Preliminary support for the oral administration of valine, isoleucine and leucine for phenylketonuria.

Recent behavioral data have demonstrated the importance of maintaining low phenylalanine concentrations beyond early childhood in patients with phenylketonuria, which can be a difficult task, particularly during adolescence. Administration of certain large neutral amino-acids (valine, isoleucine, leucine--VIL) appears to reduce phenylalanine concentrations in the cerebrospinal fluid of humans and in the brain of rats. The present study compared neuropsychological test-performance of six patients with phenylketonuria during periods of VIL administration and periods when this supplement was not given. Although individual responses to VIL were variable, there was an over-all improvement of about 1 1/2 SD in neuropsychological test performance during VIL treatment. Abstract reasoning and tactile motor problem-solving increased more than pure motor performance.