Gut, brain and sleep
The gut microbiome and mental health
Gut bacteria and the brain influence each other through nerves, hormones, the immune system and substances that bacteria make. In large population studies, people with depression have lower levels of certain gut bacteria. This is an association. It does not show that gut bacteria cause depression, or that changing them treats it. The strongest practical evidence is for diet: in one trial, improving the diet helped adults with depression who were already receiving treatment. Probiotics have shown a small effect at most. Care of the gut adds to mental health treatment and does not replace it.
In short
- The link between gut bacteria and mood is consistent in studies. Cause and effect in humans is not established.
- In a 12-week trial of 67 adults with depression, about a third of those given dietary support were in remission, compared with fewer than one in ten in the comparison group.
- In pooled trials, probiotics had a small effect on symptoms of depression and anxiety. Prebiotics had none.
- No microbiome test can diagnose a mental health condition.
How could gut bacteria affect the brain?
Four routes from the gut to the brain. The link between gut bacteria and mood is consistent in studies. Cause and effect in humans is not established.
- Through the vagus nerve, which carries signals from the gut wall to the brainstem
- Through the immune system: an unsettled gut community promotes low-grade inflammation, which affects the brain
- Through what bacteria make: short-chain fatty acids and compounds derived from the amino acid tryptophan
- Through stress hormones: in animal studies, gut bacteria shape how strongly the body responds to stress
What do human studies show?
A population study
In more than 1,000 people in Belgium, bacteria that make butyrate went with a better quality of life. Two groups, Coprococcus and Dialister, were lower in people with depression, and this held after allowing for antidepressant use.
A diet trial
Sixty-seven adults with moderate to severe depression, most of them already on treatment, were given either seven sessions of dietary support or social support. After 12 weeks, 32% of the diet group were in remission, compared with 8% of the other group. The trial was small.
Probiotic trials
A pooled analysis of controlled trials found a small benefit of probiotics for symptoms of depression and anxiety, and none for prebiotics.
Gut conditions and mood
Anxiety and depression are common in people with IBS, and each condition raises the risk of the other over time.
Animal studies
Transferring gut bacteria from people with depression into laboratory animals changes the animals’ behaviour. This supports a causal role but does not prove one in humans.
The diet trial: in remission after 12 weeks
- Dietary support32%
- Social support8%
Sixty-seven adults with moderate to severe depression, most of them already on treatment. The trial was small.
What helps in practice?
Improve the overall diet
More vegetables, fruit, pulses, whole grains, nuts and fish, and less packaged and fried food. This is the pattern used in the trial above.
Add fermented foods
In a ten-week trial, a diet high in fermented foods increased the diversity of gut bacteria and lowered markers of inflammation.
Exercise and sleep
Both have well-established benefits for mood, and both affect the gut.
Treat gut symptoms properly
Pain, bloating and an unpredictable bowel wear down mood. Treating them helps.
Continue your mental health treatment
Medicines and psychological therapy for depression and anxiety have far stronger evidence than any gut-based treatment.
From my practice
Many people come to me with a gut problem and with anxiety or low mood, and tell me that the two rise and fall together. I treat both at the same time, alongside their psychiatrist or therapist where they have one. In my experience, when long-standing constipation, bloating or bacterial overgrowth is treated properly, mood and sleep often improve as well.
This is what I see in my own patients. It is not a trial, and it is not a reason to stop a medicine for depression or anxiety. I wrote about it in my chapter of the book The Missing Links.
What should I be cautious about?
Supplements sold for mood
Products marketed as “psychobiotics” rest on small studies. A small average effect does not mean that a product will help you.
Tests that claim to read mood from stool
No microbiome test can measure brain chemistry or diagnose depression, anxiety or autism.
Stopping medicines
Do not stop an antidepressant in favour of a gut programme. If you want to reduce a medicine, plan it with the doctor who prescribed it.
Faecal transplant
For depression and autism it is experimental.
When should I see a doctor?
Get help now if
You have thoughts of harming yourself. Speak to a doctor, go to the nearest emergency department, or call Tele-MANAS, the Government of India’s free mental health helpline, on 14416.
See a doctor or a psychiatrist if low mood, loss of interest or anxiety has lasted more than two weeks or is affecting work, sleep or relationships.
Questions patients ask me
Can gut bacteria cause depression or anxiety?
They may contribute, but this has not been shown in humans. Studies show that the two go together and that the influence runs in both directions.
Can probiotics help anxiety or depression?
Pooled trials show a small benefit at most. A probiotic is not a substitute for established treatment.
Is it true that most serotonin is made in the gut?
Yes, but that serotonin acts in the gut and does not cross into the brain. The gut affects mood through nerves, immune signals and other routes.
Can improving my diet improve my mood?
In one trial of adults with depression, it did, when added to their usual treatment. A better diet is worth adopting for many reasons, and this may be one of them.
Is the gut microbiome linked to autism or Alzheimer’s disease?
Differences in gut bacteria have been reported in both. No treatment aimed at gut bacteria is established for either.
Sources
This page is written for patients and is based on the guidelines and studies below. It is for education and does not replace a consultation.
- Review of the microbiota-gut-brain axis. Cryan JF et al. Physiol Rev. 2019. PubMed 31460832
- Population study of gut bacteria, quality of life and depression. Valles-Colomer M et al. Nat Microbiol. 2019. PubMed 30718848
- Trial of dietary improvement for adults with major depression. Jacka FN et al. BMC Med. 2017. PubMed 28137247
- Meta-analysis of prebiotics and probiotics for depression and anxiety. Liu RT et al. Neurosci Biobehav Rev. 2019. PubMed 31004628
- Meta-analysis of anxiety and depression in irritable bowel syndrome. Zamani M et al. Aliment Pharmacol Ther. 2019. PubMed 31157418
- Twelve-year study showing the brain-gut pathway runs in both directions. Koloski NA et al. Gut. 2012. PubMed 22234979
- Trial of high-fibre and fermented-food diets and immune status. Wastyk HC et al. Cell. 2021. PubMed 34256014
- American Gastroenterological Association guideline on faecal microbiota-based therapies. Peery AF et al. Gastroenterology. 2024. PubMed 38395525