The primary organ responsible for regulating metabolism is the hypothalamus. The hypothalamus is located on the brain stem and forms the floor and part of the lateral walls of the third ventricle of the cerebrum. The chief functions of the hypothalamus are: control and integration of activities of the autonomic nervous system (ANS); production and regulation of feelings of rage and aggression; regulation of body temperature; and regulation of food intake. These functions form a survival mechanism that sustains the processes that BMR measures.
BMR decreases with age and with the loss of lean body mass. Increased muscle mass and cardiovascular exercise can increase BMR, even when the body is at rest. Measured in calories, metabolic rates vary with exertion, recent food ingestion, muscle exertion, environmental temperature, emotional state, body temperature, pregnancy, menstruation, level of thyroid hormones stress hormones (epinephrine and norepinephrine), fear and illness.
The human body requires energy to merely stay alive and basal metabolic rate (BMR) attempts to measure this energy. As individuals age or lose muscle mass, their basal metabolic rate decreases, which means that less energy is needed to maintain the body's functions. Often, this phenomenon is referred to as the metabolism slowing down.
BMR is measured in the calories required per day to maintain the body. Because of this, BMR can be very useful to compare caloric intake to the actual amount of calories burned by the body. If caloric intake is lower than the calories used per day, burning fat or muscle will make up the difference. A regular routine of cardiovascular exercise can increase an individual's BMR, improving health and fitness when the body's ability to burn energy gradually slows down.
BMR does not take into account the calories needed for exercise, so it is not representative of the amount of calories burned per day. There are several factors which may affect BMR, including:
Age: BMR reduces with age. After age 20, it drops about 2% per decade.
Body fat percentage: The lower the body fat percentage, the higher the BMR.
Body surface area: The greater the body surface area factor, the higher the BMR.
Diet: Starvation, eating disorders or serious abrupt calorie-reduction can dramatically reduce BMR up to 30%. Restrictive low-calorie weight loss diets may cause an individual's BMR to drop by as much as 20%.
Exercise: Since physical exercise burns calories, it influences body weight and helps raise an individual's BMR by building extra lean tissue (lean tissue is more metabolically demanding than fat tissue).
Gender: Men have a greater muscle mass and a lower body fat percentage. This means they have a higher basal metabolic rate than women.
Genes: Genetic factors play a role in the speed of a person's metabolism.
Medications: Some drugs slow down the BMR dramatically
Weight: BMR increases with weight.
Other factors: Other factors include: body temperature, health, hormones, external temperature and glands/glandular function.
An individual's BMR is typically measured after a full night's sleep in a laboratory under optimal fasting conditions of quiet, rest and relaxation.
In order to calculate an individual's approximate basal metabolic rate, the following questions may be answered: what is the gender of the individual; what is the weight in pounds of the individual; what is the height of the individual in inches; and what is the age of the individual?
Men: BMR = 66 + (6.22 x weight in pounds) + (1.96 x height in inches) - (6.8 x age in years)
Women: BMR = 655 + (4.36 x weight in pounds) + (0.71 x height in inches) - (4.7 x age in years)
Note: Keep in mind that the BMR calculator is an estimation of the calories needed for the body to perform basic functions. An individual's total daily caloric requirements are affected by muscle mass, illness, stress, food digestion and most importantly, exercise.
Only an overnight laboratory test can accurately measure an individual's actual BMR.