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, but the calorie is still the most common unit in many countries.

Fiber, fats, proteins, organic acids, polyols, and ethanol all release energy during respiration - this is often called, unhelpfully, 'food energy'. It is only when the food (providing fuel) reacts with oxygen in the cells of living things that energy is released. A small amount of energy is available through anaerobic respiration. All foods are made up of a combination of these six caloric nutrients and non-caloric nutrients. Non-caloric food includes (but not limited to) water, vitamins, minerals, antioxidants, caffeine, spices and natural flavors. Tea and coffee also cannot be respired to release calories without sugar or milk added. 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 gram, 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 fewer 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.