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. (Contrast the "small calorie" (cal), equal to 1/1000 of a food calorie, that is often used in chemistry and in physics.) 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, , respectively. Proteins and most carbohydrates have about . 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 100 kcal/g (c + 0.3 h – 0.5 o)/(12 c + h + 16 o +14 n) 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 and respectively. The amount of water, fat, and fiber in foods determines those foods' energy density.
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, , respectively. Proteins and most carbohydrates have about . 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 100 kcal/g (c + 0.3 h – 0.5 o)/(12 c + h + 16 o +14 n) 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 respectively. The amount of water, fat, and fiber in foods determines those foods' energy density.
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(Contrast the "small calorie" (cal), equal to 1/1000 of a food calorie, that is often used in chemistry and in physics.)
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