Gut brain axis: what the research shows

By UCD Neurology editorial team · Updated November 2, 2026 · Education only

3D illustration of the human small intestine on a blue background

The gut brain axis is the two-way link between your digestive system and your brain. Nerves, hormones, immune signals, and chemicals made by gut microbes all carry messages along it. The link is real, but most research on what it means for brain health is still early.

Quick facts

Signals travel both ways: brain to gut, and gut to brain.
The gut has its own nerve network, with more than 500 million nerve cells.
The vagus nerve is the main nerve link between the gut and the brain.
Much of the microbe research comes from animal studies.
A link between gut microbes and a disease does not show which one came first.

Sources: Cleveland Clinic: the gut-brain connection; Carabotti and others, Ann Gastroenterol 2015; NIDDK: gut feelings workshop summary, 2024.

What is the gut brain axis?

You have felt the gut brain axis at work. Nerves before a big event can upset your stomach. A painful gut can raise your stress. Those are messages moving between the two systems.

Doctors use the term for the network that links the brain and the gut. It includes nerves, hormones, and the immune system. It also includes the microbes that live in the gut. In recent years, research on those microbes has grown.

The basic link is well established. Researchers can trace the nerves and the chemical signals. The open question is a different one: does changing the gut change brain health in people? For most conditions, the answer is not known yet.

Sources: Cleveland Clinic: the gut-brain connection; Carabotti and others, Ann Gastroenterol 2015.

How the gut and brain send signals

Messages move along several routes at once. No single route explains the whole system.

Route	How it works	What we know
Enteric nervous system	A web of nerve cells in the gut wall that runs digestion	Some call it a "second brain," as it can work partly on its own
Vagus nerve	A long nerve from the brainstem to the gut	Carries gut news up to the brain and brain signals back down
Hormones	Chemical messages in the blood	Help signal hunger, fullness, and stress
Immune signals	Messages from immune cells in and near the gut	Help the body respond to injury or illness in the gut
Microbe products	Chemicals made by gut bacteria	Some help make brain messengers; some reach the brain through the blood

The routes also loop back. The brain can change the gut's setting, and that can change which microbes thrive. So a change in gut microbes may be a result of illness, not only a cause.

Sources: Cleveland Clinic: the gut-brain connection; Carabotti and others, Ann Gastroenterol 2015.

What the human research shows so far

Most of what we know about microbes and the brain comes from lab work. Researchers raise mice with no gut microbes, or give animals antibiotics or probiotics, and watch what changes. In people, most of the evidence is links: people with certain conditions tend to have different gut microbes.

That gap matters. This table shows what each kind of study can and cannot tell you.

Type of study	What it can show	What it cannot show
Animal or lab study	A possible way a signal works	That the same thing happens in people
Human link study	That two things show up together	Which one came first, or if one causes the other
Small or short human trial	An effect in those people, for that time	That it lasts, or works for other groups
Large controlled human trial	Whether a treatment helps a defined group	That it helps every person

Most gut brain findings sit on the first two rows. Animal studies have hinted that changing gut microbes can ease some symptoms. Results in people are still mixed. A 2024 NIH workshop noted that treatments for gut-brain disorders have had limited and variable success. It also called for more studies that test changes in people.

Sources: Carabotti and others, Ann Gastroenterol 2015; Cleveland Clinic: the gut-brain connection; NIDDK: gut feelings workshop summary, 2024.

The gut and immune diseases of the nervous system

This site focuses on neuroimmunology, the field that studies immune attacks on the nervous system. The gut matters here because the gut and the immune system work closely together.

One of the largest studies so far looked at multiple sclerosis. The study compared 576 people with MS to a healthy person living in the same home, for 1,152 people in total. People with MS had more of some bacteria and less of others. Their gut microbes also differed based on which MS medicine they took.

Comparing people who share a home helps, because they share food and surroundings. Still, the study shows links, not cause. It cannot tell whether the gut changes helped trigger MS, came from MS, or came from its treatment. Doctors diagnose MS with the exam, MRI, and other tests, not a stool test. Our page on how MS is diagnosed explains those steps.

Sources: iMSMS Consortium, Cell 2022; Cleveland Clinic: the gut-brain connection.

When gut brain signals go wrong

Some conditions are now called disorders of gut-brain interaction. Irritable bowel syndrome (IBS) is the best known. In IBS, the way the brain and gut work together seems to change. Food may move too fast or too slow. A normal amount of gas can hurt.

IBS can be painful, but it does not damage the digestive tract. Routine tests may look normal. That does not mean the symptoms are made up. It means the problem lies in how the system works, not in visible harm.

Doctors are not sure what causes IBS. Hard early life events, past gut infections, and some mental health conditions are more common in people with it. Once the gut and brain fall into a bad loop, it is hard to tell which side started it.

A new or lasting change in your gut still needs a medical check. A gut-brain label should come after other causes have been looked at, not before.

Sources: NIDDK: symptoms and causes of IBS; NIDDK: gut feelings workshop summary, 2024.

How to read a gut brain headline

Gut brain stories are everywhere, and many go further than the science. Use these steps when you read one.

Check who was studied. Was it mice, cells, or people?
Check the kind of result. Did the study find a link, or test a change?
Check the size. A trial in a few dozen people is a start, not an answer.
Check the exact product. One probiotic strain does not tell you what another strain does.
Check the outcome. A study on gut symptoms does not prove an effect on mood, memory, or MS.
Check the risk. NIH notes that harmful effects from probiotics are more likely in people with severe illness or a weak immune system, so ask a clinician first.

If a product claims to "fix" the gut brain axis, be wary. We found no good evidence that any test or product can do that.

Sources: NCCIH (NIH): probiotics, usefulness and safety; Carabotti and others, Ann Gastroenterol 2015.

Who this page is for

Read this if you have seen gut brain headlines and want to know what is solid and what is hype. It may help people with MS, autoimmune encephalitis, or another immune condition of the nervous system who hear a lot of gut advice. Curiosity is a good thing here. Just keep asking what was tested, in whom, and what changed.

Questions people ask

Is the gut-brain axis a real thing?

Yes. The gut and brain are linked by nerves, mainly the vagus nerve, and by hormones and immune signals. The gut also has its own large nerve network. Those links are well mapped. What is less clear is how much gut microbes shape brain health in people, and whether changing them helps. Much of that evidence still comes from animals.

What are the symptoms of gut-brain axis problems?

There is no single symptom list for the gut brain axis. The best-studied problems are disorders of gut-brain interaction, such as IBS. IBS often causes belly pain linked to bowel movements, along with diarrhea, constipation, or both. Bloating is common too. These symptoms have many possible causes, so a doctor should check them before blaming the gut brain link.

How do you fix the gut-brain axis?

We found no good evidence that any test or product can fix the gut brain axis as a whole. Doctors treat specific, diagnosed conditions instead. For some gut-brain disorders like IBS, doctors may consider several approaches, including medicines and talk therapies. Treatments for these disorders have had limited and variable success so far. A doctor can help decide what fits your situation.

Do probiotics help the gut-brain axis?

The evidence is not strong enough to say. Different probiotics have different effects, so a result for one strain does not apply to others. For IBS, a 2018 review of 53 studies could not draw firm conclusions about whether probiotics work or which strains to use. NIH also notes a greater risk of harm in people with severe illness or a weak immune system. Talk with a doctor first.

What does the vagus nerve do in the gut-brain axis?

The vagus nerve is the main link between the gut's own nerve network and the brain. It carries information about what is happening in the gut up to the brain. It also carries signals from the brain back down to the gut. It helps control reflexes that respond to food and to chemical changes in the gut.

Does the gut microbiome cause MS?

That is not known. A large study found that people with MS had different gut bacteria than healthy people living in the same homes. But a link is not proof of cause. The gut changes might help trigger MS, result from it, or come from its treatment. Researchers are still working to sort this out, and no gut test can diagnose MS.

Sources: Cleveland Clinic: the gut-brain connection; NIDDK: symptoms and causes of IBS; NCCIH (NIH): probiotics, usefulness and safety; Ford and others, Aliment Pharmacol Ther 2018 (PMID 30294792); iMSMS Consortium, Cell 2022.