Serotonin
A chemical messenger often called the happy chemical, though most of it is in the gut, not the brain. In the brain it plays a part in mood, sleep and appetite, but depression is not simply a shortage of it.
Serotonin is a chemical messenger, also written 5-HT, that the body makes from tryptophan, an amino acid in food. Evidence: solid More than 90% of it is in the gut, where cells in the lining release it to help keep food moving. Evidence: solid Platelets, the blood cells that plug a cut, soak it up from the blood and release it when a clot starts to form, which helps the clot along. Evidence: solid In the brain it comes from small clusters of cells in the brainstem, the raphe nuclei, and plays a part in mood, sleep, appetite and more. Evidence: solid The body also makes melatonin from it. Evidence: solid Serotonin in the blood can't get into the brain, so the gut's supply and the brain's are separate. Evidence: solid
"Happy chemical" comes from the idea that depression is a shortage of it, the chemical imbalance story. In 2022, a review of reviews led by the psychiatrist Joanna Moncrieff found no convincing evidence that depression is caused by low serotonin. Evidence: decent A group of 35 psychiatrists and researchers replied that the review left out evidence and oversimplified. Evidence: decent Their main example: lowering serotonin for a few hours with a tryptophan-free drink doesn't dent mood in most healthy people, but it does in people who've been depressed before. Evidence: decent Where that leaves it: serotonin seems to be part of the picture, and "low serotonin causes depression" is too simple. Evidence: decent
None of that means antidepressants don't work. Across 522 trials with more than 116,000 adults with major depression, all 21 antidepressants tested did better than a dummy pill. Evidence: solid On average the gain was modest, and people respond differently. Evidence: decent If you're on an antidepressant, don't stop or change it without your doctor. Evidence: solid Don't add supplements that raise serotonin on top of it without asking either; stacking them is how serotonin syndrome, a rare but serious overload, happens. Evidence: decent
Where it gets oversold: "serotonin foods". Bananas contain serotonin, but it can't cross into the brain. Evidence: solid Protein foods rich in tryptophan, turkey for one, don't raise brain serotonin either, because the other amino acids in the protein compete for the same way in. Evidence: decent Tryptophan pills are another matter, but a Cochrane review found only two trials good enough to count, 64 patients in all, too few to be sure. Evidence: decent The sunlight link rests mostly on one study: in 101 men, blood leaving the brain showed more serotonin turnover on brighter days. Evidence: decent In animals, exercise raises brain serotonin; in people, it hasn't been measured directly. Evidence: worth trying
Why it matters
Exercise eases depression in trials, whatever the chemistry turns out to be: move hard three times a week. Evidence: solid Morning daylight earns its place through the body clock, not serotonin: daylight in the first hour. Evidence: decent If you've been low for weeks, a chemical's nickname matters less than two questions, then a doctor.