Serotonin has a branding problem. Most people know it as the “happy chemical” in your brain: the thing antidepressants adjust, the biological counterpart of a feel-good social event, the reason you are or aren’t depressed.

But here’s what the branding leaves out: the overwhelming majority of your serotonin isn’t in your head at all. An estimated 90 percent of the body’s serotonin is made in the digestive tract, where it has nothing to do with mood and everything to do with keeping things moving (literally). Gut serotonin is one of the main signals that tells your intestines when to contract, which is a polite way of saying it helps you poop on schedule.

We knew that 90 percent of it is made in the digestive tract, and we knew that the gut microbiome could influence its production. But a study published in Cell Reports in October 2025 found something genuinely new about serotonin: some of it may be made by your gut bacteria.

The Discovery

Researchers at the University of Gothenburg set out to answer a question that had been hanging around microbiome science for a while. We’ve known for years that gut bacteria influence serotonin levels, but influencing production and actually producing the stuff are two different things. Could a bacterium make bioactive serotonin itself?

They identified two that can: Limosilactobacillus mucosae and Ligilactobacillus ruminis, two human gut bacteria that synthesize serotonin by converting a compound called 5-hydroxytryptophan. If that second name sounds vaguely familiar, it’s because 5-HTP is sold as a supplement. It’s the direct precursor your own body uses to build serotonin. These bacteria are running the same reaction, just inside your colon.

Does It Actually Do Anything?

This is where the honest version of the story matters, because it’s easy to oversell.

The researchers tested the bacteria in germ-free mice bred to be serotonin-deficient—essentially a blank slate with the serotonin machinery switched off. When they reintroduced the two bacteria, the results were striking. Serotonin levels rose, the density of nerve cells in the colon increased, and intestinal transit time (how long it takes food to move through it) was improved. In other words, the bacteria didn’t just raise a number on a chart. They appeared to help rebuild the local nervous wiring that controls bowel movement.

That’s a big deal for a condition like IBS, where that wiring and its serotonin signaling are thought to go haywire.

The Caveat Worth Sitting With

Here’s the part most headlines skipped. The dramatic results happened in mice. More specifically, in serotonin-deficient, germ-free mice, which are a useful laboratory tool but not a stand-in for an actual person with a functioning gut full of other microbes.

When the same research team looked at humans, the picture was murkier. Among 147 people with IBS and 27 healthy controls, there were no significant differences in fecal or circulating serotonin levels between the two groups. Some coverage reported that people with IBS carried lower levels of L. mucosae, and the study did examine that. But the serotonin levels themselves (the thing that’s supposed to matter) didn’t cleanly separate the sick from the healthy. It’s also worth knowing that the study reported industry relationships and a patent application tied to these bacteria, which is common in this kind of research but worth keeping in view when you read the enthusiastic version. Cell Reports

None of this makes the finding less interesting. It makes it prepubescent. What the researchers have shown is a plausible mechanism and a promising result in one animal model: the first step of a long staircase, not the top of it.

Why This Fits the Bigger Picture

Zoom out, and this study is really about one idea: the specific bacteria living in your gut don’t just sit there. They produce compounds that shape how your digestive system works, down to whether your intestines contract on time. Serotonin is one example. It won’t be the last.

That’s the same principle behind everything we talk about here at IgY Nutrition. A gut crowded with the wrong microbes isn’t a neutral situation. Those bacteria are metabolically active, and not in your favor. Making room for the beneficial ones isn’t just about balance for its own sake; it’s about who gets to run the chemistry in there. IgY Max works on exactly that problem, clearing dysbiotic pathogens so the beneficial residents have the space to do their jobs.

The Bottom Line

Can your gut bacteria make their own serotonin? Based on this research, it looks like at least some of them can. When they do, it may help regulate the machinery that keeps your digestion running. The human evidence is still thin, and anyone promising you a serotonin-boosting probiotic tomorrow is getting ahead of the science. But the direction is clear, and it’s one more reason to care about which bacteria are calling your gut home.

Learn more about supporting your gut at igynutrition.com.

References

  1. https://www.cell.com/cell-reports/fulltext/S2211-1247(25)01205-7
  2. https://www.news-medical.net/news/20251021/Gut-bacteria-found-to-make-serotonin-and-boost-intestinal-motility.aspx
  3. https://newatlas.com/health-wellbeing/serotonin-gut-bacteria-ibs/
  4. https://www.sciencedaily.com/releases/2026/03/260313002640.htm
  5. https://www.caltech.edu/about/news/microbes-help-produce-serotonin-gut-46495