Carbohydrate ingestion and brain serotonin synthesis: relevance to a putative control loop for regulating carbohydrate ingestion, and effects of aspartame consumption.

The ingestion of a meal of carbohydrates by fasting rats rapidly increases brain tryptophan level and serotonin (5-HT) synthesis. The rise in brain tryptophan level follows from an increase in tryptophan transport into brain, the consequence of an insulin-induced reduction in the blood levels of several amino acids that compete with tryptophan for brain uptake. In contrast, ingesting protein with carbohydrate does not stimulate brain tryptophan uptake or 5-HT synthesis, because the blood levels of tryptophan's transport competitors are increased, not reduced. These observations form the biochemical basis of a current proposal for a regulatory loop governing meal-to-meal appetite for carbohydrates. This review briefly analyzes the experimental basis for the carbohydrate appetite regulatory loop, and finds it wanting. It also considers the proposal that the ingestion of the artificial sweetener aspartame might disrupt the putative regulatory loop for carbohydrate intake regulation, and thus promote rather than help to limit carbohydrate appetite, and finds this hypothesis unrealistic as well. In general, the conclusion is that while single meals do readily influence brain tryptophan uptake and 5-HT synthesis, it is presently unclear what role such neurochemical effects of food ingestion have in the control of specific appetites.