine and norepinephrine) and thyroid hormones.
1. Age: BMR reduces with age (new-borns have double the BMR of adults). BMR reduces with age i.e. it is inversely proportional to age. Children have higher BMR than adults. After 20 years, it drops about 2 per cent, per decade.
2. Gender: males have a 10 ‐ 15% higher metabolic rate than females due to greater skeletal muscle mass. Men have a greater muscle mass and a lower body fat percentage. Thus men have a higher basal metabolic rate than women.
3. Body Size: mainly due to differences in skeletal muscle mass, hence BMR usually calculated based on body surface area
4. Body Fat Percentage: The lower the body fat percentage, the higher the BMR. The lower body fat percentage in the male body is the main reason why men generally have a 10-15% higher BMR than women.
5. Sleep: reduces BMR by 10 ‐ 15% due to a reduction in skeletal muscle and CNS activity
6. Exercise:Physical exercise raises the BMR by building extra lean tissue and by reducing fat. Note that lean tissue is more metabolically demanding than fat tissue. So more calories are burnt even when sleeping.
7. Effects of Hormones on BMR
- Thyroxine stimulates oxidation and increases heat production within cells (maximal thyroxine secretion is associated with a rise in metabolic rate by 50 ‐ 100%).
- Catecholamines increase BMR Testosterone increases BMR by 10 ‐ 15% (mainly due to its anabolic effect on skeletal muscle mass).
- Growth hormone increases BMR by 15 ‐ 20% due to direct effect on cellular metabolism
- Pregnancy and lactation: BMR increases by about 15% during the latter stages of pregnancy
8. Effects of Nutritional status on BMR
- Postprandial: BMR rises by 4 ‐ 30% for 3 ‐ 12 hours after a meal (thermogenic effect of food). This is mainly due to oxidative deamination of food in the liver. Much greater effect for protein (known as the specific dynamic action of protein) than carbohydrate or fat.
- Malnutrition and Starvation: reduces BMR by 20 ‐ 30% due to a reduction in tissue metabolism
9. Climate: BMR is 10% ‐ 20% higher for individuals living in arctic regions compared to the tropics.
10. Genetic: Some people are born with faster metabolism and some with slower metabolism. For instance, higher BMR exists in individuals living in tropical climates.
11. Pathology: BMR increases by approximately 14% per degree of fever.
12. Trauma, burns and malignancy also increase BMR
Why is it important to measure the BMR?
The following are the significance of measuring the BMR.
First, determining the BMR is the principal guide for diagnosis and treatment of thyroid disorders. If BMR is less than 10% of the normal, it suggests moderate hypothyroidism. In severe hypothyroidism, the BMR may fall 40 to 50 percent below normal.
Second, BMR enables us to know the total amount of food or calories required to maintain body weight.
Third, abnormally low levels of BMR is suggestive of under-nutrition, starvation, hypothalamic disorders, lipoid nephrosis, and Addison’s disease.
Fourth, abnormally high levels of BMR is suggestive of fever, diabetes insipidus, polycythemia, and leukemia.
How to measure BMR?
Measurement of BMR employs calorimetry:
Direct Method: Subject placed in an Atwater ‐ Benedict chamber, which measures quantity of heat liberated from a subject. Complicated and difficult to perform.
Indirect Method: This method, uses oxygen consumption to calculate metabolic rate. The utilization of one liter of oxygen is equal to the production of 4.82 kcal of energy. Under this method, we use a Benedict ‐ Roth spirometer, Douglas bag or Max Planck spirometer to measure oxygen use.
Basal Conditions for measuring BMR
1. The person must be awake but at complete mental and physical rest.
2. The person should be without food for 12 hours. This is to avoid the specific dynamic action of food stuffs, the digestion and absorption of food stuffs, and also to avoid chances of starvation.
3. The person should be in recumbent or reclining position in bed
4. There should be normal environmental conditions (i.e. temperature, pressure, and humidity). Temperature should be between 20 – 25 °C.
Under the conditions given above, energy expenditure by the individual is to maintain muscle tone, circulations, respiration, visceral functions like kidney, liver, heart, brain, and for the maintenance of body temperature.