Necessity of Complete Intake of Phenylalanine-free Amino Acid Mixture for Metabolic Control of Phenylketonuria

The importance of complete or almost complete intake of the recommended amount of phenylalanine-free amino acid mixture (AAM) for control of blood phenylalanine level in patients being treated for phenylketonuria (PKU) has not been universally appreciated. We observed the effect of complete intake of AAM on plasma phenylalanine levels during hospitalization in 6 patients with PKU (5 pregnant women with PKU and 1 child) who had poor metabolic control because of less than full compliance with prescribed AAM intake. Before hospitalization, all but 1 of the patients had blood phenylalanine levels above 1,000 μmol/L; in 1 patient the blood phenylalanine level was 703 μmol/L. During 9 periods of observation in the 6 patients, the levels of plasma phenylalanine decreased to the recommended range of below 360 μmol/L within 2 to 6 days of hospitalization. These experiences indicate a close relationship between compliance with prescribed AAM intake and control of blood phenylalanine level. We propose that hospitalization be considered when patients with PKU who are consuming a phenylalanine-restricted diet fail to maintain blood phenylalanine levels in the targeted range despite reported compliance with the prescribed intake of dietary phenylalanine and AAM. J Am Diet Assoc. 1999; 99: 1559–1563.