Food Energy

Food energy is the amount of energy obtained from food that is available through cellular respiration.

Food energy is expressed in food calories (labeling: EU kcal, US/Canada Calories) or kilojoules (kJ). Food calories, or the "Calorie" units used often in nutritional contexts, measure amounts of energy 1000 times greater than the units in scientific contexts known also as calories, or gram calories ("cal"). Food calories are thereby referred to less ambiguously in some formal contexts as kilocalories (kcal) or kilo calories. One food calorie is equal to 4.184 kilojoules. Within the European Union, both the kilocalorie (kcal) and kilojoule (kJ) appear on nutrition labels. In many countries, only one of the units is displayed.

Carbohydrates, fiber, fats, proteins, organic acids, polyols, and ethanol all release energy during respiration—this is often called 'food energy'. When nutrients react with oxygen in the cells of living things, energy is released. A small amount of energy is available through fermentation. Fats and ethanol have the greatest amount of food energy per mass, 9 and 7 kcal/g (38 and 30 kJ/g), respectively. Proteins and most carbohydrates have about 4 kcal/g (17 kJ/g) . Carbohydrates that are not easily absorbed, such as fiber or lactose in lactose-intolerant individuals, contribute less food energy. Polyols (including sugar alcohols) and organic acids have less than 4 kcal/g.

Theoretically, food energy could be measured in different ways, such as Gibbs free energy of combustion, or the amount of ATP generated by metabolizing the food. But the convention is to use the heat of the oxidation reaction, with the water substance produced being in the liquid phase. Conventional food energy is based on heats of combustion in a bomb calorimeter and corrections that take into consideration the efficiency of digestion and absorption and the production of urea and other substances in the urine. These were worked out in the late 19th century by the American chemist Wilbur Atwater. See Atwater system for more detail.

Each food item has a specific metabolizable energy intake (MEI). This value can be approximated by multiplying the total amount of energy associated with a food item by 85%, which is the typical amount of energy actually obtained by a human after respiration has been completed. In animal nutrition where energy is a critical element of the economics of meat production, a specific metabolizable energy may be determined for each component (protein, fat, etc.) of each ingredient of the feed.