The gut-brain axis: how gut health affects mood
Gut and brain talk both ways. Stress can upset your gut, and gut bacteria differ in depression. But probiotics alone didn’t ease depression in trials. In one small trial, a better diet did.
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What is the gut-brain axis?
It’s the two-way link between your gut and your brain. Your gut bacteria are part of it. Scientists say the trillions of microbes in and on us talk with the brain by several routes.
Four ways your gut and brain talk
- 1
Vagus nerve
A long nerve that carries messages between brain and gut.
- 2
Gut nerves
The gut’s own nervous system, the enteric nervous system.
- 3
Immune system
Your body’s defences also carry signals.
- 4
Bacteria chemicals
Substances gut bacteria make, like short-chain fatty acids.
Source: Cryan 2019
Think of a phone line with calls going both ways. A call can change how the other end feels. It doesn’t mean one end runs the other.
That gut nervous system is why people call the gut a second brain. You may also read that most serotonin, a mood chemical, is made in the gut. The gut does hold much of the body’s serotonin, but the researchers say it supplies the gut lining and blood.
So can fixing your gut fix your mood? Most of the proof so far comes from animals, and human studies are still going. If your mood has been low for weeks, start with your GP, not a supplement.
Do gut bacteria affect mental health?
They may. Two large studies found different gut bacteria in people with depression.
| Study | Who | What it found |
|---|---|---|
| Flemish Gut Flora Project | 1,054 adults, checked in 1,070 more | Two types of bacteria were lower in depression, even allowing for antidepressants. |
| Rotterdam Study | 1,054 adults, checked in 1,539 more | 13 types of bacteria were linked to symptoms of depression. |
| Cork transfer study | 34 people with depression, 33 without, then rats | Gut bacteria from people with depression gave rats behaviour like depression and anxiety. |
Sources: Valles-Colomer 2019; Radjabzadeh 2022; Kelly 2016
The first two show a link, not a cause. The Flemish team stresses that other factors, like medicines, can blur it.
Rats aren’t people. But the Cork team says gut bacteria may play a part in causing features of depression.
Want to look after your gut anyway? Most adults eat about 20g of fibre a day, and the NHS says it should rise to 30g. See how much fibre you need.
Can stress and anxiety upset your gut?
Yes. This is the brain-to-gut side of the line. Researchers say stress can affect the gut-brain axis at every age.
You can see it in irritable bowel syndrome (IBS). The NHS links IBS to stress and an oversensitive gut, among other things. Stress and anxiety can also trigger flare-ups of cramps, bloating and diarrhoea.
Talking therapy can help the gut too. For IBS that hasn’t eased after 12 months of medicines, NICE says to consider CBT, hypnotherapy or other talking therapy. CBT is a talking therapy that changes how you think and act.
So the NHS advice for IBS includes finding ways to relax and getting plenty of exercise. Our stress management guide has ideas to start with.
Do psychobiotics work?
A psychobiotic is a live bacterium that helps people with a mental illness when taken in large enough amounts. In other words, a probiotic aimed at mood.
| How it was taken | What 7 trials in 404 people found |
|---|---|
| With antidepressants | Eased depression symptoms. |
| On its own | No clear benefit. |
Source: Nikolova 2021
Only 7 trials qualified, and some of the reviewers had funding from a probiotic maker. Never stop or change a prescribed medicine without your prescriber.
Shop products are another worry. The NHS says we can’t always be sure they hold the bacteria on the label, or enough to work.
Have a health condition or a weak immune system? Talk to a doctor first. More in do probiotics work? and fermented foods for gut health.
Can diet help your mental health?
It may. The best test so far is the SMILES trial. It gave 67 adults with moderate to severe depression seven sessions with a dietitian over 12 weeks, or social support instead.
That’s the better diet from the top of this page. After 12 weeks, depression had lifted for about 32% of the diet group and 8% of the support group.
The SMILES trial diet, in six habits
- Eat whole grains, 5 to 8 servings a day.
- Eat 6 servings of vegetables and 3 of fruit a day.
- Have beans, lentils or chickpeas 3 to 4 times a week.
- Have fish at least twice a week, and unsalted nuts once a day.
- Use olive oil, about 3 tablespoons a day.
- Keep sweets, fried food, fast food, processed meat and sugary drinks to 3 times a week at most.
Source: Jacka 2017, full text
It’s one small trial, and people knew which group they were in, which may have biased the results. Most were already having therapy or medicine, so diet was added to treatment, not swapped in.
Already being treated? Ask your GP about seeing a dietitian. For easy swaps, see high-fibre foods.
When to speak to a GP
Book an appointment if:
- You think you may be depressed: you’ve felt sad for weeks or months, rather than a few days.
- You think you might have IBS and have had symptoms for over 4 weeks.
- Diet changes and pharmacy medicines aren’t helping your IBS.
- Ask for an urgent GP appointment or call 111 if you’ve lost a lot of weight for no reason, are bleeding from your bottom or have bloody diarrhoea, or find a hard lump or swelling in your tummy. The same goes if you’re short of breath, can feel your heartbeat and your skin is paler than usual.
- Need urgent help for your mental health? Call 111 and choose the mental health option, or call Samaritans on 116 123.
Call 999 or go to A&E if someone’s life is at risk, or you don’t feel you can keep yourself or someone else safe.
Want to rule out a physical cause of low mood? See blood tests for low mood.