Gut, brain and sleep
The gut-brain axis explained
The gut-brain axis is the two-way communication system between the digestive tract and the brain. It runs through four channels: nerves, chiefly the vagus nerve; hormones released by the gut; the immune system; and substances made by gut bacteria. It explains why stress upsets the stomach, and why a troubled gut affects mood, sleep and concentration. When this communication is disturbed, the result can be a condition such as irritable bowel syndrome, and treatment works on both ends.
In short
- The gut has its own nervous system, with 200 to 600 million nerve cells.
- Most fibres of the vagus nerve carry information upward, from the gut to the brain.
- Most of the body’s serotonin is made in the gut, where it acts locally.
- Gut bacteria take part in the conversation. In humans, most of the evidence for this is still association.
What is the gut-brain axis?
“Axis” means a line of communication. The gut-brain axis is the set of connections that lets the brain adjust digestion, and lets the gut report back to the brain.
Each end changes the other. The brain speeds up or slows down the gut, changes its blood flow and alters how sensitive it is. The gut tells the brain about hunger, fullness, stretch, pain and the presence of irritants.
How do the gut and the brain communicate?
Four channels, both directions. The gut and the brain exchange messages through nerves, hormones, the immune system and substances made by gut bacteria. Most fibres of the vagus nerve carry information upward, from the gut to the brain.
Nerves
The vagus nerve runs from the brainstem to the gut, and the gut’s own nervous system runs digestion from within the gut wall. See the vagus nerve and the enteric nervous system.
Hormones
The gut releases hormones that signal hunger and fullness. Stress hormones released on the brain’s instruction change gut movement and secretion.
The immune system
A large share of the body’s immune cells sit in the gut wall. The chemical messengers they release act on nerves and reach the brain.
Gut bacteria
Bacteria make short-chain fatty acids and other compounds that act on the gut lining, immune cells and nerve endings.
What does it do in daily life?
- The churning stomach before an examination or an interview
- The urge to pass stool when you are nervous
- Loss of appetite with worry, or comfort eating
- Nausea with fear or disgust
- The feeling of fullness that tells you to stop eating
These are normal workings of the axis.
What happens when it goes wrong?
When the signalling stays disturbed, the gut becomes over-sensitive and moves irregularly while tests remain normal. Doctors call these conditions disorders of gut-brain interaction. Irritable bowel syndrome and functional dyspepsia are the commonest. See gut-brain axis disorders.
The link runs in both directions. In a 12-year study, anxiety at the start predicted new gut disorders, and gut disorders at the start predicted new anxiety and low mood. In pooled studies, about four in ten people with IBS had symptoms of anxiety and about three in ten had symptoms of depression.
In pooled studies of people with IBS
- About 4 in 10 had symptoms of anxiety
- About 3 in 10 had symptoms of depression
What are the symptoms of a disturbed gut-brain axis?
There is no single list, because this is a mechanism and not a diagnosis. Common patterns are:
- Abdominal pain, bloating, nausea or early fullness with normal tests
- A bowel habit that changes with stress, travel or poor sleep
- Gut symptoms together with poor sleep, tiredness, headaches or anxiety
Blood in the stool, weight loss, fever and symptoms that wake you at night do not fit this pattern and need investigation.
How is it treated?
| Approach | How it works | Evidence |
|---|---|---|
| Food changes, such as a limited-period low FODMAP diet | Fewer triggering signals from the gut | Good in IBS |
| Medicines for the gut | Relieve cramps, constipation or diarrhoea | Good for each symptom |
| Low-dose nerve-calming medicines | Lower the sensitivity of gut nerves | Good. A low dose worked better than a dummy tablet in a trial of 463 people with IBS. |
| Brain-gut therapies, such as CBT and gut-directed hypnotherapy | Change how the brain processes gut signals | Good in IBS |
| Exercise | Steadies gut movement and the stress response | One randomised trial in IBS |
| Yoga and breathing practice | Shift the nervous system toward its calming branch | Small trials suggest benefit. Their quality is limited. |
| Probiotics | Act through gut bacteria | A small effect on mood in pooled trials |
How can I support my gut-brain axis?
Keep regular hours
Meals and sleep at consistent times steady the gut’s daily rhythm.
Practise slow breathing
Five to ten minutes, twice a day, at about six breaths a minute. See ways to stimulate the vagus nerve.
Move every day
A brisk walk on most days is enough to begin with.
Feed your gut bacteria
Plenty of plant foods and a fermented food each day. See how to improve your gut microbiome.
Go easy on caffeine and alcohol
Both stimulate the gut and disturb sleep.
Get help for anxiety or low mood
Treating them helps the gut, and treating the gut helps them.
When should I see a doctor?
See a doctor if gut symptoms last more than three to four weeks or interfere with daily life. See one promptly for blood in the stool, weight loss, difficulty swallowing, persistent vomiting, fever or symptoms that wake you from sleep.
Questions patients ask me
What is the gut-brain axis in simple words?
It is the two-way link between your digestive system and your brain. They send messages to each other through nerves, hormones, immune signals and substances made by gut bacteria.
Is the gut really a second brain?
The gut has its own nervous system, with hundreds of millions of nerve cells, which can run digestion without instructions from the brain. It does not think or feel emotions.
Does serotonin made in the gut affect mood?
Not directly. Most of the body’s serotonin is made in the gut, where it controls movement and sensation, and it does not cross into the brain. The gut influences mood through nerves and other signals.
Can gut problems cause anxiety?
They can contribute. In a long-term study, people with a gut disorder were more likely to develop anxiety or low mood later, and the opposite was also true.
Is there a medicine for the gut-brain axis?
There is no single one. Low-dose medicines that calm gut nerves are used when pain persists, alongside food changes and brain-gut therapies. See low-dose antidepressants for gut pain.
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
- Review of the vagus nerve in the microbiota-gut-brain axis. Bonaz B et al. Front Neurosci. 2018. PubMed 29467611
- Review of the enteric nervous system. Furness JB. Nat Rev Gastroenterol Hepatol. 2012. PubMed 22392290
- Rome IV overview of disorders of gut-brain interaction. Drossman DA. Gastroenterology. 2016. PubMed 27144617
- Twelve-year study showing the brain-gut pathway runs in both directions. Koloski NA et al. Gut. 2012. PubMed 22234979
- Meta-analysis of anxiety and depression in irritable bowel syndrome. Zamani M et al. Aliment Pharmacol Ther. 2019. PubMed 31157418
- Network meta-analysis of psychological therapies for irritable bowel syndrome. Black CJ et al. Gut. 2020. PubMed 32276950
- Trial of a low-dose tricyclic medicine for irritable bowel syndrome in primary care. Ford AC et al. Lancet. 2023. PubMed 37858323
- Meta-analysis of prebiotics and probiotics for depression and anxiety. Liu RT et al. Neurosci Biobehav Rev. 2019. PubMed 31004628
- Trial of physical activity for irritable bowel syndrome. Johannesson E et al. Am J Gastroenterol. 2011. PubMed 21206488
- Systematic review of yoga for irritable bowel syndrome. Schumann D et al. Clin Gastroenterol Hepatol. 2016. PubMed 27112106