Behavioural effects of phenylalanine-free amino acid tablet supplementation in intellectually disabled adults with untreated phenylketonuria

Aim: To evaluate the effects of phenylalanine (Phe)‐free essential amino acid (AA) tablets enriched in tyrosine and tryptophan on the performance of intellectually disabled adult patients with untreated phenylketonuria (PKU). Methods: Phe‐free AA tablets and placebo tablets were administered to 19 untreated PKU subjects on a normal diet for 6 mo in a prospective double‐blinded crossover study. The adaptive behaviour of the patients was tested prior to the study and at 6 and 12 mo after the start, using a simplified version of the Vineland Adaptive Behaviour Scale. For each sub‐domain, the patients were rated either “0” (for poor performance) or “1” (for good performance). Neurological signs and symptoms and specific behavioural characteristics were recorded monthly by caretakers. Every 6 mo, neurological examination of the patients was performed, and the caretakers were interviewed. The statistical significance of the results was tested by means of the Fisher's exact and Wilcoxon tests. Results: The most significant changes were an improved concentration and the development of a meaningful smile, which were observed in 44% and 43% of the patients on AA tablet treatment, respectively, but not patients on placebo. Other important but less significant changes included increased awareness of external stimuli (63%) and less self‐injury (43%), and 40% were smiling and laughing occasionally. The mean overall rating increased from an initial value of 6.3 to 10.1 in patients when on AA tablet treatment (p=0.002), and to 7.0 in patients when on placebo (p=0.068). The difference between active AA treatment and placebo was statistically significant (p=0.027). Conclusions: This pilot study suggests that Phe‐free AA tablets enriched in tyrosine and tryptophan may improve the quality of life in some intellectually disabled adults with untreated PKU.