Sixteen adolescents and young adults with phenylketonuria were treated with a mixture of valine, isoleucine, and leucine for four 3-month periods, and biochemical and neuropsychological tests were carried out before and after treatment at the Metabolic Disease Center, Children's Hospital Medical Center, University of Cincinnati College of Medicine. The performance of timed attentional tests and a continuous performance test was improved during the valine, isoleucine, and leucine periods compared to the control mixture periods. The attention diagnostic method, a test with strong attentional components, showed significant improvements. These results were consistent with earlier reports of improvement in specific cognitive processes with valine, isoleucine, and leucine treatment in patients who were unable to maintain low serum phenylalanine levels. 
COMMENT. Phenylalanine and other large neutral amino acids share common receptocytes on a blood brain barrier transport system. The administration of other large neutral amino acids to patients with elevated plasma phenylalanine may reduce the amount of phenylalanine reaching the brain and prevent further deficits in cognition. The amino acid mixture consisted of 150 mg/kg Valine, 150 mg/kg Isoleucine, and 200 mg/kg Leucine. This mixture was prescribed as a supplement to the low phenylalanine formula.