Food energy is chemical energy that animals derive from their food and molecular oxygen through the process of cellular respiration to sustain their metabolism and to drive their muscles.

 Organisms derive food energy from carbohydrates, fats and proteins as well as from organic acids, polyols, and ethanol present in the diet.

Energy is measured in joules (J). The kilojoule (kJ) is most often used for food-related quantities.

The "food calorie" or kilocalorie (kcal or Cal), equal to 4.184 kilojoules. "Small calorie" (cal), equal to 1/1000 of a food calorie, that is often used in chemistry.

Fats and ethanol have the greatest amount of food energy per mass, 37 and 29 kJ/g (8.8 and 6.9 kcal/g), respectively. Proteins and most carbohydrates have about half of that: 17 kJ/g (4.1 kcal/g).

Amount of Food Energy per Mass:

1. Fats: 37 kJ/g (8.8 kcal/g);
2. Ethanol: 29 kJ/g ( and 6.9 kcal/g).
3. Proteins and most carbohydrates: ~ 17 kJ/g (4.1 kcal/g);
4. Organic acids: 13 kJ/g (3.1 kcal/g);
5. Polyols, including sugar alcohols: 10 kJ/g (2.4 kcal/g);
6. Fiber: 8 kJ/g (2 kcal/g).

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 urine.

The amount of water, fat, and fiber in food determines its energy density.

About 20% of the energy is used by brain. Much of the rest is used for the basal metabolic requirements of other organs and tissues.