Your basal metabolic rate, or BMR, is the minimum calorific requirement needed to sustain life 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. 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
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.
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 increased exercise. The negative effect of dieting on BMR can be offset with a positive effect from
How to Calculate Basal Metabolic Rate (BMR)
The first step in designing a personal nutrition plan for yourself is to calculate how many calories you burn
in a day; your total daily energy expenditure (TDEE). TDEE is the total number of calories that your body expends
in 24 hours, including all activities. TDEE is also known as your "maintenance level". Knowing your maintenance level
will give you a starting reference point from which to begin your diet.