BMR can be responsible for burning up to 70% of the total calories expended, but this figure varies due to different factors. 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.

Basal metabolic rate (BMR) is the amount of energy expended while at rest in a neutrally temperate environment, in the post-absorptive state (meaning that the digestive system is inactive, which requires about twelve hours of fasting in humans). The release of energy in this state is sufficient only for the functioning of the vital organs, such as the heart, lungs, brain and the rest of the nervous system, liver, kidneys, sex organs, muscles and skin. BMR decreases with age and with the loss of lean body mass. Increased cardiovascular exercise and muscle mass can increase BMR. Illness, previously consumed food and beverages, environmental temperature, and stress levels can affect ones overall energy expenditure, and can affect one's BMR as revealed by gas analysis. An accurate BMR measurement requires that the person's sympathetic nervous system is not stimulated. Basal metabolic rate is measured under very restrictive circumstances. A more common and closely related measurement, used under less strict conditions, is resting metabolic rate (RMR).

BMR and RMR are measured by gas analysis through either direct or indirect calorimetry, though a rough estimation can be acquired through an equation using age, sex, height, and weight. Studies of energy metabolism using both methods provide convincing evidence for the validity of the respiratory quotient (R.Q.), which measures the inherent composition and utilization of carbohydrates, fats and proteins as they are converted to energy substrate units that can be converted by the body to energy.

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.
  

Short Term Factors Affecting BMR

• Age: In youth, the BMR is higher; age brings less lean body mass and slows the BMR.
  
• Height: Tall, thin people have higher BMR's.
  
• Growth: Children and pregnant women have higher BMR's.
  
• Body Composition: The more lean tissue, the higher the BMR. The more fat tissue, the lower the BMR.
  
• Fever: Fevers can raise the BMR.
  
• Stress: Stress hormones can raise the BMR.
  
• Environmental Temperature: Both the heat and cold raise the BMR.
  
• Fasting/Starvation: Fasting/starvation hormones lower the BMR.
  
• Malnutrition: Malnutrition lowers the BMR.
  
• Thyroxin: The thyroid hormone thyroxin is a key BMR regulator; the more thyroxin produced, the higher the BMR.
  

How to Calculate Basal Metabolic Rate (BMR)