Basal or Resting Metabolism Determines
Rate of Calorie Expenditure
Basal Metabolic Rate (BMR)
BMR is the largest factor in determining
overall metabolic rate and (also) 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. Because men have a greater
muscle mass and a lower body fat percentage, they generally have a higher
basal metabolic rate.
- Age. BMR is greater in childhood
than in adulthood. After 20 years, it drops about 2 per cent, per decade.
- Weight. The more you weigh the
higher your BMR will be. For example, the metabolic rate of very overweight
women is 25% higher than that of thin women.
- Height.
- 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. People with
a higher body fat percentage, have a lower BMR than those with a lower
body fat percentage - all other things being equal. Muscle cells contain
with tiny little power centers called mitochondria, which are where
calories get 'burned up.'' The greater percentage of lean muscle tissue
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 muscle. So you burn more calories even when
sleeping.
How to Calculate Basal Metabolic Rate
(BMR)
There are several methods of determining
BMR.
How to Calculate Daily Calorie Needs
Knowing your BMR helps you to calculate
your daily calorie needs, but it's not essential. You can get a ball-park
figure for calorie needs without knowing your BMR.
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