How does the brain know when carbohydrates have been or should be consumed? The answer to this question is not known, but one element in the explanation seems to be the neurotransmitter serotonin, one of a class of chemical mediators that may be released from a presynaptic neuron and that cause the serotonin-mediated transmission often have the opposite effect: they often induce carbohydrate craving and consequent weight gain.
Serotonin is a derivative of tryptophan, an amino acid that is normally present at low levels in the bloodstream. The rate of conversion is affected by the proportion of carbohydrates in an individual’s diet: carbohydrates stimulate the secretion of insulin, which facilitates the uptake of most amino acids into peripheral tissues, such the central nervous system where, in a special cluster of neurons, it is converted into serotonin.
The level of serotonin in the brain in turn affects the amount of carbohydrate an individual chooses to eat. Rats that are allowed to choose among synthetic foods containing different proportions of carbohydrate and protein will normally alternate between foods containing mostly protein and those containing mostly carbohydrate. However, if rats are their brains fail to respond when carbohydrates are eaten, so the desire for them persists.
In human beings a serotoninlike drug, d-fenfluramine (which releases serotonin into brain synapses and then prolongs its action by blocking its reabsorption into the presynaptic neuron), selectively suppresses carbohydrate snacking (and its associated weight gain) in people who crave carbohydrates. In contrast, drugs that block serotonin-mediated transmission or that interact with neurotransmitters that serotonin has other effects that may be useful indicators of serotonin levels in human beings.
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