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Many people assume that body weight changes when the amount of calories obtained through food does not match the amount of calories spent on physical activities. This view is not correct. Most of our calories spent are on basic body functions, and only around 20-30 percent fuel our physical activity.
The Basal Metabolic Rate (BMR) of a person is the number of calories required to perform basic functions of the body like pumping the blood through the body, sustaining body temperature, digesting food, and breathing. It does not include the calories required for exercises or doing other physical activities. BMR, in a way, is useful in determining the number of calories one requires to lose weight: if we know how many calories we burn daily, we can easily estimate which restrictions should be placed on our calorie intake.
How To calculate Your BMR
Basal Metabolic Rate is calculated as follows.
For Women: (4.35 * body weight in pounds) + (4.7 * height in inches) – (4.68 * age) + 655
For Men: (6.25 * body weight in pounds) + (12.7 * height in inches) – (6.76 * age) + 66
We can take the activity factor into consideration by using Harris-Benedict equation that gives the overall calorie expenditure. As per this equation, multiply the base BMR number by:
- 1.2, if you are sedentary
- 1.375, if you are a little active
- 1.55, if moderately active
- 1.725, if very active
- 1.9, if extra active
For a long time, scientists assumed that an individual BMR is more or less fixed. However, more recent research show that as one loses weight, BMR also decreases, as the body has to do less work to keep it going. Therefore, it is advisable to recalculate your BMR from time to time when you are trying to lose weight, and further reduce your calories intake to keep the process going.
However, do remember that going too low with the calorie intake may result in lethargy, weak immune system and may result in a larger gain in weight when you are off your dieting plan.
Factors That Control BMR
Body Composition: If there is more lean muscle, the BMR is higher because lean muscles are more metabolically active than fat.
Gender: Men usually have more lean body mass than women, thus having a higher BMR.
Age: There is a drop in lean body mass as we age, that is why our BMR also decreases as we grow.
Thyroid hormone levels: Hormones released from thyroid gland also affects metabolic rate.
Genes: How your body uses energy may be influenced by inherited characteristics.
Calorie Intake: Going on a strict diet to lose weight may affect your metabolic rate too. If you eat less, your ability to burn calories also goes down. A person trying to lose weight is usually advised to take 500-1000 calories less or burn 500-1000 more calories (or do a combination of two) than what is required to maintain a steady weight. However, medical experts warn that calorie levels should never drop below 1200 calories per day for women and 1800 calories per day for men as it will slow the metabolic rate too much and may be counterproductive to one’s health.
Climate and body temperature: BMR of people in cold regions is generally higher than those living in warmer regions as it requires more energy to work or exercise and, above all, to stay warm in the cold environments.