The Phenylalanine Requirement of the Normal Infant

Abstract

A study to evaluate the phenylalanine requirement of infants was carried out employing a synthetic diet, the nitrogen moiety of which consisted of a mixture of 18 amino acids, essential and non-essential, all as the natural L-isomers. By graded reduction of the phenylalanine, which was replaced by glycine, the minimal phenylalanine intake compatible with normal health was determined. It was found to be approximately 90 mg per kilogram per day. A deficiency of phenylalanine was associated with failure to gain weight, impaired nitrogen retention, due primarily to increased azoturia and hypoglobulinemia. An increase in free amino acid excretion in the urine was observed. The excretion pattern of free amino acids in the urine showed a striking decrease in phenylalanine itself and certain other consistent changes, notably an increased histidinuria.