Basal Metabolic Rate (BMR)

 
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BMR is all about energy and its level of expenditure. The basal metabolic rate (BMR) is the rate of energy expenditure that is carried out when the person is at a state of rest. It erases the variable effect of physical activities which include exercise, walking, working, etc. or anything related to the burning of energy via some physical performance. The total daily energy expenditure is calculated from knowledge of the BMR that is inclusive of a factor of physical activity.


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BMR is responsible for approximately 60% of the energy expenditure on a day to day basis. Energy in BMR includes energy that is used for normal body activities like cellular homeostasis, muscular activities, cardiac function, nerve function. Nearly 20 percent of the energy we take is used for brain functioning and metabolism. The rest is all used in basal metabolism, that is, the energy that we require in a resting state, for the body to perform functions such as blood circulation, digestion, and breathing.

How BMR is Measured?

There are various methods to measure BMR. One of them is through the evaluation of thyroid function. It measures the oxygen consumption rate under basal conditions of overnight fast and rest from mental and physical activity. Since, the standard equipment for the measurement and calculation of BMR might not be readily available, the BMR can be easily estimated and measured from the oxygen consumed over a period of time by analysing the samples of expired air. The test indirectly measures metabolic energy expenditure or the level of heat production.

Low values in the results suggest that there is hypothyroidism. High values suggest that there exists presence of thyrotoxicosis. Normal BMR percentage ranges from negative 15% to positive 5%. Most of the hyperthyroid patients hold a BMR percent of positive 20% or more and hypothyroid patients have a BMR of negative 20% or less. BMR is also altered by various clinical conditions such as viral fever, pregnancy, drugs, cancer, cholesterol, congestive heart failure, polycythaemia and Paget’s disease of the bone. These diseases are meant to increase BMR. On the other hand, overweight or obesity, Cushing’s syndrome, anorexia, adrenal insufficiency and immobilization decreases BMR.

The general formula for calculating BMR in female and male is:
Female: BMR = 655 + (4.35 x weight in pounds) + (4.7 x height in inches) – (4.7 x age in years)
Male: BMR = 66 + (6.23 x weight in pounds) + (12.7 x height in inches) – (6.8 x age in years)
BMR Variables:

Muscle mass: BMR is largely determined by the total body lean mass as lean mass requires a lot of energy to perform energetic activities. And that instant energy is released through muscle activity. Anything that reduces lean mass will result in reduction of BMR. As the muscle mass increases, body consumes more amount of energy to maintain and control its basic everyday functions, this giving BMR a well-deserved boost in the metabolic activity.

Age: The basal metabolic rate decreases linearly with increase in age number. Basal metabolic rate is dependent on body size as it is indicated by weight, age, gender. Age is a factor that has a major significance in human nutrition and diet. The new-born’s basal metabolism rate increases rapidly in proportion to the rate of growth and development of their muscles during the life span of one year. The child reaches the peak of basal heat production at two years of age. It keeps on gradually decreasing until the period of early adolescence (12-15 years of age). After puberty, the BMR gradually keeps on decreasing. The changes in BMR is different for children and adults because adults are more susceptible to the changes in the environment, whereas children are always kept protected from the environmental changes.

Genetics and hormones: On the onset of adolescence, there is a difference in the basal heat production for males and females. Girls show the pre puberty rise two years earlier than boys and when boys reach puberty stage their basal energy expenditure is higher than girls and continues to be higher throughout the life. This difference is because of the fact that men have greater muscularity and lean body mass( lower percentage of adipose tissue) than women. Genes do have a large influence on weight because of DNA and cell protein synthesis which controls the metabolism level. Genetic factors are also responsible for heredity which affects the BMR.

Pregnancy- BMR increases when a woman is pregnant. The total energy cost of pregnancy is largely due to an increased basal metabolic rate (BMR). The increase in body weight in combination with fetal weight or with the increased levels of concentration of IGF-I in serum explains approximately 60% of the variability in the increase in BMR of pregnant women.

Supplements- Supplements do not necessarily mean chemicals and sedative drugs to lose or gain weight for increase or decrease in the BMR. It involves intake of natural supplements such as

  • Caffeine which acts as a catalyst in thermogenesis.
  • Capsaicin
  • L-carnitine
  • Chromium picolinate
  • Conjugated linoleic acid (CLA)
  • Green tea
  • Resveratrol

It is a universal rule that to lose weight, one needs to burn more calories than the actual consumption. However, it is not universally accepted by all and hence people switch towards chemicals and medicines in today’s times. When the body is not fueled up with enough calories, it slows down the BMR to ensure it is storing whatever energy it can.

Weather : Climate and body temperature are critical indicators of BMR as they determine the internal environment of an individual. The BMR of people living in tropical climate is generally 5 to 20 percent higher than their counterparts living in more temperate areas. This is because it takes more energy to keep the body in a cooling state by releasing more heat. Exercise performed in hot weather also imposes an extra, additional metabolic load on the body.

Daily calorie need

Basal metabolism is usually the biggest and largest component of a person’s total caloric needs. The daily caloric need is the BMR value multiplied by a factor with a value between 1.2 and 1.9, depending on the kind of an activity that is performed. Calorie intake largely depends on factors such as age, size, height, sex, lifestyle, and overall health of a child.

Intake of calories within limits does not lead to a healthy lifestyle and a healthy diet, as different foods tend to have different effects on the body of different sets of people. For example, insulin level may rise after eating carbs as compared to eating fats or protein. Daily calorie need of a human differs from person to person.

1,000 calories a day for an infant is required and up to 3,200 calories for an active male aged between 16 to 18 years. For a woman, the daily calorie need can be in the range of 200 to 2300 depending on whether the woman’s physical activities.

As people get older, their metabolic rate slows down thus their appetite and calorie intake also goes down. It reduces the need for energy. From age 19 to 25 years, the recommended calorie intake for a women is 2,000 calories a day, but after 51 years, this falls to 1,600.

Daily calorie needs largely depend upon the Activity factor. After calculating the BMR, multiply the result with an activity factor to know about the daily calorie needs.

Activity Level >Daily Calorie Requirement
Sedentary Lifestyle: little or no exercise BMR * 1.2
Slighty Active Lifestlye:Exercise 1-3 times/week BMR * 1.375
Moderately Active Lifestyle: Exercise 4-5 times/week BMR * 1.55
Active Lifestyle:Daily exercise or intense exercise 3-4 times/week BMR * 1.725
Very Active Lifestyle: Intense exercise 6-7 times/week BMR * 1.9

Exercise: 15-30 minutes of elevated heart rate activity.
Intense exercise: 45-120 minutes of elevated heart rate activity.
Very intense exercise: 2+ hours of elevated heart rate activity.
Adopt a healthy lifestyle and start exercising to lead a healthy and happy life.

References

Harris J, Benedict F. 1919. A biometric study of basal metabolism in man. Washington DC: Carnegie Institute of Washington.

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