# Atwater system

The Atwater system (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 no real 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 (GEp), fat (GEf) and carbohydrate (GEcho) (by difference) in the proximate system.

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

• Variations in food composition: Foods are biological mixtures and as such show considerable variation in composition, particularly in respect of water and fat content. This means that compositional values quoted for representative samples of foods in food composition tables do not necessarily apply to individual samples of foods. In studies where great accuracy is required, samples of the food consumed must be analysed.
• Measurements of food intake: In estimating energy intakes, measurements of food intake are made, and these are known to be subject to considerable uncertainty. Even in studies under very close supervision the errors in weighing individual food items are rarely less than ± 5%. A certain degree of pragmatism must therefore be used when assessing procedures for calculating energy intakes, and many authors impute greater accuracy to quoted calculated energy intakes than is justifiable.
• Individual variation: Variations in individuals are seen in all human studies, and these variations are not allowed for in most calculations.
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