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) 11years (6-16years). 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-AAs. 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, 275molL(-1); CGMP-AA, 317mol L-1; P=0.02), a decrease in tyrosine concentrations (pre-study, 50molL(-1); CGMP-AA, 40molL(-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 325mol/L: study 280mol/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.