Protein requirements in adults with phenylketonuria and bioavailability of glycomacropeptide compared to an l-amino acid-based product.

Background: Phenylketonuria (PKU) is caused by phenylalanine hydroxylase deficiency. Treatment is primarily a low-Phe diet combined with l-amino acid-based products (l-AA). Protein requirements in adults with PKU have not been directly determined. A formula with glycomacropeptide (GMP) and low phenylalanine is available, yet untested for optimal protein synthesis.

Objectives: To determine the protein requirements in adults with PKU and the bioavailability of GMP-AA in the same patients using the indicator amino acid oxidation (IAAO) technique.

Methods: Each participant was allocated to 7 separate l-AA intakes (range: 0.1-1.8 g/kg/day) in Experiment 1. In Experiment 2, the same patients participated in 4 GMP-AA intakes (range: 0.1-0.9 g/kg/day). The IAAO method with l-[1-¹³C]-lysine as the indicator amino acid and its oxidation to ¹³CO₂ was used as the primary indicator of protein synthesis. Protein requirements were identified with a breakpoint, and bioavailability was determined by comparing ¹³CO₂ slope from GMP-AA versus l-AA.

Results: Six adults with PKU (4 M: 2F) completed a total of 54 study days over the 2 experiments. The estimated average requirement (EAR) for protein was determined to be 1.11 g/kg/day (R² = 0.20). The bioavailability of protein from GMP-AA was determined to be 99.98%, which was high and near to 100% comparable to l-AA; although, the results apply only to the tested GMP-AA blend.

Conclusions: To our knowledge, this is the first study to directly define a quantitative protein requirement and indicates that current PKU protein recommendations for adults with PKU may be underestimated. The bioavailability of protein in the GMP-AA blend was high and optimal for protein synthesis in adults with PKU.