Rather than using growth in an animal species to indicate protein quality, a variety of methods have been developed to assign a quantitative value to the pattern of amino acids in a nutritional formula or to a particular dietary protein source. Thus, assignment is based on the amounts and importance of the individual amino acids in a formula. These "scoring" methods can be used to define protein quality in terms of amino acid content for any species. Block and Mitchell pointed out in 1946 that all amino acids have to be provided simultaneously at the sites of protein synthesis in the body in the same proportions that go into the protein ( 177). Assuming that any nonessential amino acid would not be limiting, they proposed that the value of a protein could be determined from the essential amino acid most limiting in abundance relative to the optimum amount needed. From this idea of "most limiting amino acid" came the concept of chemical scoring, which has been incorporated into the important reports assessing dietary needs of humans (157, 158, 178). The key to the method is that the test protein is defined "against" a reference protein, deemed to be of the "highest quality" in terms of amino acid composition. Historically, proteins that support maximal growth in animals were considered the proteins of "highest" quality. Those proteins from the most available sources for human consumption—eggs and cow milk—were consequently used as reference proteins. In 1973, the FAO/WHO adopted the amino acid requirement pattern recommended for various age groups ( Table.2.14.) as the basis for calculating chemical scores (158, 179). Some of the scoring patterns that have been used are shown in Table. 2.1.5,. Typically, data for the 2- to 5-year-old child are taken as most representative.
The scoring system is easy to apply because no animal studies or clinical studies are required to compare different nutritional formulations. The chemical score of a protein is calculated in two steps. First a score is calculated for each essential amino acid (EAA) in the protein against the reference protein:
(cQiiltn! of tjicEAA 1» the tnt proteln/mlKlure) ^ (concent of iIn- EAA in nAnmproteln/nilxmi^:)
Next the lowest score is selected; the amino acid with the lowest score is defined as the limiting amino acid. The chemical score of the limiting amino acid is the protein's score. Typically, the limiting amino acids in dietary proteins are lysine, the sulfur-containing amino acids, threonine, and/or tryptophan. The BCAAs and phenylalanine/tyrosine are not usually limiting. The scoring method points out the obvious: proteins not balanced among the essential amino acids are not as good as those that are.
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