<P> Theoretically, one could measure food energy in different ways, using (say) the Gibbs free energy of combustion, or the amount of ATP generated by metabolizing the food . However, the convention is to use the heat of the oxidation reaction producing liquid water . 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 urea and other substances in the urine . The American chemist Wilbur Atwater worked these corrections out in the late 19th century (see Atwater system for more detail). Based on the work of Atwater, it became common practice to calculate energy content of foods using 4 kcal / g for carbohydrates and proteins and 9 kcal / g for lipids . The system was later improved by Annabel Merrill and Bernice Watt of the United States Department of Agriculture, who derived a system whereby specific calorie conversion factors for different foods were proposed . </P> <P> Many governments require food manufacturers to label the energy content of their products, to help consumers control their energy intake . In the European Union, manufacturers of packaged food must label the nutritional energy of their products in both kilocalories and kilojoules, when required . In the United States, the equivalent mandatory labels display only "Calories", often as a substitute for the name of the quantity being measured, food energy; an additional kilojoules figure is optional and is rarely used . In Australia and New Zealand, the food energy must be stated in kilojoules (and optionally in kilocalories as well), and other nutritional energy information is similarly conveyed in kilojoules . The energy available from the respiration of food is usually given on labels for 100 g, for a typical serving size (according to the manufacturer), and / or for the entire pack contents . </P> <P> The amount of food energy associated with a particular food could be measured by completely burning the dried food in a bomb calorimeter, a method known as direct calorimetry . However, the values given on food labels are not determined in this way . The reason for this is that direct calorimetry also burns the dietary fiber, and so does not allow for fecal losses; thus direct calorimetry would give systematic overestimates of the amount of fuel that actually enters the blood through digestion . What are used instead are standardized chemical tests or an analysis of the recipe using reference tables for common ingredients to estimate the product's digestible constituents (protein, carbohydrate, fat, etc .). These results are then converted into an equivalent energy value based on the following standardized table of energy densities . However "energy density" is a misleading term for it once again assumes that energy is IN the particular food, whereas it simply means that "high density" food needs more oxygen during respiration, leading to greater transfer of energy . </P> <P> Note that the following standardized table of energy densities is an approximation and the value in kJ / g does not convert exactly to kcal / g using a conversion factor . </P>

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