The gut-brain axis explained simply

In 1997, Australian researchers asked 1,775 adults about their gut and their mood. Twelve years later, they asked again. People who were anxious at the start were more likely to develop new gut problems. People with gut problems at the start became more anxious and low over time.

The traffic ran both ways. That two-way link is the gut-brain axis. It explains why your belly knots before an exam, and why a bad gut day can ruin your mood. Here is how it works, in plain words, and what you can do with it.

What is the gut-brain axis?

The gut-brain axis is the two-way line of communication between your gut and your brain. Signals travel up and down through nerves, hormones, the immune system and the bacteria living in your gut.

Think of it as a busy phone line that never hangs up. Your brain tells your gut when to speed up, slow down or squeeze. Your gut sends back reports about fullness, pain, inflammation and what the bacteria are doing.

Doctors now call conditions like IBS “disorders of gut-brain interaction”. That name came from the Rome Foundation, the group that sets the rules for diagnosing these conditions. Its global study surveyed 73,076 adults in 33 countries. Of those who answered online, 40.3% met the criteria for at least one of these disorders.

How do the gut and brain talk to each other?

They use four main routes. Each one carries messages in both directions.

• The vagus nerve. This long nerve runs from the brainstem to the gut. It carries many messages from the gut up to the brain, not only from the brain down.
• The gut’s own nervous system. Your gut wall holds a large network of nerve cells that can run digestion with some independence. It is sometimes called the second brain.
• Hormones and chemical messengers. Stress hormones such as cortisol change how the gut moves. The gut also makes chemical messengers, including serotonin.
• The immune system. Much of your immune system sits around the gut. Inflammation there can send signals that affect mood and energy.

Do gut bacteria really affect your mood?

They seem to, but most of the strongest proof comes from animals. In people, the evidence shows links, and early trials are promising but not settled.

A well-known 2011 study fed mice a strain of Lactobacillus rhamnosus. The mice showed fewer anxious behaviours and lower stress hormone levels. When researchers cut the vagus nerve, the effect disappeared. This suggests bacteria can send messages to the brain through that nerve.

A 2016 study in Ireland went further. Researchers took gut bacteria from people with depression and moved them into rats with very few gut bacteria of their own. The rats began to show depression-like and anxiety-like behaviours. People with depression also had less variety in their gut bacteria than healthy people.

In 2019, a Belgian study looked at the gut bacteria of 1,054 adults. Two types of bacteria that make butyrate, a fatty acid that feeds the gut lining, were linked to better quality of life. One of these, Coprococcus, was lower in people with depression, even after the researchers allowed for antidepressants. This shows a link, not proof that the bacteria cause the mood change.

Your gut cells also make much of your body’s serotonin. A 2015 study in mice showed that certain gut bacteria help gut cells make it. Serotonin in the gut mainly controls gut movement, though. It does not travel straight into the brain, so this is not a simple “happy chemical” shortcut.

Is the gut-brain axis why IBS and anxiety go together?

Yes, it is a big part of the reason. The 12-year Australian study found the link runs in both directions. Anxiety predicted new gut problems, and gut problems predicted new anxiety and low mood.

The study followed 1,002 people to the end. Among people with no gut disorder at the start, higher anxiety made a new gut disorder more likely 12 years later. Among people with no anxiety at the start, those with a gut disorder became more anxious and more depressed over time.

I see this in my own clinic and I lived it during my years with diarrhoea-type IBS. Worry made my gut worse, and my gut made me worry more. My guide to IBS and anxiety explains how to break that loop.

The same link shows up in people with inflammatory bowel disease. Many still have IBS-type symptoms when their inflammation is controlled, and those people tend to have higher anxiety scores. I explain this in IBS vs IBD.

Can you calm the gut by calming the brain?

Yes. Some of the best-tested IBS treatments work on the brain side of the gut-brain axis.

A 2025 review in The Lancet Gastroenterology and Hepatology pooled 67 trials with 7,441 adults with IBS. Several talking therapies beat a waiting list, including cognitive behavioural therapy and gut-directed hypnotherapy. Hypnotherapy alone was tested in 12 trials with 1,507 people.

A 2026 review in the journal Gut pooled 68 trials with 6,694 adults. Low-dose medicines that act on gut nerves, such as some older antidepressants, helped IBS symptoms. So did several talking therapies. The certainty of the evidence was low for most comparisons, and none of these suit everyone.

These medicines are used at low doses for gut pain, not because a doctor thinks the pain is “all in your head”. Never start or stop a medicine without your doctor.

Can food help the gut-brain axis?

Food can help, and this is where I see real change with clients. I strongly believe gut health plays a major role in nearly every health condition, and mood is no exception.

The SMILES trial in Australia tested this in 2017. It enrolled 67 adults with moderate to severe depression. One group had 7 sessions with a dietitian over 12 weeks to move towards a Mediterranean-style diet. After 12 weeks, 32.3% of the diet group were in remission, compared with 8.0% of the social support group. The trial was small and did not measure gut bacteria.

In a 2021 Stanford trial, 36 healthy adults ate more fermented foods or more fibre for 17 weeks. The fermented food group gained more variety in gut bacteria and had lower markers of inflammation. Mood was not the main goal, so this is a clue, not proof.

A 2026 review of 38 trials with 2,338 adults with major depression looked at gut-targeted treatments. Probiotics and prebiotics, the fibres that feed good bacteria, lowered depression scores more than placebo. The authors rated the certainty as low to moderate and said the results should be read with caution.

How can you support your gut-brain axis every day?

Small daily habits send calm signals in both directions. In my clinic, I always ask clients about their sleep hours, sleep quality, gut health and their poop, because all of them are connected.

Try this.

• Breathe slowly after meals. Breathe in for 4 seconds and out for 6 to 8 seconds, for 5 minutes. My Breathe With Me exercise guides you.
• Eat a wide range of plants. Aim for vegetables, fruit, oats, beans, lentils, nuts and seeds through the week. Grow your count in the Gut Garden game.
• Add a fermented food. Try a small pot of yoghurt, a glass of kefir or a spoon of sauerkraut. Start small if you have IBS.
• Raise fibre slowly. Too much too fast can cause bloating. My guide to fibermaxxing shows a gentle way to do it.
• Keep regular meal times. Your gut likes rhythm. Long gaps and very large meals can both stir symptoms.
• Protect your sleep. Aim for 7 to 9 hours. Poor sleep often makes gut pain and low mood feel worse.
• Move every day. A 20 to 30 minute walk is enough to start. Gentle yoga is safe for most people too.
• Ask about gut-focused therapy. If your gut and worry feed each other, ask your doctor about CBT or gut-directed hypnotherapy.

When should you see a doctor?

See a doctor if gut symptoms come with warning signs. These include blood in your stool or symptoms that wake you at night. Losing weight without trying, such as 3 kg (about 7 lb) or more, also needs a check.

Also seek help soon if low mood or anxiety stops you from working, sleeping or enjoying life. If you ever have thoughts of harming yourself, contact a doctor or an emergency service straight away. Your gut and your mind both deserve care.

Frequently asked questions

Is the gut really a second brain?

In a way, yes. The gut has its own large network of nerve cells that can run digestion with some independence. It does not think like your brain, but it sends constant messages to it, mostly through the vagus nerve.

Can probiotics fix anxiety or depression?

Not on their own. Some trials show a small benefit for depression, but the evidence is low to moderate in certainty. Talk to your doctor before you change any treatment.

How long does it take to improve the gut-brain axis?

Give it weeks, not days. In the trials above, people changed their diet for 12 to 17 weeks. Give new habits at least a month before you judge them.

Does stress damage your gut?

Stress changes how fast your gut moves and how sensitive it feels. It can make IBS symptoms worse. Daily calm habits, good sleep and support from a professional can all ease that effect.

What is the best food for the gut-brain axis?

No single food is best. A varied diet with plenty of plants, fibre, olive oil, fish and fermented foods has the most support. Fewer ultra-processed foods is a good goal too.

What I want you to remember

The gut-brain axis is a two-way line between your gut and your brain. Stress can upset your gut, and an upset gut can lower your mood. Gut bacteria seem to take part, though most of the strongest proof still comes from animals.

You can send calmer signals both ways with slow breathing, varied plant foods, fermented foods, sleep and movement. Test your knowledge in the Daily Myth game, and join my free 30-Day Calm Gut Challenge to build these habits one day at a time.