Recent Research on the Behavioral Effects of Tryptophan and Carbohydrate

Animal and human studies indicate that diet can alter plasma and brain concentrations of neurotransmitter precursors, with possible implications for the synthesis and release of brain neurotransmitters. The best known example is serotonin, whose synthesis is limited by the availability of its precursor, tryptophan, in the brain. Consuming trypto-phan or a carbohydrate-rich, protein-poor meal increases brain levels of tryptophan and serotonin. Although a carbohydrate meal itself lacks tryptophan, the meal causes insulin to be secreted. Insulin, in turn, decreases plasma levels of large neutral amino acids that would ordinarily compete with tryptophan for transport across the blood-brain barrier. Resulting brain changes in serotonin provide a plausible mechanism whereby diet could affect behaviour. Research on human subjects suggests that ingesting tryptophan or carbohydrate can reduce subjective alertness and possibly influence some aspects of objective performance. Effects on sleep latency and on pain perception have also been detected. Behavioral effects may come about via the action of tryptophan on brain serotoninergic pathways, although other mechanisms may operate and must still be ruled out before the mechanism is certain. NUTRITIONAL INFLUENCES ON SEROTONIN SYNTHESIS Until recently it was assumed that short-term fluctuations in nutritional intake had little effect on the brain. It appeared that the brain occupied a privledged position in relation to the rest of the body, having its metabolic needs guaran-teed before that of any other bodily organ. Further, it seemed that the blood-brain barrier protected the brain against short-term nutritional changes. This inference seemed logical because fluctuations in brain concentrations of single amino acids did not closely parallel changes in plasma levels. Short-tenn Effects of Nutrients on the Brain The assumption that the brain is unaffected by nutritional intake has now been questioned due in large pa,rt to the research of Hartmann (1981), Wurtma