Amino Acid Availability and Brain Function in Health and Disease

Abstract

Until the early seventies, it was generally believed that normal brain function would be unaffected by the metabolic changes associated with the ingestion of different foods. This assumption was effectively undermined by the finding that the protein-to-carbohydrate ratio of a meal did affect the brain concentration of one particular neuro-transmitter, serotonin. The following theory was proposed to explain this phenomenon: the ingestion of carbohydrates stimulates the release of insulin. This insulin release causes an increase in the uptake of amino acids into tissues. Because this increase is less for tryptophan, there is an increase in plasma of tryptophan relative to the other large neutral amino acids (LNAA: tryptophan, phenylalanine, tyrosine, valine, leucine and isoleucine). The LNAA in the blood compete for a common transport mechanism in order to cross the blood-brain barrier. The relative increase in plasma tryptophan results in an increased occupation of the transport mechanism with tryptophan and, therefore, an increase in the brain tryptophan levels. Because tryptophan is a rate limiting substrate for serotonin synthesis, the increase in brain tryptophan leads to an increase in brain serotonin.