This paper describes a method for determining the nutritional value of d -amino acids, d -peptides, and amino acid derivatives using a growth assay in mice fed a synthetic all-amino acid diet. A large number of experiments were carried out in which a molar equivalent of the test compound replaced a nutritionally essential amino acid such as l -lysine (l -Lys), l -methionine (l -Met), l -phenylalanine (l -Phe), and l -tryptophan (l -Trp) as well as the semi-essential amino acids l -cysteine (l -Cys) and l -tyrosine (l -Tyr). The results show wide-ranging variations in the biological utilization of test substances. The method is generally applicable to the determination of the biological utilization and safety of any amino acid derivative as a potential nutritional source of the corresponding l -amino acid. Because the organism is forced to use the d -amino acid or amino acid derivative as the sole source of the essential or semi-essential amino acid being replaced, and because a free amino acid diet allows better control of composition, the use of all-amino acid for such determinations may be preferable to protein-based diets.
