Your gut and brain really talk, but many mood claims started in mice

Key takeaways

• In a survey of 28,771 adults in 15 countries, finished in early 2025, about 41% met the criteria for at least one disorder of gut-brain interaction, the family of conditions that includes IBS.
• More than 90% of your body’s serotonin is made in the gut, but the jobs scientists have documented for gut-made serotonin are outside the brain, such as moving food along and helping blood clot.
• A gut bacterium that calmed mice stopped working when their vagus nerve was cut, and the same strain did nothing for 29 healthy men in a 2017 trial.
• A 2025 review of 67 trials found that gut-focused talk therapy and hypnotherapy eased IBS symptoms better than comparison care, including when delivered by app or online.
• Pooled trials suggest probiotics may ease depression as an add-on for people with a diagnosis, but results vary widely, and effects in healthy people are small.

In 2011, researchers fed healthy mice a strain of the gut bacterium Lactobacillus rhamnosus called JB-1¹².

The mice became bolder in standard anxiety tests, gave up less quickly in a stress test and released less stress hormone¹². Then came the striking part. In mice whose vagus nerve, a main cable between gut and brain, had been cut, the calming effect disappeared¹.

It looked like proof that a gut microbe could change behavior by sending signals up a nerve. Researchers now use the word “psychobiotics” for gut-targeted products meant to affect mood or behavior³.

Then researchers tested the same strain in people. In a randomized, placebo-controlled trial published in 2017, 29 healthy men took JB-1 during one period and a placebo during another². The bacterium did no better than the placebo on mood, anxiety, stress, sleep, memory, attention, brain waves or inflammation². The researchers titled their paper “Lost in translation?”².

That two-step story sums up the gut-brain axis. The connection is real and busy. But many of its most exciting findings come from mice, and people are harder to change. This guide sorts out what’s established, what’s promising and what’s still a hunch.

Your gut and brain trade messages on four lines

Your digestive tract has its own nervous system, called the enteric nervous system, woven through the walls of your gut. It holds more nerve cells than any part of your body except the brain⁴. That’s where its nickname, the “second brain,” comes from.

The American College of Gastroenterology (ACG) notes that the gut and the brain grow from the same tissue before birth, use some of the same chemical messengers and are wired to each other by nerves⁵. Messages travel along at least four kinds of lines:

• Nerves. The vagus nerve runs from the brainstem through the neck and chest into the abdomen⁴. Gut microbes and the substances they make can signal to it directly, partly through gut-lining cells nicknamed neuropods³.
• Chemical messengers. Cells in the gut lining release signaling molecules such as serotonin, which act on gut nerves and help move food along⁶.
• Immune signals. The vagus nerve helps run a reflex that calms inflammation³. That pathway may matter for depression tied to inflammation³.
• Microbial products. Better vagus nerve function goes along with more of the bacteria that make short-chain fatty acids, compounds microbes produce when they ferment fiber³.

Traffic runs both ways. The ACG describes a loop in irritable bowel syndrome (IBS): a stressful stretch can flare belly symptoms, and months of pain can feed anxiety or low mood⁵. If your stomach has ever knotted before a big talk, you’ve likely felt this wiring at work.

The 90% serotonin fact is real, but the mood leap isn’t

You may have seen the claim that most of your serotonin is made in your gut, often followed by “so your gut controls your happiness.”

The first half holds up. A 2015 Caltech study in the journal Cell reported that more than 90% of the body’s serotonin is made in the gut, mostly by enterochromaffin cells in the gut lining⁶. Microbes help drive that production. In mouse experiments, gut microbes regulated about 64% of colon serotonin and about half of the serotonin in blood⁶.

The second half is a leap. The jobs documented for gut-made serotonin are outside the brain. It stimulates the gut nerves that move food along, and platelets carry it to help blood clot⁶. It also plays roles in immune responses, bone development and heart function⁶.

So the 90% figure tells you where most serotonin is made. It doesn’t tell you where your mood comes from. The Caltech paper didn’t study mood at all⁶. Researchers looking for a gut-mood link focus largely on the other lines, especially the vagus nerve and the microbial products and immune signals that reach it³.

IBS has a new name that puts the gut-brain link up front

The Rome Foundation, the international group that writes the diagnostic criteria for gut disorders, classes IBS and dozens of related conditions as disorders of gut-brain interaction, or DGBIs⁷⁸.

A 2026 review of the Rome process in the journal Gastroenterology describes these as gut symptoms driven by some mix of five things: disturbed gut movement, an oversensitive gut, altered immune activity in the gut lining, altered gut microbes and changes in how the brain processes gut signals⁷. The paper says the newest criteria, called Rome V, build on earlier efforts to drop imprecise or stigmatizing terms⁷.

These conditions are common. For a 2026 study in the journal Gut, researchers surveyed 28,771 adults online in 15 countries, including the U.S., with data collection finished in early 2025⁸. About 41% met the criteria for at least one DGBI, and the figure for Americans was about 42%⁸. The Rome Foundation’s research arm funded the survey⁸.

Rome V also loosened the IBS rules, counting discomfort as well as pain and lowering the minimum to three days a month⁸. Under the new rules, 8.5% of people surveyed had IBS, more than double the 3.9% under the older Rome IV rules⁸.

The ACG’s patient page puts the U.S. figure at 10–15% of adults with IBS symptoms, but only 5–7% with a diagnosis⁵. A prevalence number depends partly on where experts draw the line, and on whether people have seen a doctor.

According to Mayo Clinic, hard experiences, above all in childhood, are tied to more IBS symptoms⁹. Stress can make a flare worse, but Mayo says it doesn’t cause the condition. And for some people, IBS begins after a severe gut infection that causes diarrhea⁹.

For many readers, this framing is a relief. IBS isn’t “all in your head.” It’s a real disorder in which both the gut and the brain play parts⁷. Diet, fiber and other IBS treatments are covered in our article on gut-healing claims; this one focuses on the brain side.

Talk therapy for the gut has dozens of trials behind it

This is a twist many people don’t expect: some of the best-tested IBS treatments work through the brain.

Two stand out. Cognitive behavioral therapy (CBT) for IBS teaches your brain new ways of reacting to the signals coming up from your gut⁵. Gut-directed hypnotherapy pairs relaxation with mental imagery aimed at turning down pain⁵. Neither asks you to believe your symptoms are imaginary.

In 2020, a team including Alexander Ford pooled 41 randomized trials with 4,072 people, using a method that compares many therapies at once¹⁰. CBT, whether face to face or mostly self-guided, and gut-directed hypnotherapy all beat comparison care, and none clearly beat the others¹⁰. With face-to-face CBT, the chance of still having symptoms after treatment was about 38% lower than with comparison care. With hypnotherapy, it was about 33% lower¹⁰.

A 2025 update of that review, published in The Lancet Gastroenterology & Hepatology, pooled 67 trials with 7,441 participants¹¹¹². Ford, of the University of Leeds, was its corresponding author¹². Again, these brain-gut therapies did better than routine care, education or diet advice, whether delivered in person or by app or internet¹².

Both reviews were frank about weak spots. The 2020 authors judged the risk of bias high and said the benefits were likely overestimated¹⁰. The 2025 team said publication bias and trial quality limit how confidently the therapies can be ranked¹¹.

Our rating: Strong evidence that these therapies ease IBS symptoms, with real effects that are probably somewhat smaller than the trials suggest. That’s a step up from the more cautious rating in our earlier article on gut-healing claims: the 2025 analysis added 26 more trials and kept finding a benefit, which has strengthened the case. For readers who favor mind-body care, this is one place where the research is on your side.

Many headline findings come from mice

Most brain-gut research has used animals, a researcher told the APS Observer⁴. Some of the most dramatic results come from germ-free mice, animals raised in sterile conditions with no microbes at all.

In one line of work, germ-free mice overreacted to stress, and colonizing their guts with a probiotic microbe brought that reaction back to normal, according to a 2021 APS Observer summary of the research⁴. Mice also let scientists do what they can’t do in people, such as cutting the vagus nerve to see what changes¹.

The catch is in the setup. An animal with no microbes doesn’t exist outside a lab, and a mouse’s brain and gut differ from yours. The JB-1 authors said their failed trial shows how hard it is to carry lab findings over to people, and they called for trials in people with stress-related disorders².

That trial also shows why disclosures matter in both directions. The probiotic and placebo came from Alimentary Health Ltd., a company one coauthor was affiliated with, and several authors reported research ties to food and microbiome companies². Even so, they published a clear negative result.

Probiotics for mood: a real signal, with a lot of noise

Probiotic trials for mood now fill several reviews. What they show depends on who was studied.

People with a diagnosis. A 2025 review in Nutrition Reviews pooled randomized trials in adults diagnosed with depression or anxiety, 23 trials in all¹³. Probiotics were linked with what the authors called a substantial drop in depression scores and a moderate drop in anxiety scores¹³. Prebiotics, fibers that feed gut microbes, didn’t make a clear difference for depression¹³.

Results varied widely from trial to trial, and 7 of the 23 trials were at high risk of bias¹³. The authors suggested probiotics might work as an add-on to standard care and called for larger trials¹³.

Healthy people. A 2025 review in BMC Psychology looked at 12 trials in healthy working adults, mostly health care workers and mostly women¹⁴. Probiotics had a small effect on combined depression, anxiety and stress scores and lowered the stress hormone cortisol slightly, but didn’t change a blood marker of inflammation¹⁴. Ten of the 12 trials tested commercial products, and the authors called their findings preliminary¹⁴.

The big picture. A 2026 umbrella review, which pulls together earlier reviews, gathered 30 meta-analyses¹⁵. It found a moderate average effect of probiotics on depressive symptoms and a small, inconsistent one on anxiety¹⁵. Many of those reviews were rated low quality, and one leaned on a study that had since been retracted¹⁵. Which strain, what dose and how long all remain open questions¹⁵.

Our rating: Promising but early for probiotics as an add-on in diagnosed depression, and small, early effects in healthy people. Never stop or cut back an antidepressant to try a probiotic without talking with your prescriber. Choosing and judging probiotic products gets its own article later in this series, and our Mediterranean eating article covers the best-known diet trial for depression.

If you’re in a mental health crisis or thinking about suicide, call or text 988 to reach the Suicide & Crisis Lifeline.

Vagus nerve “resets”: the nerve is real, the hacks aren’t tested

Social media is full of vagus nerve “resets”: splashing ice water on your face, humming, gargling or long, slow exhales.

The nerve deserves the attention. It’s a major gut-brain channel, and in animal studies it carries some microbe effects to the brain¹³. But that doesn’t mean a 30-second trick changes how it works.

When we looked for controlled trials testing humming, gargling or face-splashing for digestion or mood, we found wellness blogs and product pages, not studies. Until trials exist, these rate as Not enough evidence. If one feels soothing, it can serve as a brief pause. It isn’t treatment.

Slow breathing has a better claim, because relaxation is built into gut-directed hypnotherapy⁵. But on its own, the 2025 Leeds review found confidence limited for stress-reduction techniques outside the main brain-gut therapies¹². That makes slow breathing Promising but early for gut symptoms.

A gut-brain claims check

Claim you’ll hear	What research shows	Rating
“Your gut has its own nervous system.”	It has more nerve cells than anywhere but the brain4	Strong evidence
“Most serotonin is made in the gut, so your gut sets your mood.”	True about where it’s made; its known gut jobs are digestion and clotting6	Not enough evidence for the mood part
“IBS is all in your head.”	DGBIs involve gut movement, sensitivity, immunity, microbes and brain processing7	Evidence says no
“Hypnotherapy or CBT can calm IBS.”	Benefit across 41, then 67 trials, though effects may be overstated1011	Strong evidence
“Probiotics can ease diagnosed depression.”	Moderate to substantial pooled effects, with wide variation and many weak trials1315	Promising but early
“A daily probiotic lifts a healthy person’s mood.”	Small pooled effects; one strain failed outright214	Promising but early
“A probiotic that calms mice will calm people.”	Mouse results often don’t carry over; JB-1 calmed mice and failed in healthy men12	Not enough evidence
“Humming or ice water resets your vagus nerve.”	No controlled human trials found	Not enough evidence

A calm-digestion routine you can test

Many natural-health traditions treat digestion and emotion as one system. Gut-brain science gives that view real support. The trick is to use the parts with evidence and keep the rest as low-stakes experiments.

1. Check for warning signs first. Use the box below. A routine is for ordinary gut-brain symptoms, not for symptoms that need testing.
2. Keep a two-week log. Note meals, stress, sleep and symptoms each day. You’re looking for your own patterns, not proof.
3. Take a two-minute breathing pause before meals. Breathe slowly, with a longer exhale than inhale. Rating: Promising but early⁵¹².
4. Sit down to eat, and slow down. It costs nothing, though we found no trials of it for gut-brain symptoms. Rating: Not enough evidence, low risk.
5. If you have IBS, ask about CBT or gut-directed hypnotherapy. App and online programs count¹². Rating: Strong evidence¹⁰¹¹.
6. If you try a probiotic for mood, run it as a test. Tell your prescriber, pick one product, set an end date and track mood the same way before and after. Rating: Promising but early¹³¹⁴.
7. Skip the “reset” hacks as treatment. Use them as a pause if you like. Rating: Not enough evidence.

Questions readers ask

The bottom line

Your gut and brain are in constant conversation through nerves, chemical messengers, immune signals and microbes. That conversation helps explain why stress upsets digestion and why IBS is a real disorder of gut-brain interaction, not an imaginary one.

The best-tested ways to use that connection today are brain-based therapies for IBS, which have dozens of trials behind them. Probiotics for mood are a promising add-on, not a replacement for care, and quick vagus nerve “resets” haven’t been tested. Lean on the strongest evidence, and treat the rest as careful experiments with an end date.

Related reading

• Leaky gut can be measured, but most “heal your gut” protocols haven’t been tested
• A classic Mediterranean diet trial used canola oil, so the pattern travels well
• Only 1 in 20 treatments tested in animals wins approval for people
• Stress can reach your immune cells, but positive thinking still can’t cure cancer
• The preventive care that costs you nothing, and the exceptions

Sources

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3. Mörkl S, Butler MI, Wagner-Skacel J. “Gut-Brain-Crosstalk: The Vagus Nerve and the Microbiota-Gut-Brain Axis in Depression. A Narrative Review.” Journal of Affective Disorders Reports, 2023 (DOAJ record). doaj.org
4. Thayer L. “The Gut and Brain, Inextricably Linked.” APS Observer, Association for Psychological Science, 2021. psychologicalscience.org
5. American College of Gastroenterology. “Irritable Bowel Syndrome: Overview.” ACG patient information, updated 2026. gi.org
6. Yano JM, Yu K, Donaldson GP, et al. “Indigenous Bacteria From the Gut Microbiota Regulate Host Serotonin Biosynthesis.” Cell, 2015 (Caltech authors repository). authors.library.caltech.edu
7. Drossman DA, Chang L, Tack J. “Disorders of Gut-Brain Interaction and the Rome V Process.” Gastroenterology, 2026 (abstract). nutrition-evidence.com
8. Sperber AD, Hreinsson JP, Simrén M, et al. “Rome V Global Epidemiology and Validation Survey: Prevalence of Disorders of Gut–Brain Interaction and Comparison With the Rome IV Global Epidemiology Study.” Gut, 2026 (Rome Foundation copy). theromefoundation.org
9. Mayo Clinic. “Irritable Bowel Syndrome: Symptoms and Causes.” Mayo Clinic. mayoclinic.org
10. Black CJ, Thakur ER, Houghton LA, Quigley EMM, Moayyedi P, Ford AC. “Efficacy of Psychological Therapies for Irritable Bowel Syndrome: Systematic Review and Network Meta-Analysis.” Gut, 2020. pubmed.ncbi.nlm.nih.gov
11. Epocrates. “Which Behavioral Treatments Work Best for IBS?” (summary of Thakur ER, et al., The Lancet Gastroenterology & Hepatology, 2025). Epocrates, October 2025. epocrates.com
12. IANS. “Behavioural Therapies May Be Effective at Treating Irritable Bowel Syndrome” (report on Thakur ER, et al., The Lancet Gastroenterology & Hepatology, 2025). IANS, October 10, 2025. ianslive.in
13. Asad A, Kirk M, Zhu S, Dong X, Gao M. “Effects of Prebiotics and Probiotics on Symptoms of Depression and Anxiety in Clinically Diagnosed Samples: Systematic Review and Meta-Analysis of Randomized Controlled Trials.” Nutrition Reviews, 2025. pubmed.ncbi.nlm.nih.gov
14. “Probiotic Intake and Mental Health in Healthy Working Adults: A Systematic Review and Meta-Analysis of Randomized Controlled Trials.” BMC Psychology, 2025. link.springer.com
15. Sá Filho AS, et al. “‘Attacking’ the Gut–Brain Axis With Psychobiotics: An Umbrella Review of Depressive and Anxiety Symptoms.” Pharmaceuticals, 2026. pubmed.ncbi.nlm.nih.gov