We know that there’s a huge connection between the gut and the brain—so much so that it’s got its own term: the gut-brain axis. We’ve also known for awhile that there is a definite relationship between the gut and mood regulation.

Serotonin is the “happy” neurotransmitter best known for its ability to lift depression in the form of SSRIs. So we think of it as a brain chemical—and it is. Yet 90% of our serotonin is made in the gut.

Serotonin and Your Gut

The cells in the gut that produce serotonin are called enterochromaffin (EC) cells, and they’re scattered throughout the small intestine and colon. According to this article, the EC cells partner with the good bacteria in the gut to produce serotonin. The study demonstrated that mice depleted of gut flora lost at 60% of their serotonin, and that this as reversible when the good gut flora was replaced.

Why does the gut produce serotonin? It helps to maintain gut motility via the migrating motor complex (MMC). There’s a bit of a catch-22 here, as proper gut motility is necessary to prevent SIBO, and SIBO can likewise set up a dysbiotic environment in the gut that suppresses serotonin production.

Serotonin and Blood Sugar

Serotonin also works in concert with the gut flora to regulate blood sugar levels as well. Perhaps this is because it decreases carb intake by modulating stress eating, or perhaps there is a more direct effect. But it stands to reason that gut flora disruption, depression, and insulin resistance can easily go together.

The Upshot

If you’ve struggled with depression, and it fits the picture of a low serotonin type of depression (rather than a low catecholamine type of depression), consider whether the problem might, at least in part, be coming from your gut.