The Harris Benedict Equation is a formula that can help an individual determine his or her daily caloric expenditure. Knowing how many calories someone burns in a day can be very helpful if that person is trying to gain or lose weight. It can be very important, then, for athletes who are trying to build muscle mass or for individuals who are trying to shed extra pounds. There are limits to the overall accuracy of the Harris Benedict Equation, but it is a useful measure of how many calories someone burns in a day.
Before utilizing the Harris Benedict Equation, a subject must first determine his or her basal metabolic rate (BMR), which is a measure of the number of calories that would be burned in a day if he or she were to remain at rest throughout the day. There are other methods of calculating this value, but BMR is effective because it accommodates for several variables that can affect the body's metabolism, such as weight, height, gender and age. The BMR does not, though, account for the subject's ratio of lean muscle mass to body weight, which can have a very large effect on the resting energy expenditure (REE). The BMR is limited in this regard, so the resulting Harris Benedict Equation values are similarly limited.
Despite this limitation, the Harris Benedict Equation can be very useful. The equation uses the BMR as a baseline, then multiplies those caloric values by a variable that depends on the subject's amount of activity during the day. This results in a measure of daily calorie use that is more useful than BMR, because most subjects don't remain at rest all day.
After determining the BMR, using the Harris Benedict Equation is quite simple because it requires only a single calculation. For those who get little or no exercise beyond basic daily activities, the formula is BMR x 1.2. For those who get light activity, such as light exercise one to three days a week, the calorie calculation is BMR x 1.375. For moderately active individuals who exercise three to five days a week, the value is BMR x 1.55. Very active individuals should multiply the BMR by 1.725. Those who are extremely active, such as those who combine exercise with a physical job or who work out multiple times a day, the value is the BMR x 1.9.