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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Atwater system | 1/2 | https://en.wikipedia.org/wiki/Atwater_system | reference | science, encyclopedia | 2026-05-05T14:14:55.347304+00:00 | kb-cron |
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).
Metabolisable Energy
=
(
Gross Energy in Food
)
−
(
Energy lost in Faeces, Urine, Secretions and Gases
)
.
{\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.
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{\displaystyle {GE}={{GE}_{\mathrm {p} }+{GE}_{\mathrm {f} }+{GE}_{\mathrm {cho} }}.}
Atwater considered the energy value of feces in the same way.
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{\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.
Digestible energy
=
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{\displaystyle {\text{Digestible energy}}={{GE}_{\mathrm {p} }(D_{\mathrm {p} })}+{{GE}_{\mathrm {f} }(D_{\mathrm {f} })}+{{GE}_{\mathrm {cho} }(D_{\mathrm {cho} })},}
where Dp, Df, and Dcho are respectively the digestibility coefficients of protein, fat and carbohydrate calculated as
intake
−
faecal excretion
intake
{\displaystyle {\frac {{\text{intake}}-{\text{faecal excretion}}}{\text{intake}}}}
for the constituent in question. Urinary losses were calculated from the energy to nitrogen ratio in urine. Experimentally this was 7.9 kcal/g (33 kJ/g) urinary nitrogen and thus his equation for metabolisable energy became
M
E
=
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G
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7.9
6.25
)
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{\displaystyle {ME}=\left({GE}_{\mathrm {p} }-{\frac {7.9}{6.25}}\right)D_{\mathrm {p} }+{GE}_{\mathrm {f} }D_{\mathrm {f} }+{GE}_{\mathrm {cho} }D_{\mathrm {cho} }.}
=== Gross energy values === Atwater collected values from the literature and also measured the heat of combustion of proteins, fats and carbohydrates. These vary slightly depending on sources and Atwater derived weighted values for the gross heat of combustion of the protein, fat and carbohydrate in the typical mixed diet of his time. It has been argued that these weighted values are invalid for individual foods and for diets whose composition in terms of foodstuffs is different from those eaten in the US in the early 20th century.
=== Apparent digestibility coefficients === Atwater measured a large number of digestibility coefficients for simple mixtures, and in substitution experiments derived values for individual foods. These he combined in a weighted fashion to derive values for mixed diets. When these were tested experimentally with mixed diets they did not give a good prediction, and Atwater adjusted the coefficients for mixed diets.
=== Urinary correction === 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.
== Modified system == 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. 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.