Efficiency of Phenylpyruvic and Phenyllactic Acids as Substitutes for Phenylalanine in the Diet of the Growing Rat

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Abstract
Male weanling albino rats fed a diet containing all nutrients for optimal growth, including 45.4 µmoles of phenylalanine/g, gained weight at an average of 6.5 g/day. Removal of phenylalanine caused an average weight loss of 1.1 g/day. Addition of graduated increments of phenylalanine led to progressive increases in growth rate; the increases being proportional to the dietary content of this amino acid. When phenylpyruvic or L-phenyllactic acids were added isonitrogenously to the phenylalanine-free diet, growth rate also increased. Percent efficiency of these amino acid analogues as dietary substitutes for phenylalanine, calculated as varied from 50% to 70% for phenylpyruvic acid and 65% to 95% for L-phenyllactic acid. Efficiency increased as dose of analogue increased. Non-isonitrogenous substition did not change growth rates. Supplementation with a megadose of pyridoxine did not increase the efficiency of phenylpyruvic acid. D-phenyllactic acid and cinnamic acid were completely ineffective as substitutes for phenylalanine. When tyrosine was withdrawn from the diet, efficiency of phenylpyruvic acid was 70% at several doses. The efficiency of L-phenyllactic acid was 65 to 40%, decreasing with increasing dose.