food energy

Food energy is the amount of energy in food that is available through digestion. The values for food energy are expressed in kilocalories (kcal) and kilojoules (kJ). One food Calorie is the amount of digestively available food energy (heat) that will raise the temperature of one kilogram of water one degree Celsius. Some advocate the convention of the capitalizing the C in these so that one Calorie is equal to 1000 lowercase calories, but that is not a convention generally followed. The large Calorie is sometimes abbreviated kcal, to indicate clearly that is 1000 times as large as the calorie. Consequently, the prefix kilo- is not used with large Calories. Food calories are also more specifically called kilocalories on the basis of the small calorie usage. This term, which makes it clear that large Calories are intended, is widely used by professional nutritionists when speaking in terms of calories rather than joules, but the term kilocalorie for 1,000 small calories is less often used by laypersons. The International System of Units unit kilojoule is becoming more common. In some countries (Australia, for example) only the kilojoule is normally used. Some types of food contain more food energy per gram than others: fats and ethanol have particularly high values for food energy density: 9 and 7 kcal/gram, respectively. Sugars and proteins have about 4 kcal/gram. One Calorie is approximately equal to 4.1868 joules. Each food item has a specific metabolizable energy intake (MEI). For a normal human this value is obtained by multiplying the number of kilocalories or kilojoules contained in a food item by 85%, which is the amount of energy actually obtained by a human after the digestive processes have been completed.
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