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Basal Metabolic Rate (BMR) can be explained as the minimum amount of calories required to sustain the body's functions and processes when the body is resting. It is responsible for the consumption of about 70% of total calories used up by the body. BMR is regulated by a hormone called thyroxin. This hormone is produced by the thyroid gland and helps to control the body's metabolic activities. Thyroxin affects the heart rate, body weight, muscle strength, and blood cholesterol levels.

BMR generally decreases with age. It is affected by certain body functions, such as respiration, circulation, and maintenance of constant body temperature. Exercise, calorie consumption, and lean body tissue can also affect BMR. Body weight, height, and environmental temperature also play a significant role in increasing the BMR. If it is increased, then there would be more calorie consumption, which results in burning excess fats and remarkable weight loss. Increase in BMR offers a number of health benefits, such as reducing the risk of developing heart disease, stroke, heart attack, and diabetes. An average BMR for adults ranges between 1200 and 1800 kcal.
Factors Affecting BMR
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BMR determines the overall metabolic rate and calories required to lose, gain, or maintain the body weight. BMR is determined by using genetic and environmental factors. These factors are explained below.
  • Genetics: Some people are born with a higher metabolism rate, while some have slow metabolism.
  • Age: BMR generally decreases with age. After the age of 20 years, it reduces about 2% every decade.
  • Gender: Men have a lower body fat percentage and a greater muscle mass. As a result, they have a higher BMR.
  • Body Surface Area: Your height and weight contribute a lot in determining your BMR. The greater is your body surface area, the higher is your BMR. Thus, thin and tall people have a higher BMR.
  • Weight: If your body weight is more, then your BMR would be higher. The metabolic rate of obese women is about 25% higher than the metabolic rate of slim women.
  • Body Fat Percentage: If the body fat percentage is lower, then the BMR will be higher. Generally, men have a lower body fat percentage, and hence they have 10-15% higher BMR than women.
  • Body Temperature: With an increase of 0.5°C in the internal body temperature, BMR increases by about 7%. The chemical reactions in the body take place more quickly at higher temperatures.
  • Diet: Because of starvation or serious abrupt calorie reduction, BMR can drop by about 30%. The low-calorie weight loss diets can cause dropping of BMR to up to 20%.
  • External Temperature: External temperature also affects the BMR. Exposure to low temperature leads to an increase in the BMR in order to maintain body's internal temperature.
  • Glands: Thyroxine accelerates the metabolic activity of the body. More the thyroxine produced, higher is the BMR. In case of thyrotoxicosis (excess production of thyroxin), BMR may be doubled.
  • Exercise: Physical exercise decreases body weight by burning calories as well as increases BMR by building extra lean tissue.

Some short-term factors affecting BMR are high levels of stress hormones, certain illnesses such as fever, and increase or decrease in the environmental temperature. These factors cause an increase in the BMR. Starvation, fasting, or malnutrition results in lowering the BMR.
How To Calculate BMR
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While designing a personal nutrition plan, you need to calculate the calories that you burn in a day. Total daily energy expenditure (TDEE) is the total number of calories that are spent by the body within 24 hours of doing all the activities. TDEE is also known as the maintenance level. The calorie expenditure can vary from person to person and it is much higher in extremely active people and athletes. There are various formulae to determine the calorie maintenance level considering the factors such as age, weight, height, sex, activity level, and lean body mass. Any formula that involves your lean body mass would give an accurate determination of your energy expenditure.

Quick Method Based on Total Body Weight:
It is a quick and easy method to determine the calorie requirements. It involves the use of total current body weight times a multiplier.

Fat loss = 12-13 calories per lb of body weight
Maintenance (TDEE) = 15-16 calories per lb of body weight
Weight gain = 18-19 calories per lb of body weight

This method doesn't consider the activity levels or body consumption. The extremely active people may need much more calories than this formula indicates.

Equations Based on BMR:Determine the basal metabolic rate using various factors such as age, weight, height, and sex is more accurate method to calculate TDEE. Then, multiply the BMR by an activity factor to calculate TDEE.

The Harris-Benedict Formula:
This is a calorie formula using factors such as age, sex, weight, and height to determine the BMR. The lean body mass is not taken into consideration in this method.

Men: BMR = 66 + (13.7 x wt in kg) + (5 x ht in cm) - (6.8 x age in years)

Women: BMR = 655 + (9.6 x wt in kg) + (1.8 x ht in cm) - (4.7 x age in years)