BMR and Its Importance

Basal metabolic rate or BMR is the amount of daily energy expended by humans and other animals at rest. It is the minimum calorific requirement needed to sustain life in a resting individual. The rest is defined as existing in a neutrally temperate environment while in the post-absorptive state. It is the number of calories that is burned if one stayed in bed all day, when the subject is at complete mental and physical rest, but not sleeping. The release and use of energy in this state is sufficient only for the functioning of the vital organs, the heart, lungs, nervous system, kidneys, liver, intestine, sex organs, muscles, and skin. Normally calories are being burned by body processes such as respiration, circulation of blood, and maintenance of body temperature.

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BMR is an important factor in determining overall metabolic rate and how many calories one needs to maintain, lose, or gain weight. The BMR values are measured in Kcal or Kj per square meter of body surface per hour. The normal adult BMR value for a healthy male is 40 Kcal (168 Kj)/sq m/hr and in healthy female it is 37 Kcal (155Kj)/sq m/hr.

BMR is influenced by a combination of genetic and environmental factors.

The Factors Influencing BMR Values Are

1. Age: The BMR decreases with age. In childhood the BMR is high and in old age it tends to fall. After about 20 years it drops by 2% per decade.

2. Sex: BMR is higher in males than in females. Men have greater muscle mass and lower body fat percentage and so higher basal metabolic rate.

3. Weight: The heavier one is, the higher will be the BMR. Metabolic rate in obese people are higher than the metabolic rate in slim.

4. Genetics: People different in the rate of metabolism and it is determined by the genetics. Some people have faster metabolism and BMR and some have slower BMR.

5. Diet: In foods that contains excess protein the BMR value increases due to the specific dynamic action of the food protein. Starvation and severe reduction in calorie intake can reduce BMR, and so low calorie weight loss diets tend to drop the BMR.

6. Body Surface area: The increase in the body surface area tend to increase the BMR. Body surface area is also a reflection of a persons height and weight. Tall and thin people have higher BMR.

7. Body Fat Percentage: The higher the body fat, the lesser the BMR and so increase in the body fat decreases the BMR. Men tend to have lesser body fat and so higher BMR.

8. Climate: Colder climate increases the BMR and vice versa. 

9. Body Temperature: Every increase in internal body temperature the BMR increases. It is because the chemical reactions in the body increases with higher temperature.

10. Endocrine Factors: The BMR is related to thyroid activity and thyroxin produced by the thyroid gland speeds up metabolic activity and increases the BMR. In thyrotoxicosis there is elevated BMR and in hypothyroidism the BMR falls.

11.  Exercise: Physical exertion burns more calories and thus increases BMR. Exercise also helps to build lean body tissues, which is metabolically more demanding that body fat tissues.

​12. Pregnancy: In later stages of pregnancy the BMR increases as the maternal BMR also includes that of the fetus.

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Physiology

The primary organ which is responsible for regulating metabolism is the hypothalamus. It is situated on the diencephalon and forms the floor and part of the lateral walls of the third ventricle of the cerebrum. The chief functions of the hypothalamus are

1. Control and integration of activities of the autonomic nervous system(ANS). The ANS regulates contraction of smooth muscle and cardiac muscle, along with secretions of endocrine organs such as thyroid gland. It regulates the visceral activities such as heart rate, movement of food through the gastrointestinal tract, contraction of urinary bladder etc.

2. Production and regulation of rage and aggression.

3. Regulation of body temperature.

4. Regulation of food intake through the 2 centers, which is the feeding center and thirst center.

All these functions sustain the body process and maintain the BMR.

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Calculating BMR​

The basal metabolic rate is usually expressed in terms of daily rates of energy expenditure. We need to calculate how many calories that is burned in a day, which is the total daily energy expenditure (TDEE). Total energy expenditure is the total number of calories that the body expends in 24 hours. The caloric maintenance values can be derived using age, sex, height, weight, body surface area, activity level, and the oxygen and carbon dioxide measures taken from calorimetry. There are many formulas for determining the BMR taking into consideration the various factors and varying in accuracies.

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The Harris-Benedict formula​

This is a calorie formula taken using the factors of height, weight, age, and sex to determine the BMR. It is more accurate than determining calorie needs based on body weight alone. The only variable that it does not take into account is the lean body mass. So this equation will be very accurate in all individuals except extremely muscular or extremely fat.

For men: BMR = 66 + (13.7 X wt in kg) + (5 X ht in cm) - (6.8 X age in years)

For women: BMR = 655 + (9.6 X wt in kg) + (1.8 X ht in cm) - (4.7 X age in years)

Note:

1 inch = 2.54 cm.

1 kilogram = 2.2 lbs.​

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