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.

Available energy (as used by Atwater) is equivalent to the modern usage of the term metabolisable energy (ME).

$${\displaystyle {\text{Metabolisable Energy}}=\left({\text{Gross Energy in Food}}\right)-\left({\text{Energy lost in Faeces, Urine, Secretions and Gases}}\right).}$$

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_p), fat (GE_f) and carbohydrate (GE_cho) (by difference) in the proximate system.

$${\displaystyle {GE}={{GE}_{\mathrm {p} }+{GE}_{\mathrm {f} }+{GE}_{\mathrm {cho} }}.}$$

Atwater considered the energy value of feces in the same way.

$${\displaystyle {GE}^{\mathrm {F} }={{GE}_{\mathrm {p} }^{\mathrm {F} }+{GE}_{\mathrm {f} }^{\mathrm {F} }+{GE}_{\mathrm {cho} }^{\mathrm {F} }}.}$$

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