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.
What this question is testing
Topic
The author is answering a biology question: how does the brain know when you've had enough carbs (or that you need more)? Serotonin is the key.
Framework
Highlight Noteworthy. The author isn't arguing against opponents — they're building up an explanation step by step, with chemistry, animal studies, and human studies.
Main Point
Here's the simpler version: when you eat carbs, your body releases insulin, which clears most amino acids out of the bloodstream — but not tryptophan. That gives tryptophan more room to enter the brain, where it gets converted into serotonin. Serotonin then signals to the brain that you've had enough carbs. Drugs that boost serotonin reduce carb craving; drugs that block it leave you craving more. Studies in rats and in humans both back this up.
P1: The big finding
Drugs that increase serotonin tend to cause weight loss. Drugs that block it cause carb craving and weight gain. So serotonin is part of the brain's answer to "have I had enough carbs?"
P2: The chemistry, in plain terms
Eating carbs triggers insulin. Insulin clears most amino acids from the blood — but tryptophan stays. With less competition, more tryptophan crosses into the brain, where the brain converts it into serotonin.
P3: Rats prove it works
Given a choice, rats normally bounce between protein and carb foods. If you give them serotonin-boosting drugs, they cut back on carbs. If you block serotonin, the brain never registers the carbs they did eat, so they keep craving them.
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