Glycomacropeptide for nutritional management of phenylketonuria: a randomized, controlled, crossover trial

Am J Clin Nutr. 2016 Aug;104(2):334-45. doi: 10.3945/ajcn.116.135293. Epub 2016 Jul 13.


Background: To prevent cognitive impairment, phenylketonuria requires lifelong management of blood phenylalanine (Phe) concentration with a low-Phe diet. The diet restricts intake of Phe from natural proteins in combination with traditional amino acid medical foods (AA-MFs) or glycomacropeptide medical foods (GMP-MFs) that contain primarily intact protein and a small amount of Phe.

Objective: We investigated the efficacy and safety of a low-Phe diet combined with GMP-MFs or AA-MFs providing the same quantity of protein equivalents in free-living subjects with phenylketonuria.

Design: This 2-stage, randomized crossover trial included 30 early-treated phenylketonuria subjects (aged 15-49 y), 20 with classical and 10 with variant phenylketonuria. Subjects consumed, in random order for 3 wk each, their usual low-Phe diet combined with AA-MFs or GMP-MFs. The treatments were separated by a 3-wk washout with AA-MFs. Fasting plasma amino acid profiles, blood Phe concentrations, food records, and neuropsychological tests were obtained.

Results: The frequency of medical food intake was higher with GMP-MFs than with AA-MFs. Subjects rated GMP-MFs as more acceptable than AA-MFs and noted improved gastrointestinal symptoms and less hunger with GMP-MFs. ANCOVA indicated no significant mean ± SE increase in plasma Phe (62 ± 40 μmol/L, P = 0.136), despite a significant increase in Phe intake from GMP-MFs (88 ± 6 mg Phe/d, P = 0.026). AA-MFs decreased plasma Phe (-85 ± 40 μmol/L, P = 0.044) with stable Phe intake. Blood concentrations of Phe across time were not significantly different (AA-MFs = 444 ± 34 μmol/L, GMP-MFs = 497 ± 34 μmol/L), suggesting similar Phe control. Results of the Behavior Rating Inventory of Executive Function were not significantly different.

Conclusions: GMP-MFs provide a safe and acceptable option for the nutritional management of phenylketonuria. The greater acceptability and fewer side effects noted with GMP-MFs than with AA-MFs may enhance dietary adherence for individuals with phenylketonuria. This trial was registered at as NCT01428258.

Keywords: executive function; inborn errors of amino acid metabolism; medical food; phenylalanine; sapropterin dihydrochloride; threonine; tyrosine.

Publication types

  • Comparative Study
  • Randomized Controlled Trial

MeSH terms

  • Adolescent
  • Adult
  • Analysis of Variance
  • Caseins / chemistry
  • Caseins / therapeutic use*
  • Cross-Over Studies
  • Dietary Proteins / chemistry
  • Dietary Proteins / therapeutic use*
  • Feeding Behavior
  • Female
  • Foods, Specialized*
  • Gastrointestinal Diseases / etiology
  • Gastrointestinal Diseases / prevention & control
  • Humans
  • Hunger
  • Male
  • Middle Aged
  • Patient Satisfaction
  • Peptide Fragments / chemistry
  • Peptide Fragments / therapeutic use*
  • Phenylalanine* / administration & dosage
  • Phenylalanine* / blood
  • Phenylketonurias / blood
  • Phenylketonurias / diet therapy*
  • Young Adult


  • Caseins
  • Dietary Proteins
  • Peptide Fragments
  • caseinomacropeptide
  • Phenylalanine

Associated data