Protein metabolism in adult patients with phenylketonuria

Objective Protein intake recommendations in phenylketonuria (PKU) are frequently the subject of discussion. For healthy adults, the recommended daily allowance (RDA) is 0.8 g · kg−1 · d−1, which is generally lower than that observed in the general Western population. We investigated whether whole-body protein metabolism in patients with PKU is comparable to that of healthy controls at a RDA rate of protein intake. Methods Six adult patients with well-controlled PKU and six healthy subjects of comparable age, height, and weight were studied using a primed continuous infusion of [1-13C]-valine for 8 h after an overnight fast before and during frequent meals. Normal protein was given to controls, whereas patients with PKU received a combination of an amino acid mixture and natural protein. Results No significant differences were observed between patients with PKU and controls in preprandial and prandial rates of valine appearance and oxidation and protein breakdown, protein synthesis, and net protein balance. Feeding resulted in a significant (P < 0.01) decrease in protein breakdown (PKU: 94 ± 15 μmol · kg−1 · h−1 preprandial to 49 ± 10 μmol · kg−1 · h−1 prandial; controls: 97 ± 10 μmol · kg−1 · h−1 preprandial to 55 ± 10 μmol · kg−1 · h−1 prandial), whereas no effects were observed in protein synthesis (PKU: 77 ± 10 μmol · kg−1 · h−1 preprandial to 73 ± 7 μmol · kg−1 · h−1 prandial; controls: 76 ± 8 μmol · kg−1 · h−1 preprandial to 71 ± 5 μmol · kg−1 · h−1 prandial). Net protein balance increased from negative prandial to positive preprandial values (PKU: −17 ± 6 μmol · kg−1 · h−1 preprandial to +23 ± 8 μmol · kg−1 · h−1 prandial; controls: −21 ± 4 μmol · kg−1 · h−1 preprandial to +16 ± 9 μmol · kg−1 · h−1 prandial). Conclusion Whole-body protein metabolism in adult patients with PKU is fully comparable to that in healthy controls at the RDA level of protein intake.