To compare the energy we require from food on a common standard the concept of the ‘basal metabolic rate’ was invented. The basal metabolic rate or BMR is the amount of energy which is needed to maintain the body in energy equilibrium while lying at rest in a fasted state. It is measured as the rate of energy consumption in kcal/day.


A person’s BMR is the energy expended by the body at rest to maintain normal bodily functions. This continual work makes up about 60-70% of the calories we use ("burn" or expend) and includes the heart beat, respiration, and the maintenance of body temperature. The BMR is governed by a number of factors, including age, weight, height, gender, environmental temperature, dieting, and exercise habits. Because of the increased activity of cells undergoing division, the young have a higher (faster) metabolism. Also the taller and heavier a person is, the faster their metabolism. Because of the greater percentage of lean muscle tissue in the male body, men generally have a 10-15% faster BMR than women. People living in tropical or very cold environments generally have BMR's 5-20% higher than those living in more temperate climates. In general, depending on the intensity and duration, consistent exercise can also increase BMR.

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.