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

Like other forms of energy, food energy is expressed in calories or joules. Some countries use the food calorie, which is equal to 1 kilocalorie (kcal), or 1,000 calories. In the context of nutrition, and especially food labeling, the calories are large calories approximately equal to 4.1868 kilojoules (kJ). The kilojoule is the unit officially recommended by the World Health Organization and other international organizations. In some countries only the kilojoule is normally used on food packaging, while in others the calorie is the most common unit.

Fiber, fats, proteins, organic acids, polyols, and ethanol all release energy during respiration — this is often called 'food energy'. When the food (providing fuel) reacts with oxygen in the cells of living things energy is released. A small amount of energy is available through anaerobic respiration. Nutritionists usually talk about the number of calories in a gram of a nutrient, but this implies that the food actually 'contains' energy. It's better to say that each gram of food (fuel) is associated with a particular amount of energy (released when the food is respired). 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.

Each food item has a specific metabolizable energy intake (MEI). Normally this value is obtained 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.

The amount of food energy associated with a particular food could be measured by completely burning the dried food in a bomb calorimeter, a method known as direct calorimetry. However, the values given on food labels are not determined this way, because it overestimates the amount of fuel that actually enters the blood through digestion because it also burns the indigestible dietary fiber so that not all food eaten is actually absorbed by the body (fecal losses). Instead, standardized chemical tests or an analysis of the recipe using reference tables for common ingredients are used to estimate the product's digestible constituents (protein, carbohydrate, fat, etc.). These results are then converted into an equivalent energy value based on a standardized table of energy densities.

Calorimetry is the science of measuring the heat of chemical reactions or physical changes. Calorimetry is performed with a calorimeter. The word calorimetry is derived from the Latin word calor, meaning heat. Scottish physician and scientist Joseph Black, who was the first to recognize the distinction between heat and temperature, is said to be the founder of calorimetry.

Indirect calorimetry calculates heat that living organisms produce from their production of carbon dioxide and nitrogen waste (frequently ammonia in aquatic organisms, or urea in terrestrial ones), OR from their consumption of oxygen. Lavoisier noted in 1780 that heat production can be predicted from oxygen consumption this way, using multiple regression. The Dynamic Energy Budget theory explains why this procedure is correct. Of course, heat generated by living organisms may also be measured by direct calorimetry, in which the entire organism is placed inside the calorimeter for the measurement.

Constant-volume calorimetry is calorimetry performed at a constant volume. This involves the use of a constant-volume calorimeter. No work is performed in constant-volume calorimetry, so the heat measured equals the change in internal energy of the system.

See also: Food Energy

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