validate their use as predictors of protein digestibility in humans.

Calculations and Examples

The EAA composition and protein digestibility of the food or mixed diet being tested are determined. Then the percentage or fractional value of the most limiting EAA (noncorrected amino acid score) is multiplied by the percentage or fractional value of 'true' protein digestibility to obtain the corrected score, which is equivalent to protein quality. This value can be used as such or it can be expressed in relation to the corrected amino acid score of a reference protein or food, usually casein or an animal food (milk, egg, or beef).

Proteins that have no limiting amino acids are assigned an amino acid score of 100% (or 1.00) that must be only corrected for digestibility.

Table 6 Calculation of amino acid scores of single protein sources corrected for digestibility and in relation to the protein quality of cow's milk

Food Most limiting amino acid Noncorrected amino True protein Corrected amino Protein quality relative to acid score digestibility acid score milk

Polished rice Lysine 36 mg per g protein (36/58) x 100 = 62% x88% = 55% (55/95) x 100 = 58%

Egg white None >100! 100% x97% = 97% (97/95) x 100 = 102%

Similarly, if the clinical or experimental assessment of 'true' protein digestibility gives a value greater that 100% (generally due to experimental variability), a digestibility correction factor of 100% (or 1.00) is applied to the amino acid score. Table 6 shows examples of calculations for a single food as protein source. The same procedure can be used for food mixtures using a weighted average procedure based on the protein content, amino acid composition, and digestibility of the individual components. Table 7 shows an example of those calculations. For simplicity, the example uses only the four EAAs that are most often limiting.

Protein Concentration

Protein concentration or density (i.e., the amount of protein per unit of food) is another factor of a food's protein quality. Protein-dense foods are especially important for young infants, whose small gastric capacity limits the amount they can eat, and for elderly people with poor appetite. Evaluation of a food's protein concentration must be done for ready-to-eat preparations because food processing and cooking can result in significant changes relative to raw foods. Meats, poultry, and fish usually have a higher concentration of protein after cooking or frying, whereas vegetable food preparations contain more water and less protein than the raw products (Table 8).

Protein/Energy Ratio

The percentage of protein energy in the diet (P/E ratio) has been used to describe whether a diet provides adequate amounts of protein. The reasoning is that energy requirements are the main driving force for food intake. Therefore, a diet is adequate if it satisfies the requirements for all nutrients when it is eaten in amounts that will satisfy energy needs.

P/E ratio is calculated by dividing the amount of metabolizable energy derived from dietary protein (grams of protein x 16.7kJ or 4kcal) by the total amount of metabolizable energy in the diet, multiplied by 100 to avoid using fractional values. However, the use of P/E ratio as an index of food's protein adequacy may be misleading because it only gives information about protein concentration and does not indicate the biological value or quality of the proteins. Its usefulness improves when amino acid score is taken into account to calculate what can be defined as a desirable P/E ratio, as in the examples discussed later.

The P/E ratio indicates the amount of protein that the diet provides relative to energy and does not imply a constant relationship between protein and energy requirements. For example, the lower limit of the desirable P/E ratio of a diet with an amino acid score of 85% is 6.2 for a young child whose daily requirements are 16 g protein and 5.1 MJ energy ((16g x 16.7kJ/0.85)/ 5100 kJ x 100). For an adult male with daily requirements of 55 g protein and 12.8 MJ, the desirable P/E ratio is 8.4 ((55 x 16.7/0.85)/ 12,800 x 100).

Diets, especially those eaten by adults, often provide protein in amounts that surpass requirements, which elevates the P/E ratio. For example, almost all adult populations eat diets with P/E ratios between 10 and 15%. This is related to culture and food availability and does not reflect a biologically optimal ratio. Consistent with the calculations in the preceding paragraph, P/E ratios of 10 and 15 are adequate and it cannot be argued that one is nutritionally better than the other.

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