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Improved nutritional management of phenylketonuria by using a diet containing glycomacropeptide compared with amino acids

¹ From the Waisman Center, the University of Wisconsin, Madison, WI (SCvC, JAW, and DMN); the Department of Nutritional Sciences, the University of Wisconsin, Madison, WI (ELM, STG, and DMN); the Department of Food Science, the University of Wisconsin, Madison, WI (MRE); and the Departments of Statistics and Plant Pathology, the University of Wisconsin, Madison, WI (MKC).

² SCvC and ELM contributed equally to this work.

³ Supported by NIH grants R03-DK-071534 and P30-HD-03352; the Michaux Family Foundation; and the College of Agricultural and Life Science, the University of Wisconsin-Madison. Supported by grant 1UL1RR025011 from the Clinical and Translational Science Award program of the National Center for Research Resources, the National Institutes of Health.

⁴ Address reprint requests and correspondence to DM Ney, Department of Nutritional Sciences, 1415 Linden Drive, University of Wisconsin, Madison, WI 53706. E-mail: ney{at}nutrisci.wisc.edu.

Background: Phenylketonuria (PKU) requires a lifelong low-phenylalanine diet that provides the majority of protein from a phenylalanine-free amino acid (AA) formula. Glycomacropeptide (GMP), an intact protein formed during cheese production, contains minimal phenylalanine.

Objective: The objective was to investigate the effects of substituting GMP food products for the AA formula on acceptability, safety, plasma AA concentrations, and measures of protein utilization in subjects with PKU.

Design: Eleven subjects participated in an inpatient metabolic study with two 4-d treatments: a current AA diet (AA diet) followed by a diet that replaced the AA formula with GMP (GMP diet) supplemented with limiting AAs. Plasma concentrations of AAs, blood chemistries, and insulin were measured and compared in AA (day 4) and GMP diets (day 8).

Results: The GMP diet was preferred to the AA diet in 10 of 11 subjects with PKU, and there were no adverse reactions to GMP. There was no significant difference in phenylalanine concentration in postprandial plasma with the GMP diet compared with the AA diet. When comparing fasting with postprandial plasma, plasma phenalyalanine concentration increased significantly with the AA but not with the GMP diet. Blood urea nitrogen was significantly lower, which suggests decreased ureagenesis, and plasma insulin was higher with the GMP diet than with the AA diet.

Conclusions: GMP, when supplemented with limiting AAs, is a safe and highly acceptable alternative to synthetic AAs as the primary protein source in the nutritional management of PKU. As an intact protein source, GMP improves protein retention and phenylalanine utilization compared with AAs.