Benefits of normalizing plasma phenylalanine: Impact on behaviour and health. A case report

An elderly man with mental retardation who had never received dietary treatment for his phenylketonuria was placed on a phenylalanine‐restricted diet. Social skills and walking gait improved and a new interest in the objects in his environment developed spontaneously. A 2‐year analysis of diet, blood plasma phenylalanine levels and behavioural state indicated that small differences in phenylalanine intake impacted his well‐being. Of significant note, leg tremor and spasm that precipitated severe self‐injury were only reversible when plasma blood phenylalanine concentrations were titrated to near normal ranges and daily phenylalanine intake was strictly controlled. This case may offer a potential explanation for some of the late treatment failures that have been reported and suggest new avenues to explore in the late treatment of PKU.