The body requires energy to support normal functions and physical activity, growth, and repair of damaged tissues. Energy is provided by oxidation of dietary protein, fat, carbohydrate, and alcohol. Oxidation of 1 g of each provides 4 kcal of energy from protein and carbohydrate, 9 kcal from fat, and 7 kcal from alcohol.
In healthy adults, energy expenditure is primarily determined by three factors: basal energy expenditure (BEE), thermic effect of food (TEF), and physical activity. The BEE is the amount of energy required to maintain basic physiologic functions. It is measured while the subject is resting in a warm room, not having eaten for 12 hours. In healthy persons, the BEE (in kcal/24 h) can be estimated by the Harris—Benedict equation, which will correctly predict measured BEE in 90% ± 10% of healthy subjects. In clinical practice, patients rarely meet the strict criteria for BEE measurement. Instead, energy expenditure is measured in individuals at rest without food for 2 hours. This measurement, the resting energy expenditure (REE), is about 10% greater than BEE. TEF, the amount of energy expended during and following the ingestion of food, averages approximately 10% of the BEE.