Food energy is chemical energy that animals (including humans) derive from their food and molecular oxygen through the process of cellular respiration. Cellular respiration involves either the process of joining oxygen from air with the molecules of food (aerobic respiration) or the process of reorganizing the atoms within the molecules (anaerobic respiration).
Humans and other animals need a minimum intake of food energy to sustain their metabolism and to drive their muscles. Foods are composed chiefly of carbohydrates, fats, proteins, water, vitamins, and minerals. Carbohydrates, fats, proteins, and water represent virtually all the weight of food, with vitamins and minerals making up only a small percentage of the weight. (Carbohydrates, fats, and proteins comprise ninety percent of the dry weight of foods.) Organisms derive food energy from carbohydrates, fats and proteins as well as from organic acids, polyols, and ethanol present in the diet. Some diet components that provide little or no food energy, such as water, minerals, vitamins, cholesterol, and fiber, may still be necessary to health and survival for other reasons. Water, minerals, vitamins, and cholesterol are not broken down (they are used by the body in the form in which they are absorbed) and so cannot be used for energy. Fiber, a type of carbohydrate, cannot be completely digested by the human body. Ruminants can extract food energy from the respiration of cellulose because of bacteria in their rumens.
Using the International System of Units, researchers measure energy in joules (J) or in its multiples; the kilojoule (kJ) is most often used for food-related quantities. An older metric system unit of energy, still widely used in food-related contexts, is the calorie; more precisely, the "food calorie", "large calorie" or kilocalorie (kcal or Cal), 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; in the US and Canada labels spell out the unit as "calorie" or as "Calorie".
Fats and ethanol have the greatest amount of food energy per mass, 37 and 29 kJ/g (9 and 7 kcal/g), respectively. Proteins and most carbohydrates have about 17 kJ/g (4 kcal/g). The differing energy density of foods (fat, alcohols, carbohydrates and proteins) lies mainly in their varying proportions of carbon, hydrogen, and oxygen atoms: For food of elemental composition CcHhOoNn, the heat of combustion underlying the food energy is given by the empirical formula
to a good approximation (±3%). 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 contribute 10 and 13 kJ/g (2 and 3 kcal/g) respectively. The amount of water, fat, and fiber in foods determines those foods' energy density. Theoretically, one could measure food energy in different ways, using (say) the Gibbs free energy of combustion, or the amount of ATP generated by metabolizing the food. However, 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. The American chemist Wilbur Atwater worked these corrections out in the late 19th century. (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, researchers may determine a specific metabolizable energy for each component (protein, fat, etc.) of each ingredient of the feed.