Atwater system

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.

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.

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.

The experimental evidence for the magnitude of this variation is very limited, but as the heats of combustion of the individual amino-acids are different it is reasonable to expect variations between different proteins. An observed range of from 5.48 for conglutin (from blue lupin) to 5.92 for Hordein (barley) was reported, which compares with Atwaters' range of 5.27 for gelatin to 5.95 for wheat gluten. It is difficult to calculate expected values for a protein from amino-acid data, as some of the heats of combustion are not known accurately. Preliminary calculations on cow's milk suggest a value of around 5.5 kcal/g (23.0 kJ/g).

Analogously the experimental evidence is limited, but since the fatty acids differ in their heats of combustion one should expect fats to vary in heats of combustion. These differences are, however, relatively small – for example, breast milk fat has a calculated heat of combustion of 9.37 kcal/g (39.2 kJ/g) compared with that of cow milk fat of 9.19 kcal/g (38.5 kJ/g).

Monosaccharides have heats of combustion of around 3.75 kcal/g (15.7 kJ/g), disaccharides 3.95 kcal/g (16.5 kJ/g) and polysaccharides 4.15 to 4.20 kcal/g (17.4 to 17.6 kJ/g). The heat of hydrolysis is very small and these values are essentially equivalent when calculated on a monosaccharide basis. Thus 100 g sucrose gives on hydrolysis 105.6 g monosaccharide and 100 g starch gives on hydrolysis 110 g glucose.

The human digestive tract is a very efficient organ, and the faecal excretion of nitrogenous material and fats is a small proportion (usually less than 10%) of the intake. Atwater recognised that the faecal excretion was a complex mixture of unabsorbed intestinal secretions, bacterial material and metabolites, sloughed mucosal cells, mucus, and only to a small extent, unabsorbed dietary components. This might be one reason why he chose to use availability rather than digestibility. His view was that these faecal constituents were truly unavailable and that his apparent disregard of the nature of faecal excretion was justifiable in a practical context.

The ratio ${\displaystyle {\frac {{\text{intake}}-{\text{faecal excretion}}}{\text{intake}}},}$ 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.

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

• 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.

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.