soshu8 how does the brain know when carbohydrates have been or should be consumed? 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 the answer to this question is not known, but one element in the explanation seems to be the neurotransmitter serotonin (serotonin: explanation seems to be the neurotransmitter serotonin (serotonin: n.含于血液中的复合胺), one of a class of chemical mediators that may be n. han yu xue ye zhong de fu he an ), one of a class of chemical mediators that may be released from a presynaptic neuron and that cause the transmission of a nerve impulse across a synapse released from a presynaptic neuron and that cause the transmission of a nerve impulse across a synapse to an adjacent postsynaptic neuron. in general, it has been found that drugs to an adjacent postsynaptic neuron. in general, it has been found that drugs that selectively facilitate serotonin-mediated neurotransmission that selectively facilitate serotonin-mediated neurotransmission tend to cause weight loss, whereas drugs that block serotonin-mediated tend to cause weight loss, whereas drugs that block serotonin-mediated transmission often have the opposite transmission often have the opposite effect: they often induce carbohydrate craving and effect: they often induce carbohydrate craving and consequent weight gain. consequent weight gain. serotonin is a derivative of tryptophan (tryptophan: serotonin is a derivative of tryptophan (tryptophan: n.[生化]色氨酸), an amino acid that is normally present n.[ sheng hua ] se an suan ), an amino acid that is normally present at low levels in the bloodstream. at low levels in the bloodstream. the rate of conversion is affected by the proportion of carbohydrates in an the rate of conversion is affected by the proportion of carbohydrates in an individual’s diet: carbohydrates stimulate the secretion of individual s diet: carbohydrates stimulate the secretion of insulin, which facilitates the uptake of most amino acids into insulin, which facilitates the uptake of most amino acids into peripheral tissues, such as muscles. blood tryptophan levels, peripheral tissues, such as muscles. blood tryptophan levels, however, are unaffected by insulin, however, are unaffected by insulin, so the proportion of tryptophan in the blood relative to the other so the proportion of tryptophan in the blood relative to the other amino acids increases when carbohydrates are consumed. since amino acids increases when carbohydrates are consumed. since tryptophan competes with other amino acids for transport across the tryptophan competes with other amino acids for transport across the blood-brain barrier into the brain, insulin secretion indirectly blood-brain barrier into the brain, insulin secretion indirectly speeds tryptophan’s entry into the central nervous system (central nervous system: speeds tryptophan s entry into the central nervous system (central nervous system: n. n. 中枢神经系统) where, in a special cluster of neurons, it is zhong shu shen jing xi tong ) where, in a special cluster of neurons, it is converted into serotonin. soshu8 converted into serotonin. soshu8 the level of serotonin in the brain in turn affects the amount of carbohydrate 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 an individual chooses to eat. rats that are allowed to choose among synthetic foods containing different proportions of carbohydrate synthetic foods containing different proportions of carbohydrate and protein will normally alternate between foods containing mostly and protein will normally alternate between foods containing mostly protein and those containing mostly carbohydrate. however, if rats are given drugs that protein and those containing mostly carbohydrate. however, if rats are given drugs that enhance the effect of serotonin, the rats’ carbohydrate intake is enhance the effect of serotonin, the rats carbohydrate intake is reduced. on the other hand, reduced. on the other hand, when rats are given drugs that interrupt serotonin-mediated when rats are given drugs that interrupt serotonin-mediated neurotransmission, their brains fail to respond when carbohydrates neurotransmission, their brains fail to respond when carbohydrates are eaten, so the desire for them persists. are eaten, so the desire for them persists. in human beings a serotoninlike drug, d-fenfluramine in human beings a serotoninlike drug, d-fenfluramine （弗苯丙胺）(which release serotonin into brain synapses and fu ben bing an (which release serotonin into brain synapses and then prolong its action by blocking its reabsorption into the then prolong its action by blocking its reabsorption into the presynaptic neuron), selectively suppresses carbohydrate snacking (and its presynaptic neuron), selectively suppresses carbohydrate snacking (and its associated weight gain) in people who crave carbohydrates. associated weight gain) in people who crave carbohydrates. in contrast, drugs that block in contrast, drugs that block serotonin-mediated transmission or that interact with serotonin-mediated transmission or that interact with neurotransmitters other than serotonin have the opposite effect: they often induce carbohydrate neurotransmitters other than serotonin have the opposite effect: they often induce carbohydrate craving and subsequent weight gain. people who crave carbohydrates craving and subsequent weight gain. people who crave carbohydrates report feeling refreshed and invigorated after eating a report feeling refreshed and invigorated after eating a carbohydrate-rich meal (which would be expected to increase brain carbohydrate-rich meal (which would be expected to increase brain serotonin levels), in contrast, serotonin levels), in contrast, those who do not crave carbohydrates become sleepy following a those who do not crave carbohydrates become sleepy following a high-carbohydrate meal. these findings suggest that serotonin has other effects that may be useful high-carbohydrate meal. these findings suggest that serotonin has other effects that may be useful indicators of serotonin levels in human beings. indicators of serotonin levels in human beings. soshu8