Your BMR (basal metabolic rate) is the number of calories your body needs each day to perform basic functions. From the time you go to sleep at night until you go to sleep the next night, your body is using calories to fuel the body. Nearly 75 percent of the calories you eat each day are used by the body for this purpose.

You expend energy no matter what you're doing -- even when you are sleeping. It takes calories to breathe, build new red and white blood cells, build muscle tone, pump blood throughout the body, think, raise or lower your body temperature and all other basic body functions -- not to mention the calories needed for moving around, working, reading and everything else you do in a day.

If you've noticed that it becomes harder every year to eat whatever you want and stay slim, you've learned that your BMR decreases as you age. Likewise, depriving yourself of food in hopes of losing weight also decreases your BMR, as your body adjusts how it burns fuel depending upon the amount of fuel it is given.

If you eat more calories than your body metabolism needs, you will gain weight. There are 3,500 calories in every pound of body fat. So, if you eat 500 calories more a day than your body needs, you will gain one pound every week.

Knowing your BMR can help you maintain your weight, because you will know approximately how many calories you need each day to perform basic bodily functions. This can also help you determine your exercise needs.

Your BMR is influenced by many factors:

Gender -- Men have a greater muscle mass and a lower body fat percentage. This means they have a higher basal metabolic rate.

Medications -- Some drugs slow down BMR dramatically.

Genes -- Some people are born with faster metabolisms, some with slower metabolisms; this genetic metabolic fact cannot be changed.

Age -- BMR reduces with age. After age 20, it drops about 2 percent per decade.

Exercise -- Physical exercise influences body weight by burning calories, but 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.

Weight -- The more your weight, the higher your BMR; for example: the metabolic rate of obese women is 25 percent higher than the metabolic rate of thin women.

Body Surface Area -- The greater your body surface area factor, the higher your BMR, i.e., tall, thin people have higher BMRs.

Body Fat Percentage -- The lower your body fat percentage, the higher your BMR; the higher body fat percentage in the male body is one reason why men generally have a 10-15 percent higher BMR than women.

Diet -- Starvation, eating disorders 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 by as much as 20 percent.

Other Factors -- Other factors include body temperature and health, hormones, external temperature and glands/ glandular function.