The basal metabolic rate (BMR) is the energy used by the body’s mechanical activities necessary to sustain life, such as breathing and blood circulation. There are several different ways to calculate BMR, also known as basal energy expenditure (BEE). The most accurate method is calorimetry, which requires monitoring the amount of heat produced by a subject while in a resting and fasting state in a thermally neutral environment (neither hot nor cold). This method requires expensive equipment and is only available in certain facilities.
Generally, an estimate of the BEE is sufficient, and usually obtained from standard tables and equations. The most common method used by health professionals, such as dietitians, is the Harris-Benedict formula. Men 66 + (6.3 X weight in lbs) + (12.9 X height in inches) – (6.8 X age in years) Women 655 + (4.3 X weight in lbs.) + (4.7 X height in inches) – (4.7 X age in years).
Because the metabolic rate is affected by factors such as sex, age, weight, height, muscle mass, and hormones, better accuracy is obtained by knowing the value of these variables. There is, however, an abbreviated formula that can estimate the BEE of persons of normal height and weight: Men Weight in kg. X 1 X 24 hrs Women Weight in kg. X 0.95 X 24 hrs. Note: 1 kg. = 2.2 lbs. There are also tables available that provide depicted by the formulas in a format that is easier to interpret. These are, however, too extensive to reproduce here.
As I already mentioned, metabolic rates vary from one person to another and are affected by many factors. BMR/BEE is increased with regular physical activity that promotes the build up of lean body mass (muscle). It is higher in tall, thin people, children and pregnant women. It increases with fever, a hyperactive thyroid gland and many diseases. It decreases with age, fasting or starvation, malnutrition and when sleeping. For this reason, it is not realistic to set a norm based on age and sex alone. The formulas given can give you this information when provided by the variables required.
Be aware that BMR/BEE alone does not provide you with your daily caloric requirement. Other factors such as amount of physical activity and certain medical conditions must be taken into account. A qualified health professional, such as a registered dietitian, can provide a more accurate assessment of your individual energy requirements and guide you toward achieving your particular goals.