Plasma phenylalanine levels in phenylketonuric heterozygous and normal adults administered aspartame at 34 mg/kg body weight

Following administration of aspartame (34 mg/kg body wt) in orange juice, plasma concentrations of free amino acids were measured in 12 female subjects known to be heterozygous for phenylketonuria and 22 normal subjects (12 male, 10 female). No change in fasting plasma aspartate concentrations were noted after aspartame loading in either group. In normal male subjects, the mean (±S.D.) plasma phenylalanine concentration increased from a fasting value of 5.86 ± 1.25 μmol/dl to a mean peak value of 12.0 ± 3.79 μmol/dl. Plasma phenylalanine levels in normal female subjects increased from a mean fasting concentration of 4.83 ± 0.84 μmol/dl to a mean peak value of 8.95 ± 1.49 μmol/dl suggesting a more rapid absorption, metabolism, and/or clearance of phenylalanine by females. In female heterozygous subjects, the mean peak plasma phenylalanine concentration was significantly higher than in normal females. Plasma phenylalanine values increased from a mean fasting value of 5.92 ± 1.51 μmol/dl to a mean peak value of 15.1 ± 4.67 μmol/dl. Similarly, the area under the plasma phenylalanine concentration-time curve was significantly greater in heterozygous female subjects (21.36 ± 5.10 IU) than in normal female subjects (10.84 ± 2.32 IU). However, peak plasma phenylalanine levels were well below those associated with toxic effects in all cases.