Calorie expenditure is the amount of energy the body uses in a certain period of time. Everything the body does requires energy. Even while sleeping, the body is still performing myriad processes, including breathing, digestion, and healing. The calorie expenditure is different for each activity and each individual, depending upon a number of factors including age, weight, and gender.

A “calorie” is simply a unit of measure used in relation to the body’s energy usage, with that energy coming from food intake. When a food is listed as having 300 calories, that means it contains 300 units of energy. When an activity is said to burn 300 calories, that means the body uses 300 units of energy to complete the activity.

Estimating calorie expenditure is important for many different reasons. Doctors use it to determine how much nutritional support critically ill patients need. Bodybuilders use it to determine how much food to eat in order to promote muscle growth, and dieters use it to determine how much food to eat to promote fat loss. The base calculation is called the Total Daily Energy Expenditure (TDEE), and is the total energy the body needs to function throughout a 24 hour period. A caloric intake below the TDEE will result in weight loss, and a caloric intake above the TDEE will result in weight gain.

There are many different methods of determining TDEE, with varying degrees of accuracy. The easiest method for individuals looking for a rough estimate is to multiply the body weight by 15. The result is a very basic estimation of TDEE that is most likely not very accurate. Many factors are important to consider when calculating caloric needs and expenditure, including body composition.

The Harris Benedict equation is the one widely used by doctors and fitness professionals because of its overall accuracy. This method takes many factors into account, including basal metabolic rate (BMR), which is the body’s calorie expenditure just for performing basic bodily functions, excluding physical activity. In addition, the calculation involves an “activity multiplier.” which fine-tunes for physical activity.

First, the BMR is calculated differently for men and women:

Men: BMR= 66+ (13.7 x weight in kg) + ( 5 x height in cm) - (6.8 x age in years)

Women: BMR= 655+ (9.6 x weight in kg) + (1.8 x height in cm) - (4.7 x age in years)

Next, the BMR is multiplied by the activity multiplier, which ranges on a scale from a value of 1.2 for a sedentary person to a value of 1.9 for a professional athlete. For example, if a 32-year-old woman was 5’4” ( about 1.6 meters) tall, weighed 130 pounds (about 59 kg), and did moderate exercise four days per week, the equation would be:

BMR=655 + (9.6 x 59 kg) + (1.8 x 162.6 cm) - (4.7 x 32) = 1363.68

TDEE = 1363.68 x 1.6 = 2181.888

The Harris Benedict equation is accurate for most people, but because it does not take lean body mass into account, it is not accurate for the extremely muscular and the extremely obese. For these two groups, doctors use the Katch-McArdle formula, which is the same for both men and women:

BMR = 370 + (21.6 x kg lean mass)

The Katch-McArdle formula relies on doctor-determined body composition data to form a more accurate picture of the daily caloric needs of these patients.