By measuring coefficients of availability or in modern terminology apparent digestibility, Atwater derived a system for calculating faecal energy losses.

Apparent digestibility coefficients of protein, lipid and carbohydrate in practical diets fed to paddlefish, "polyodon spathula" (Walbaum).

Both these effects however lead to reductions in apparent digestibility, and therefore in the Atwater system produce small changes in the proper energy conversion factors for those diets.

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 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.