A Preliminary Investigation of the Role of the Phenylalynine: Tyrosine Ratio in Children With Early and Continuously Treated Phenylketonuria: Toward Identification of "Safe" Levels

Children with early and continuously treated phenylketonuria (ECT-PKU) remain at risk of developing executive function (EF) deficits. There is some evidence that a high phenylalanine to tyrosine ratio (phe:tyr) is more strongly associated with impaired EF development than high phenylalanine alone. This study examined EF in a sample of 11 adolescents against concurrent and historical levels of phenylalanine, phe:tyr, and tyrosine. Lifetime measures of phe: tyr were more strongly associated with EF than phenylalanine-only measures. Children with a lifetime phe: tyr less than 6 demonstrated normal EF, whereas children who had a lifetime phe: tyr above 6, on average, demonstrated clinically impaired EF.