<P> The energy / nitrogen ratio in urine shows considerable variation and the energy / organic matter is less variable, but the energy / nitrogen value provided Atwater with a workable approach although this has caused some confusion and only applies for subjects in nitrogen balance . </P> <P> 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 USDA, who derived a system whereby specific calorie conversion factors for different foods were proposed . This takes cognizance of the fact that first the gross energy values of the protein, fats and carbohydrates from different food sources are different, and second, that the apparent digestibility of the components of different foods is different . </P> <P> This system relies on having measured heats of combustion of a wide range of isolated proteins, fats and carbohydrates . It also depends on data from digestibility studies, where individual foods have been substituted for basal diets in order to measure the apparent digestibility coefficients for those foods . This approach is based on the assumption that there are no interactions between foods in a mixture in the intestine, and from a practical view point, such studies with humans are difficult to control with the required accuracy . </P> <P> The carbohydrate by difference approach presents several problems . First, it does not distinguish between sugars, starch and the unavailable carbohydrates (roughage, or "dietary fibre"). </P>

In the atwater system what is the average value used for protein