Atwater system
The Atwater system, named after Wilbur Olin Atwater, or derivatives of this system are used for the calculation of the available energy of foods. The system was developed largely from the experimental studies of Atwater and his colleagues in the later part of the 19th century and the early years of the 20th at Wesleyan University in Middletown, Connecticut. Its use has frequently been the cause of dispute, but few alternatives have been proposed. As with the calculation of protein from total nitrogen, the Atwater system is a convention and its limitations can be seen in its derivation . Derivation. Available energy (as used by Atwater) is equivalent to the modern usage of the term metabolisable energy (ME). formula_1 In most studies on humans, losses in secretions and gases are ignored. The gross energy (GE) of a food, as measured by bomb calorimetry is equal to the sum of the heats of combustion of the components – protein (GE), fat (GE) and carbohydrate (GE) (by difference) in the proximate system. formula_2 Atwater considered the energy value of feces in the same way. formula_3 wherever faecal excretion is small, will approximate to unity and thus these "coefficients" have a low variance  and have the appearance of constants. This is spurious since faecal excretion is variable even on a constant diet, and there is no evidence to suggest that faecal excretion is in fact related to intake in the way implied by these coefficients. Practical considerations in calculations of energy value of foods and diets. The calculation of energy values must be regarded as an alternative to direct measurement, and therefore is likely to be associated with some inaccuracy when compared with direct assessment. These inaccuracies arise for a number of reasons The theoretical and physiological objections to the assumptions inherent in  the Atwater system are likely to result in errors much smaller than these practical matters. Conversion factors were derived from experimental studies with young infants, but these produced values for metabolisable energy intake that were insignificantly different from those obtained by direct application of the modified Atwater factors.