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Basal Metabolic Rate - What is BMR and what factors affect it?
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Basal Metabolic Rate

What is Basal Metabolic Rate (BMR)?

Basal metabolic rate (BMR) is basically the amount of energy that keeps your body functioning at rest. This energy is used for basic life maintaining processes like maintaining heartbeat, temperature, blood circulation, nerve function and breathing.

Why do we need to measure Basal Metabolic Rate?

Basal Metabolic Rate is measured as it affects the rate at which a person burns calories. 60 to 75% of the calories expended daily are due to it. Thus BMR is used to calculate how many calories you need to maintain your current weight, or decrease it.

How do we measure BMR?

Basal Metabolic Rate is measured in a dark room just after waking from sleeping at least 8 hours. The subject must not eat for 12 hours and should rest in a reclining position. The heat given off by the body is measured in a given amount of time and is expressed in calories burnt/unit body mass/unit time. However, when this procedure cannot be carried out, some equations have been developed which can be used instead.

BMR Equations

Several formulas exist for the estimation of BMR. However the most notable two are the Harris Benedict Equation and the Mifflin St Jeor Equation.

According to the Harris Benedict Equation:

  • For men, BMR = (13.75 x Weight in kg) + (5 x Height in cm) – (6.76 x Age) + 66
  • For women, BMR = (9.56 x Weight in kg) + (1.85 x Height in cm) – (4.68 x Age) + 655

According to the Mifflin St Jeor Equation:

  • For men: (10 x Weight in kg) + (6.25 x Height in cm) – (5 x Age) + 5
  • For women: (10 x Weight in kg) + (6.25 x Height in cm) – (5 x Age) – 161

The Harris Benedict Equation has been the standard since as far back as 1919, but it is losing its validity according to today’s tests standards. It gives a more reliable measurement of Resting Metabolic State instead of Basal Metabolic Rate. The Mifflin St Jeor Equation is an improvement of the Harris Benedict Equation that gives a more accurate measurement of BMR and increases its applicability.

(For a quick calculation of your basal metabloic rate you can also use a BMR calculator.)

What is Resting Metabolic Rate?

Resting Metabolic Rate is measured under less restraining conditions than BMR. While calculating RMR the person is supposed to rest in a reclining position, but it can be taken at any time throughout the day. Fasting or sleeping beforehand is not required. Hence it delivers a more realistic result than BMR.

What factors affect BMR?

  • Age – BMR decreases about 5% per decade after turning 40. This is due to the loss of lean muscle mass thus less energy is required to maintain the body’s vital processes.
  • Gender – Men have a BMR 5 to 10% higher than women. This can be attributed to their increased lean muscle mass and less fat in their body.
  • Muscle mass – More muscle burns more calories and results in a higher BMR.
  • Hormones – Certain hormones are responsible for increasing or decreasing metabolism. For example a decrease in thyroid hormone may lead to approximately a 15% decrease in metabolism. Similarly, there may be a 5% increase in body’s metabolism during a woman’s luteal phase of the menstrual cycle.
  • Caffeine, Tobacco – Research shows that consuming caffeine in different amounts between 5 to 100 mg per day may increase body’s metabolism by 7%. Even tobacco can cause upto3-7% increase in body’s BMR.
  • Stress/ emotions – stress may lead to an increased production of stress hormones like epinephrine and norepinephrine. This may also lead to an increased heart rate and respiratory rate and an increased metabolism may be required to fulfill these needs.
  • Exercise: Various forms of exercise can have different effects on our body. Strength training can increase our body’s metabolism chronically. This is because it causes an increase in the lean body mass. Aerobic exercise and cardiovascular exercise do not have a significant role in chronically increasing body’s basal metabolic rate.
  • Calorie restriction – Restricitve diets below 100 calories per day may create a problem with the basal metabolic rate. There can be an acute decrease in basal metabolic rate which can be harmful for the individual.

How to calculate daily calorie needs?

To measure the ideal amount of calorie intake, your BMR value is multiplied by a value between 1.2 and 1.9 depending on your physical activity level, with 1.2 for no exercise and 1.9 for extremely active individuals.