Serotonin is something which everyone should at least have some sort of concern about – as it is a substance that affects a lot of things. Serotonin is a neurotransmitter, or one of the chemicals that signals to your brain and indicates certain information or conditions. Serotonin is largely secreted and stored in enterchromaffin cells, which as contained in the digestive tract, believe it or not – and this is leading scientists to believe that there is a link between serotonin levels, eating habits, and weight gain or loss. It is also the chemical that's involved in anti-depressant medications. It's one of the major players in the brain. If levels of serotonin aren't correct, other things won't work as well – and medications might send you out for payday loans to get a refill.