Energy metabolism

Man covers energy expenditure by the oxidation of the nutrients carbohydrate, protein, fat, and sometimes alcohol. Energy expenditure is assessed with indirect calorimetry, measuring oxygen consumption and/or carbon dioxide production, and with direct calorimetry, measuring heat loss from nutrient oxidation. Energy expenditure is a function of (1) body composition determining resting energy expenditure (REE), (2) food intake determining the energy cost of food processing, and (3) body movement determining activity-induced energy expenditure (AEE). Energy expenditure, and thus energy requirement, is generally higher in larger individuals, e.g., overweight and obese subjects, through a higher REE. Subjects in negative energy balance show reduced energy expenditure, mainly through a reduction of AEE. Physical activity is rather a function than a determinant of energy balance. The main determinant of energy balance is energy intake.