The Effects of Aspartame on Human Mood, Performance, and Plasma Amino Acid Levels

Abstract

Consumption of the artificial sweetener aspartame raises plasma phenylalanine levels, thereby increasing brain phenylalanine and, conceivably, affecting the syntheses of monoaminergic brain neurotransmitters known to underlie various types of behavior. We have thus assessed the effects of single aspartame doses on certain types of behavior, particularly those relating to mood and performance. In a double-blind, placebo-controlled, crossover study, a single dose of aspartame (60 mg/kg), or the same dose in combination with 37 g of carbohydrate, was administered to 20 male volunteers. [Carbohydrates enhance the entry of circulating phenylalanine into the brain by lowering plasma levels of competing large neutral amino acids (LNAA)]. A lower dose of aspartame (20 mg/kg) was also tested. Aspartame alone, or in combination with carbohydrate, did not alter any aspect of behavior that we assessed, nor did it produce detectable side effects. The ratios of plasma phenylalanine and tyrosine concentrations to those of the other LNAA were significantly increased by administration of aspartame. Since anecdotal reports of aspartame-associated neurological or behavioral side effects almost always describe effects as occurring after multiple aspartame exposures, it would be important to repeat our study using a protocol involving repeated aspartame administration.