Serotonin is an ancient molecule that is found in plants as well as in the human body. As a result of its long evolutionary history, it has been used to achieve multiple functions in multiple physiological pathways.  It is a major neurotransmitter found in the brain, but is mostly produced in the gut. Serotonin is made from an essential amino acid called tryptophan that the body cannot produce. Tryptophan must be obtained through one’s diet. Foods such as chocolate, nuts, cheese, and poultry are particularly high in tryptophan.