Amino acids may be grouped according to their transport affinities or essential role in animal nutrition or on the basis of catabolic fate of the carbon skeleton. Additional subsets are also recognized, based on common structural features. Thus, leucine, isoleucine and valine are referred to as the branched-chain amino acids (BCAA), whereas phenylalanine and tyrosine are categorized as the aromatic amino acids. Assessments of requirements for

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sulphur amino acids (SAA) are invariably based on a summation of methionine and cyst(e)ine needs, but it will be recalled that homocysteine is another member of this group. Another subset, the excitatory amino acids, includes glutamate and aspartate.

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