Basal Metabolic Rate
Basal Metabolic Rate (BMR) can be explained as minimum amount of calories required to sustain the body’s functions and processes, when the body rests. It is responsible for 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 thyroid gland, which helps to control the body’s metabolic activities. Thyroxin affects the heart rate, body weight, muscle strength and blood cholesterol levels.
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. Obviously the body will burn more calories on top of those burned due to BMR.
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.
- 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.
Some short term factors affecting BMR are high levels of stress hormones, certain illnesses such as fever and increase or decrease in environmental temperature. These factors cause an increase in BMR. Starvation, fasting or malnutrition results in lowering BMR.
Methods of determining caloric needs
There are many different formulas you can use to determine your caloric maintenance level by taking into account the factors of age, sex, height, weight, lean body mass, and activity level. Any formula that takes into account your lean body mass (LBM) will give you the most accurate determination of your energy expenditure, but even without LBM you can still get a reasonably close estimate.
The “quick” method (based on total bodyweight)
A fast and easy method to determine calorie needs is to use total current body weight times a multiplier.
Fat loss = 12-13 calories per lb. of bodyweight
Maintenance (TDEE) = 15-16 calories per lb. of bodyweight
Weight gain = 18-19 calories per lb. of bodyweight
This is a very easy way to estimate caloric needs, but there are obvious drawbacks to this method because it doesn’t take into account activity levels or body composition. Extremely active individuals may require far more calories than this formula indicates. In addition, the more lean body mass one has, the higher the TDEE will be. Because body fatness is not accounted for, this formula may greatly overestimate the caloric needs if someone is extremely overfat. For example, a lightly active 50 year old woman who weighs 235 pounds and has 34% body fat will not lose weight on 3000 calories per day (235 X 13 as per the “quick” formula for fat loss).
Equations based on BMR.
A much more accurate method for calculating TDEE is to determine basal metabolic rate (BMR) using multiple factors, including height, weight, age and sex, then multiply the BMR by an activity factor to determine TDEE. BMR is the total number of calories your body requires for normal bodily functions (excluding activity factors). This includes keeping your heart beating, inhaling and exhaling air, digesting food, making new blood cells, maintaining your body temperature and every other metabolic process in your body. In other words, your BMR is all the energy used for the basic processes of life itself. BMR usually accounts for about two-thirds of total daily energy expenditure. BMR may vary dramatically from person to person depending on genetic factors. If you know someone who claims they can eat anything they want and never gain an ounce of fat, they have inherited a naturally high BMR. BMR is at it’s lowest when you are sleeping undisturbed and you are not digesting anything. It is very important to note that the higher your lean body mass is, the higher your BMR will be. This is very significant if you want to lose body fat because it means that the more muscle you have, the more calories you will burn. Muscle is metabolically active tissue, and it requires a great deal of energy just to sustain it. It is obvious then that one way to increase your BMR is to engage in weight training in order to increase and/or maintain lean body mass. In this manner it could be said that weight training helps you lose body fat, albeit indirectly.
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 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.
Now that you know your BMR, you can calculate TDEE by multiplying your BMR by your activity multiplier from the chart below:
Katch-McArdle formula (BMR based on lean body weight)
BMR (men and women) = 370 + (21.6 X lean mass in kg)
To determine TDEE from BMR, you simply multiply BMR by the activity multiplier:
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.
Adjust your caloric intake gradually
It is not advisable to make any drastic changes to your diet all at once. After calculating your own total daily energy expenditure and adjusting it according to your goal, if the amount is substantially higher or lower than your current intake, then you may need to adjust your calories gradually. For example, if your determine that your optimal caloric intake is 1900 calories per day, but you have only been eating 900 calories per day, your metabolism may be sluggish. An immediate jump to 1900 calories per day might actually cause a fat gain because your body has adapted to a lower caloric intake and the sudden jump up would create a surplus. The best approach would be to gradually increase your calories from 900 to 1900 over a period of a few weeks to allow your metabolism to speed up and acclimatize.
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 more calories 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.