Brain tryptophan concentrations and serotonin synthesis remain responsive to food consumption after the ingestion of sequential meals.

The response of brain tryptophan concentration and serotonin synthesis to the ingestion of two sequential meals was examined in rats. Fasted rats ingested a carbohydrate meal followed 2 h later by a protein-containing meal and were examined 2 or 4 h after the first meal. Other rats ingested a protein meal first, followed by a carbohydrate meal. When the carbohydrate meal was fed first, brain tryptophan concentrations and serotonin synthesis increased at 2 h; these changes were reversed at 4 h if the second meal contained protein. When the protein meal was fed first, there were no changes in brain tryptophan or serotonin at 2 h, and a second carbohydrate meal at 2 h did not raise brain tryptophan or serotonin 2 h later. Carbohydrate ingestion 3 h after a protein meal, however, did raise brain tryptophan and serotonin 2 h later. Brain tryptophan concentrations and serotonin synthesis are thus responsive to the sequential ingestion of protein and carbohydrate meals if there is a sufficient interval between meals.