Abstract
Two groups of amino acids—the aromatic and the acidic amino acids—are reputed to influence
brain function when their ingestion in food changes the levels of these amino acids
in the brain. The aromatic amino acids (tryptophan, tyrosine, phenylalanine) are the
biosynthetic precursors for the neurotransmitters serotonin, dopamine, and norepinephrine.
Single meals, depending on their protein content, can rapidly influence uptake of
aromatic amino acid into the brain and, as a result, directly modify their conversion
to neurotransmitters. Such alterations in the production of transmitters can directly
modify their release from neurons and, thus, influence brain function. The acidic
amino acids glutamate and aspartate are themselves brain neurotransmitters. However,
they do not have ready access to the brain from the circulation or the diet. As a
result, the ingestion of proteins, which are naturally rich in aspartate and glutamate,
has no effect on the level of acidic amino acid in the brain (or, thus, on brain function
by this mechanism). Nevertheless, the food additives monosodium glutamate and aspartame
(which contains aspartate) have been reputed to raise the level of acidic amino acid
in the brain (when ingested in enormous amounts), to modify brain function, and even
to cause neuronal damage. Despite such claims, a substantial body of published evidence
clearly indicates that the brain is not affected by ingestion of aspartame and is
affected by glutamate only when the amino acid is administered alone in extremely
large doses. Therefore, when consumed in the diet neither compound presents a risk
to normal brain function.
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© 1993 The American Dietetic Association. Published by Elsevier Inc. All rights reserved.
