Sources of food energy may be grouped into the categories of proteins, fats and carbohydrates. The calorie content of these food groups varies according to the system used to measure them. A single value may be used for each energy source, but a more exact measurement of calories depends on the specific type of food.
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Scientific measurements of the energy in food are made in joules, but dieticians and nutritionists still use the kilocalorie, which is 1,000 calories. The calorie is not a scientific unit of measure and is, therefore, not formally standardized. The calorie most often used in the context of food energy is based on the thermochemical calorie which has been defined as 4.184 joules.
Atwater General Factor System
The Atwater general factor system uses a single factor to determine the energy in each of the major sources of food energy. This value is based on the heat of combustion and accounts for the energy losses due to absorption, digestion and excretion. The Atwater general factor system provides a value of 4.1 kilocalories per gram (kcal/g) for protein, 8.8 kcal/g for fat, 4.1 kcal/g for carbohydrates and 6.9 kcal/g for alcohol. This system includes fiber in the carbohydrate energy value.
Atwater Specific Factor System
The Atwater specific factor system provides values for proteins, fats and carbohydrates that are based on specific foods. This system accounts for the fact that these energy sources have different heats of combustion and digestibility depending on the foods they’re found in. The Atwater specific factor system consists of a set of tables that provide the energy content for each of the listed foods. The energy content of protein in this system ranges from 2.44 kcal/g to 4.36 kcal/g for protein. Fat has 8.37 kcal/g to 9.02 kcal/g and carbohydrates have 2.70 kcal/g to 4.16 kcal/g.
General Factor System
The general factor system is a modification of the Atwater general factor system that separates fiber from carbohydrates. This system uses a value of 3.75 kcal/g for monosaccharides (sugar) and a value of 2.0 kcal/g for dietary fiber.
Manufacturers generally use values of 4 kcal/g for proteins and carbohydrates and 9 kcal/g for fat when calculating the calories in food. The total caloric content of food may then be given as (grams of protein) x 4 +(grams of carbohydrates) x 4 + (grams of fat) x 9. The manufacturer may also subtract the caloric value of indigestible fiber from this total.