Carbohydrate craving, obesity and brain serotonin.
One mechanism through which the brain obtains information about the composition of the diet involves food-induced changes in the plasma amino acid pattern (principally the "plasma tryptophan ratio"), which then cause increases or decreases in brain tryptophan levels, and in the synthesis of a neurotransmitter, serotonin, which is formed from the tryptophan. A carbohydrate-rich, protein-poor meal stimulates insulin secretion; this diminishes plasma levels of the amino acids which compete with tryptophan for transport into the brain (e.g., leucine, isoleucine and valine), thus increasing tryptophan's flux across the blood-brain barrier and its brain levels. In contrast, a high-protein meal contributes so much more of these latter amino acids to the blood stream than of the relatively-scarce tryptophan that it diminishes tryptophan's entry into the brain. This article reviews evidence that the brain actually utilizes the food-induced changes in brain serotonin in order to make choices about what to eat at the next meal. It also discusses the likelihood that a disturbance in this mechanism is involved in producing the "carbohydrate-craving" that is frequently associated with obesity. (This behavior which has been studied by allowing hospitalized subjects to choose freely among isocaloric meals and snacks of varying protein/carbohydrate ratios, typically manifests itself as a propensity to consume 30 per cent or more of the total daily calorie intake in the form of sweet or starchy snacks, usually at a characteristic time of day.) D-Fenfluramine, a drug that selectively enhances serotonin-mediated neurotransmission, also selectively suppresses "carbohydrate-craving" in these subjects.(ABSTRACT TRUNCATED AT 250 WORDS)
D000596:Amino Acids / Q000097:blood; D001923:Brain Chemistry; D004040:Dietary Carbohydrates / Q000494:pharmacology; D004041:Dietary Fats / Q000494:pharmacology; D004044:Dietary Proteins / Q000494:pharmacology; D005247:Feeding Behavior / Q000502:physiology*; D006801:Humans; D006815:Hunger / Q000502:physiology*; D007328:Insulin / Q000378:metabolism; D000078790:Insulin Secretion; D009765:Obesity / Q000209:etiology* / Q000503:physiopathology; D012701:Serotonin / Q000032:analysis*
D000596:Amino Acids; D004040:Dietary Carbohydrates; D004041:Dietary Fats; D004044:Dietary Proteins; D007328:Insulin; D012701:Serotonin