Tryptophan is an amino acid mainly found in the gastrointestinal tract (90%), the blood cells (8%) and the brain (2%). After hydroxylation, tryptophan becomes 5-hydroxytryptophan, commonly known as 5-HTP, which is subsequently converted into serotonin by an enzyme called aromatic L-amino acid decarboxylase. This biotransformation occurs mainly in the liver and the brain. After being transformed into serotonin, the amino acid influences the receptors that are responsible for the modulation of emotional and dietary behavior, cognitive function, body temperature, pain sensitivity, sexual behavior, and the regulation of sleep. Maintaining adequate serotonin levels additionally helps regulate appetite, and may therefore help control body weight.