Plasma amino acid concentrations in normal adults administered aspartame in capsules or solution: lack of bioequivalence

Aspartame: review of safety

Over 20 years have elapsed since aspartame was approved by regulatory agencies as a sweetener and flavor enhancer. The safety of aspartame and its metabolic constituents was established through extensive toxicology studies in laboratory animals, using much greater doses than people could possibly consume. Its safety was further confirmed through studies in several human subpopulations, including healthy infants, children, adolescents, and adults; obese individuals; diabetics; lactating women; and individuals heterozygous (PKUH) for the genetic disease phenylketonuria (PKU) who have a decreased ability to metabolize the essential amino acid, phenylalanine. Several scientific issues continued to be raised after approval, largely as a concern for theoretical toxicity from its metabolic components--the amino acids, aspartate and phenylalanine, and methanol--even though dietary exposure to these components is much greater than from aspartame. Nonetheless, additional research, including evaluations of possible associations between aspartame and headaches, seizures, behavior, cognition, and mood as well as allergic-type reactions and use by potentially sensitive subpopulations, has continued after approval. These findings are reviewed here. The safety testing of aspartame has gone well beyond that required to evaluate the safety of a food additive. When all the research on aspartame, including evaluations in both the premarketing and postmarketing periods, is examined as a whole, it is clear that aspartame is safe, and there are no unresolved questions regarding its safety under conditions of intended use.

Effect of aspartame and protein, administered in phenylalanine-equivalent doses, on plasma neutral amino acids, aspartate, insulin and glucose in man

Six human males each received 0.56 g phenylalanine (Phe) in the form of 1.0 g aspartame or 12.2 g bovine albumin in 200 ml water or water alone. Venous blood samples collected before consumption and during the following 4 hr were assayed for plasma levels of large, neutral amino acids (LNAA), aspartate, insulin and glucose. The area under the curve for plasma Phe was 40% greater, although not significant, after aspartame compared with albumin intake. The indicated increased clearance rate of plasma Phe after albumin may be caused by the significant increase of insulin, on which aspartame had no effect. There was a significant main effect of aspartame for plasma tyrosine but not for tryptophan, valine, isoleucine or leucine. Plasma aspartate was significantly increased at 0.25 hr after the aspartame intake. The percentage Phe/LNAA decreased slightly in response to albumin but increased 55% after aspartame and remained significantly increased for 2 hr. Tyrosine/LNAA increased and tryptophan/LNAA decreased modestly after aspartame intake. The study showed that the intake of aspartame in a not unrealistically high dose produced a marked and persistent increase of the availability of Phe to the brain, which was not observed after protein intake. The study indicated, furthermore, that Phe was cleared faster from the plasma after consumption of protein compared with aspartame.

Bioavailability of phenylalanine and aspartate from aspartame (20 mg/kg) in capsules and solution

Aspartame (L-aspartyl-L-phenylalanine methyl ester) was given in capsules or solution to compare the bioavailability of its constituent amino acids, aspartate and phenylalanine. Twenty healthy subjects received a single 20 mg/kg dose of aspartame in capsules or solution in a randomized, crossover design. Plasma amino acid concentrations and the phenylalanine to large neutral amino acid ratios (Phe/LNAA) were determined. Plasma aspartate concentrations did not increase with either treatment. For plasma phenylalanine following capsule ingestion, there was a smaller peak plasma concentration (Cmax; 103.3 v 126.6 mumol/L), a longer time to peak concentration (tmax; 108.6 v 36.6 minutes), but no significant difference in the area under the plasma concentration-time curve (AUC) (7,656 v 7,200 mumol.min/L) when compared with solution ingestion. The maximum plasma Phe/LNAA ratio was smaller (0.16 v 0.19) with capsules. The changes for plasma tyrosine were similar to those seen with phenylalanine. There were no significant differences in the plasma concentrations of the other LNAAs between capsule and solution ingestion. Thus, given the small effect on phenylalanine Cmax and Phe/LNAA and no effect on the extent of absorption of phenylalanine, aspartame ingested in capsules at doses up to 20 mg/kg is a suitable dosage form for blinded clinical studies, provided that the slower rate of absorption of phenylalanine from capsules is taken into account.

Stimulation of pituitary hormone secretion by neurotransmitter amino acids in humans

The effects of several neurotransmitter amino acids on pituitary hormone secretion were examined in normal humans. Oral administration of 10 g of glutamic acid stimulated the secretion of prolactin (PRL) and cortisol to approximately twice baseline values, with no effect on GH, TSH or LH. Aspartic acid (10 g), taurine (5 g), and cysteine (5 or 10 g) had no consistent effect on any hormone measured, although the lack of effect of aspartic acid may relate to the modest increments in serum concentration achieved. Glutamic acid may be an important modulator of PRL and ACTH secretion in humans.