Basal Metabolic Rate - BMR
Your basal metabolic rate, or BMR, is the minimum calorific requirement needed just to sustain life and maintain composition (muscle weight, fat weight, water weight, etc) in a resting individual.
It can be looked at as being the amount of energy (measured in calories) expended by the body to remain in bed asleep all day!
BMR can be responsible for burning up to 70% of the total calories expended, but this figure varies due to different factors (see below). Calories are burned by bodily processes such as respiration, the pumping of blood around the body and maintenance of body temperature and nutrition. Obviously the body will burn more calories on top of those burned due to BMR through daily activities, complicated ones such as exercise, breastfeeding, playing & housework to simple ones, like going from the bed to the bathroom, walking to the fridge and letting the dog out – everything we do requires calories (fuel) .
BMR is the largest factor in determining overall metabolic rate and how many calories you need to maintain, lose or gain weight. BMR is determined by a combination of genetic and environmental factors, as follows:
- Genetics. Some people are born with faster metabolisms; some with slower metabolisms.
- Gender. Men have a greater muscle mass and a lower body fat percentage. This means they have a higher basal metabolic rate.
- Age. BMR reduces with age. After 20 years, it drops about 2 per cent, per decade.
- Weight. The heavier your weight, the higher your BMR. Example: the metabolic rate of obese women is 25 percent higher than the metabolic rate of thin women.
- Body Surface Area. This is a reflection of your height and weight. The greater your Body Surface Area factor, the higher your BMR. Tall, thin people have higher BMRs. If you compare a tall person with a short person of equal weight, then if they both follow a diet calorie-controlled to maintain the weight of the taller person, the shorter person may gain up to 15 pounds in a year.
- Body Fat Percentage. The lower your body fat percentage, the higher your BMR. The lower body fat percentage in the male body is one reason why men generally have a 10-15% faster BMR than women.
- Diet. Starvation or serious abrupt calorie-reduction can dramatically reduce BMR by up to 30 percent. Restrictive low-calorie weight loss diets may cause your BMR to drop as much as 20%.
- Body Temperature/Health. For every increase of 0.5C in internal temperature of the body, the BMR increases by about 7 percent. The chemical reactions in the body actually occur more quickly at higher temperatures. So a patient with a fever of 42C (about 4C above normal) would have an increase of about 50 percent in BMR. (That is why steam rooms but more so, saunas, can cause immediate weight reduction)
- External temperature. Temperature outside the body also affects basal metabolic rate. Exposure to cold temperature causes an increase in the BMR, so as to create the extra heat needed to maintain the body's internal temperature. A short exposure to hot temperature has little effect on the body's metabolism as it is compensated mainly by increased heat loss. But prolonged exposure to heat can raise BMR.
- Glands. Thyroxin (produced by the thyroid gland) is a key BMR-regulator which speeds up the metabolic activity of the body. The more thyroxin produced, the higher the BMR. If too much thyroxin is produced (a condition known as thyrotoxicosis) BMR can actually double. If too little thyroxin is produced (myxoedema) BMR may shrink to 30-40 percent of normal. Like thyroxin, adrenaline also increases the BMR but to a lesser extent.
- Exercise. Physical exercise not only influences body weight by burning calories, it also helps raise your BMR by building extra lean tissue. (Lean tissue is more metabolically demanding than fat tissue.) So you burn more calories even when sleeping.
**Short Term Factors Affecting BMR
Illnesses such as a fever, high levels of stress hormones in the body and either an increase or decrease in the environmental temperature will result in an increase in BMR. Fasting, starving or malnutrition all result in a lowering of BMR. This lowering of BMR can be one side effect of following a diet and nothing else. Solely dieting , i.e. reducing the amount of calories the body takes on, will not be as affective as dieting and exercise combined.
The Harris-Benedict formula (BMR based on total body weight)
The Harris Benedict equation is a calorie formula using the factors of height, weight, age, and sex to determine YOUR basal metabolic rate (BMR). This makes it more accurate than determining calorie needs based on total bodyweight alone. The only variable it does not take into consideration is lean body mass. Therefore, this equation will be very accurate in all but the extremely muscular (will underestimate caloric needs) and the extremely overfat (will overestimate caloric needs).
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)
Note: 1 inch = 2.54 cm.
1 kilogram = 2.2 lbs.
Once you determine your BMR number, choose an accurate “normal” activity level for yourself and multiply as follows for your final BMR:
Activity Multiplier
Sedentary = BMR X 1.2 (little or no exercise, desk job)
Lightly active = BMR X 1.375 (light exercise/sports 1-3 days/wk)
Mod. active = BMR X 1.55 (moderate exercise/sports 3-5 days/wk)
Very active = BMR X 1.725 (hard exercise/sports 6-7 days/wk)
Extr. active = BMR X 1.9 (hard daily exercise/sports & physical job or 2X day training, i.e marathon, contest etc.)
Katch-McArdle formula (BMR based on lean body weight)
If you have had your body composition tested and you know your lean body mass, then you can get the most accurate BMR estimate of all. This formula from Katch & McArdle takes into account lean mass and therefore is more accurate than a formula based on total body weight. The Harris Benedict equation has separate formulas for men and women because men generally have a higher LBM and this is factored into the men's formula. Since the Katch-McArdle formula accounts for LBM, this single formula applies equally to both men and women.
BMR (men and women) = 370 + (21.6 X lean mass in kg)
Example:
You are female
You weigh 120 lbs. (54.5 kilos)
Your body fat percentage is 20% (24 lbs. fat, 96 lbs. lean)
Your lean mass is 96 lbs. (43.6 kilos)
Your BMR = 370 + (21.6 X 43.6) = 1312 calories
To determine TDEE from BMR, you simply multiply BMR by the activity multiplier:
Example:
Your BMR is 1312
Your activity level is moderately active (work out 3-4 times per week)
Your activity factor is 1.55
Your TDEE = 1.55 X 1312 = 2033 calories
As you can see, the difference in the TDEE as determined by both formulas is statistically insignificant (2075 calories vs. 2033 calories) because the person we used as an example is average in body size and body composition. The primary benefit of factoring lean body mass into the equation is increased accuracy when your body composition leans to either end of the spectrum (very muscular or very obese).
Adjust your caloric intake according to your goal
Once you know your TDEE (maintenance level), the next step is to adjust your calories according to your primary goal. The mathematics of calorie balance are simple: To keep your weight at its current level, you should remain at your daily caloric maintenance level. To lose weight, you need to create a calorie deficit by reducing your calories slightly below your maintenance level (or keeping your calories the same and increasing your activity above your current level). To gain weight you need to increase your calories above your maintenance level. The only difference between weight gain programs and weight loss programs is the total number of calories required.
Calorie deficit thresholds: How low is too low?
It is well known that cutting calories too much slows down the metabolic rate, decreases thyroid output and causes loss of lean mass, so the question is how much of a deficit do you need? There definitely seems to be a specific cutoff or threshold where further reductions in calories will have detrimental effects. The most common guideline for calorie deficits for fat loss is to reduce your calories by at least 500, but not more than 1000 below your maintenance level. For some, especially lighter people, 1000 calories may be too much of a deficit. The
Example 1:
Your weight is 120 lbs.
Your TDEE is 2033 calories
Your calorie deficit to lose weight is 500 calories
Your optimal caloric intake for weight loss is 2033 - 500 = 1533 calories
Example 2:
Your calorie deficit to lose weight is 20% of TDEE (.20% X 2033 = 406 calories)
Your optimal caloric intake for weight loss = 1627 calories
Measure your results and adjust calories accordingly
These calculations for finding your correct caloric intake are quite simplistic and are just estimates to give you a starting point. You will have to monitor your progress closely to make sure that this is the proper level for you. You will know if you’re at the correct level of calories by keeping track of your caloric intake, your bodyweight, and your body fat percentage. You need to observe your bodyweight and body fat percentage to see how you respond. If you don't see the results you expect, then you can adjust your caloric intake and exercise levels accordingly. The bottom line is that it’s not effective to reduce calories to very low levels in order to lose fat. In fact, the smaller, more frequent (balanced and “clean”) meals you consume the better, as long as a deficit is created through diet and exercise. **The best approach is to reduce calories only slightly and raise your daily calorie expenditure by increasing your frequency, duration and or intensity of exercise**.
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