Plasma amino acids in term neonates and infants with phenylketonuria before and after institution of the diet.

The plasma amino acid concentrations were measured in 12 neonates with phenylketonuria (PKU). Blood samples were taken when fasting and two hours after a meal. The individual amino acid concentrations, the glycine/valine ratios, and the total amino acid concentrations were mutually compared and also compared to reference intervals. When the diet was instituted and plasma phenylalanine decreased, we observed a normalization of the postprandial response of isoleucine, leucine, tyrosine, and valine.