Serotonin

While we think of serotonin as a brain chemical, a staggering 90% of your body's serotonin is actually produced in your gut by enterochromaffin cells lining your intestinal walls. This gut serotonin regulates digestion, bowel movements, and communicates with your brain via the vagus nerve—explaining why gut health profoundly affects mood and why extreme stress can trigger digestive chaos. The catch? Most gut-produced serotonin can't cross the blood-brain barrier, meaning your gut and brain maintain largely separate serotonin economies despite their constant communication.

For decades, depression was marketed as a "chemical imbalance" caused by low serotonin—a narrative that sold billions in SSRI prescriptions. But a 2022 umbrella review examining 17 studies found no convincing evidence that depression is caused by lower serotonin activity or concentrations, sending shockwaves through psychiatry and public understanding. This doesn't mean SSRIs don't help some people, but it suggests they work through mechanisms we still don't fully understand, highlighting how a compelling but oversimplified story became medical dogma before the science caught up.

Serotonin got its name in 1948 when researchers isolated a serum substance that regulated vascular tone—literally "serum" + "tonic," describing its ability to constrict blood vessels. Its association with happiness came much later, after drug companies developed SSRIs in the 1980s and marketed them with the "chemical imbalance" narrative. The irony? This molecule discovered for controlling bleeding became culturally synonymous with emotional well-being through pharmaceutical marketing, not scientific revelation.

Here's a puzzle: classic psychedelics like psilocybin and LSD work by activating serotonin receptors (specifically 5-HT2A), yet they produce radically different effects than SSRIs which increase serotonin availability. Recent studies show psilocybin can alleviate depression more effectively than SSRIs for some patients, despite working on the same neurotransmitter system through opposite mechanisms—activation versus accumulation. This paradox is forcing researchers to reckon with how little we truly understand about serotonin's role in consciousness and mental health.

You've probably heard that turkey makes you sleepy because it contains tryptophan, serotonin's precursor amino acid—but this is mostly myth. Turkey contains no more tryptophan than chicken or beef, and dietary tryptophan competes with other amino acids to enter the brain, making post-feast drowsiness far more attributable to carb-loading and overeating. That said, combining tryptophan-rich foods with carbohydrates does facilitate its brain entry, which is why a banana with almond butter might genuinely support serotonin synthesis better than a protein-heavy meal alone.

Seasonal Affective Disorder (SAD) represents one of the clearest natural experiments linking serotonin to mood: brain serotonin levels measurably drop with reduced sunlight exposure, and light therapy effectively treats SAD with response rates comparable to antidepressants. The mechanism involves light hitting your retina and triggering a cascade that affects serotonin production and regulation. This gives us a practical, evidence-based intervention—morning bright light exposure of 10,000 lux for 20-30 minutes—that can genuinely shift mood without pharmaceuticals, especially during winter months.