Serotonin

Serotonin is a mechanism term: it names a signaling molecule, not an outcome. Its best-known roles are in mood, sleep and anxiety in the brain, and in gut movement (motility) in the rest of the body [1].

Almost all serotonin outside the brain comes from specialized cells lining the gut, called enterochromaffin cells [1]. Two forms of the enzyme tryptophan hydroxylase make it: TPH1 mainly in those gut cells and TPH2 mainly in neurons of the brainstem [1].

Because serotonin cannot readily cross the blood-brain barrier, the brain's supply and the body's supply are kept apart and act in their own ways [1].

The split between the two pools is a useful check on claims. Because the central and peripheral pools are kept apart [1], a serotonin measure taken in blood or gut is not a direct measure of serotonin in the brain.

One experimental way to change serotonin levels, in the brain and in the body, is to deplete or add tryptophan, either once or over time [2]. A 2016 review of that work, which draws on both preclinical and clinical studies, reports that low brain serotonin is associated with poorer memory and lower mood [2]. That is an association from experiments on brain chemistry, not a finding about any supplement.

A separate line of research looks at the gut-brain axis, a two-way system linking the brain and the digestive tract [2].

Claims that a food or supplement "raises serotonin" are mechanism claims. Our compound pages grade measured outcomes, such as mood ratings compared with a placebo, not serotonin itself.

• Neurotransmitter
• Blood-brain barrier
• Dopamine