Glycomacropeptide in children with phenylketonuria : does its phenylalanine content affect blood phenylalanine control?

Background In phenylketonuria ( PKU ), there are no data available for children with respect to evaluating casein glycomacropeptide ( CGMP ) as an alternative to phenylalanine‐free protein substitutes [Phe‐free L‐amino acid ( AA )]. CGMP contains a residual amount of phenylalanine, which may alter blood phenylalanine control. Methods In a prospective 6‐month pilot study, we investigated the effect on blood phenylalanine control of CGMP ‐amino acid ( CGMP ‐ AA ) protein substitute in 22 PKU subjects (13 boys, nine girls), median age (range) 11 years (6–16 years). Twelve received CGMP ‐ AA and nine received Phe‐free L‐ AA , (1 CGMP ‐ AA withdrawal). Subjects partially or wholly replaced Phe‐free L‐ AA with CGMP ‐ AA . If blood phenylalanine exceeded the target range, the CGMP ‐ AA dose was reduced and replaced with Phe‐free L‐amino acids. The control group remained on Phe‐free L‐ AA s. Phenylalanine, tyrosine and Phe : Tyr ratio concentrations were compared with the results for the previous year. Results In the CGMP ‐ AA group, there was a significant increase in blood phenylalanine concentrations (pre‐study, 275 μmol L −1 ; CGMP ‐ AA , 317 μmol L −1 ; P  = 0.02), a decrease in tyrosine concentrations (pre‐study, 50 μmol L −1 ; CGMP ‐ AA , 40 μmol L −1 ; P  = 0.03) and an increase in Phe : Tyr ratios (pre‐study, Phe : Tyr 4.9:1; CGMP ‐ AA , Phe : Tyr 8:1; P  = 0.02). In the control group there was a non‐significant fall in phenylalanine concentrations (pre‐study 325μmol/L: study 280μmol/L [p = 0.9], and no significant changes for tyrosine or phe/tyr ratios [p = 0.9]. Children taking the CGMP‐AA found it more acceptable to L‐AA. Conclusions Blood phenylalanine control declined with CGMP ‐ AA but, by titrating the dose of CGMP ‐ AA , blood phenylalanine control remained within target range. The additional intake of phenylalanine may have contributed to the change in blood phenylalanine concentration. CGMP ‐ AA use requires careful monitoring in children.