Effect of aspartame loading upon plasma and erythrocyte amino acid levels in phenylketonuric heterozygotes and normal adult subjects

Abstract

Aspartame is a dipeptide (L-aspartyl-L-phenylalanyl-methyl ester) with a sweetening power 180 to 200 times that of sucrose. Questions about aspartame safety have arisen because of concern about potential toxic effects of its constituent amino acids, aspartate and phenylalanine, particularly in subjects heterozygous for phenylketonuria. Plasma and red cell amino acid levels were measured in eight female subjects known to be heterozygous for phenylketonuria and 12 subjects assumed to be normal (six male, six female), after aspartame administration (34 mg/kg) in orange juice. No changes in either plasma or erythrocyte aspartate levels were noted at any time after aspartame loading in either group, indicating rapid metabolism of the aspartate administered. In the normal subjects, plasma phenylalanine levels (mean ± SD) increased from fasting levels (5.66±1.21 μmoles/100 ml) to values in the normal postprandial range (11.11±2.49 μmoles/100 ml) and essentially returned to baseline by 8 hours. In the heterozygous subjects, mean peak plasma phenylalanine levels were higher (16.03±2.25 μmoles/100 ml) and the plasma concentration-time curve broader. However, maximum plasma phenylalanine levels were only slightly above values noted postprandially in the human infant and adult. Erythrocyte phenylalanine levels showed similar, but smaller changes. The data indicate slightly slower, but adequate metabolism and clearance of the phenylalanine portion of aspartame by the phenylketonuric heterozygote.