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Basal Metabolic Rate
Our bodies need a basic amount of energy to function every day, even when we’re sleeping. To perform bodily functions, such as breathing and circulation, our body needs energy in the form of calories. Your Resting Metabolic Rate or Basal Metabolic Rate is number of calories your body needs to just exist.
How Many Calories Do I Need?
Since most of us do more than sleep 24 hours a day, our daily calorie needs will be higher than our Resting Metabolic Rate calculation: To maintain your current weight, your daily calorie consumption should equal 1.5 x your BMR.
To lose weight at a healthy rate (i.e., lose fat mass instead of muscle mass & water weight) you need to be between your minimum calories (i.e., BMR) and your maintenance calories (i.e., 1.5 x BMR). The best way to reach your ideal weight is to add some physical activity into each day and make healthier food choices.
To gain weight at a healthy rate, consume more daily calories than needed for maintenance. Your basal metabolic rate, or BMR, is the minimum calorific requirement needed to sustain life in a resting individual. It may be looked at as being the amount of energy (measured in calories) expended by the body to remain in bed asleep all day!
BMR may 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 may 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 may 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 may 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.