You have nothing to worry about please read the following: One of the mechanisms for regulating satiety in the brain is through certain serotonin receptors. When serotonin binds to these receptors, you feel satiated. If you have too much serotonin binding to these receptors, you not only feel satiated, but can even become nauseated. That is why people get nauseated frequently when they start taking any of the above medications. The SSRIs markedly increase serotonin in the brain, and the serotonin binds to the receptors responsible for satiety. You now feel stuffed/nauseated because there is more serotonin than normal, which the body reads as being grossly overfed. Your brain tells you that you are stuffed based on the binding of serotonin in this case, and this feeling has nothing to do with having anything in your stomach.
That all sounds great, and initially it is. You lose weight since you neurochemically feel well fed. Usually, when you are taking enough of an SSRI, you not only lose weight, but lose your craving for carbohydrates. Eating carbohydrates--sugar, bread, cookies, pasta, potatoes, chocolate, candy-- pumps tryptophan into your brain. Tryptophan is what serotonin is made out of. So, when you have enough serotonin in your brain, you stop eating near as much carbohydrate.
Unfortunately, your brain does something called down regulation of serotonin receptors. You grow less of them in your brain if it is exposed to an SSRI. Since you have to have a certain number of these receptors occupied to feel satiated, and you have fewer to occupy, you need to have even higher levels of serotonin in your brain. Thus, you eat more, and it is carbohydrates that you eat to pump in tryptophan to make serotonin. When you eat more, you gain weight, and it is an ongoing process since your brain will always down-regulate the number of serotonin receptors in the presence of an SSRI.
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