The consumption of a carbohydrate-rich, protein-poor meal or snack can increase the synthesis of the brain neurotransmitter serotonin; proteins block this effect. The mechanism of the rise in brain serotonin involves the secretion of insulin, and the decrease that the hormone produces in plasma levels of certain amino acids that compete with tryptophan, serotonin's precursor, for transport across the blood-brain barrier. The rise in serotonin can thus be produced by any carbohydrate that elicits insulin secretion, independent of its sweetness. Pharmacologic treatments that amplify serotonin-mediated neurotransmission can selectively decrease the consumption of carbohydrate (i.e., in relation to that of protein). A group of diseases seems to exist in which depressive symptoms are associated with "carbohydrate-craving", and the consumption of large quantities of carbohydrate-rich, protein-poor snacks. Patients describe positive subjective responses to the dietary carbohydrates which are unrelated to hunger. These responses may be mediated by the rise in brain serotonin.