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BMR or Basal Metabolic Rate Explained

Wednesday, 13 January 2010

A person’s basal metabolic rate, or BMR, is the amount of calories a person consumes a day without being physically active. The rate only includes basic metabolic functions like sustaining internal body temperature, cell repair, pumping blood, breathing, etc. In other words, the energy your body needs if you would do nothing else, but relax on the couch all day.

BMR is important in determining your overall metabolic rate and the amount of calories you need to maintain, lose, or gain weight. BMR results are different for everyone and are influenced by factors such as:

-          Genetics. Everyone’s metabolism is different.

-          Gender. Generally, men are more muscular and have a lower body fat percentage. Their basal metabolic rate is therefore often higher.

-          Age. BMR decreases with age, and will drop about 2 per cent by decade after the age of 20.

-          Weight. The heavier you are, the higher your BMR.

-          Height-Weight Ratio. The more body surface area, the higher your BMR.

-          Body Fat. People with less body fat will have a higher BMR.

-          Diets. Serious calorie-reduction can dramatically reduce your BMR. Restrictive, low-calorie weight loss regimens can cause a BMR drop of as much as 20%.

-          Internal Temperature. When you have a fever, your BMR will grow; any body temperature increase of 0.5C will boost your basal metabolic rate by about 7 percent.

-          External temperature. The body’s surrounding temperature also affects your BMR. Cold will increase your BMR, because more energy is needed to maintain your normal body temperature.

-          Thyroxin. Thyroxin is produced by the thyroid gland and a key BMR-regulator.

The more thyroxin is produced, the higher your BMR. 

-          Exercise. Physical activities like exercise and sports will help to burn calories and raise your BMR. It also builds up extra lean tissue.

Short term BMR fluctuations

BMR can be influenced temporarily by factors such as:

-          Fever

-          High stress levels

-          Increase or decrease of environmental temperature

-          Fasting and starvation

Methods to determine BMR

You can determine your caloric maintenance level through a number of scientific formulas, all of which will take your age, gender, height, weight, activity level, and lean body mass (LBM) into account. Keep in mind that your calculations are only estimates, as no equation can accurately calculate your personal BMR. If you are on a diet, adjust your calorie intake according to the amount of weight you want to lose in a specific time frame. Make sure you use the activity multiplier. Remember, a BMR is your calorie usage without any physical activity!

Activity Multiplier                                                                                          

-          Little or no exercise, desk job = BMR X 1.2

-          Light exercise/sports 1-3 days/wk = BMR X 1.375

-          Moderate exercise/sports 3-5 days/wk= BMR X 1.55

-          Hard exercise/sports 6-7 days/wk= BMR X 1.725

-          Hard daily exercise/sports & physical job or 2X training/day= BMR X 1.9

The following are popular BMR estimation formulas.

The Harris-Benedict formula

These calculations are based on total body weight, but also uses of height, weight, age, and gender to determine basal metabolic rate.

-          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)

MD Mifflin and ST St Jeor Equation

These formulas are based on body weight, and do not take into account the difference in metabolic activity between lean body mass and body fat. They are less suited for heavy people.

-          Men: RMR = (9.99 X wt in kg) + (6.25 X ht in cm) – (4.92 X age in years) + 5

-          Women: RMR = (9.99 X wt in kg) + (6.25 X ht in cm) – (4.92 X age in years) – 161

Katch-McArdle formula

This equation is based on lean body weight, which means you need to know your lean body mass (LBM). Because of the added factor, this formula is more accurate than others.

-          BMR for men & women = 370 + (21.6 X LBM in kg)

Adjust your caloric intake according to your goal

Once you know the amount of calories you need per day to stay healthy and be active, you can adjust your calorie intake according to your goals. To keep your weight stable, you should remain at your daily caloric maintenance level. If you want to lose a few kilos, you need to create a calorie deficit and reduce your daily calorie intake according to your objectives. The fewer calories you take in, the quicker you will lose the weight. If you need to gain weight, add to your maintenance level calorie intake. Adjust your diet gradually to avoid health risks and throwing your metabolism out of whack.

Keep track of your results, and adjust your eating plan if progress is too slow or too quick. If any health problems arise, consult your physician immediately.

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