The gut-brain axis, often called the gut-brain connection, is a two-way communication network linking your gut and your brain.
It involves nerves, hormones, immune signals and biochemical messengers, and scientists have studied it extensively.
You might notice the gut-brain connection in your everyday life. For example, your stomach might feel queasy when you get nervous before a big event, or you might get butterflies when you’re excited.
There’s growing interest in what the gut-brain axis means for mood and mental health. But it’s not as simple as ‘fixing your gut to treat mental health conditions’.
In this article, we’ll cover how the gut-brain axis works, how gut microbes and mood are linked, and what stress does to the gut. We’ll also look at what the evidence says about diet and probiotics.
What is the gut-brain axis?
The gut-brain axis is a network of communication routes between your brain and your gut, including the trillions of microbes that live there. Communication can go both ways, from your gut to your brain and vice versa.
The gut-brain axis is very important. It’s involved in regulating digestion, immune function and your body’s response to stress. There’s also some evidence that it’s linked to mood and mental health.
How do the gut and the brain communicate?
Communication between the gut and the brain occurs across a complex network of pathways. Here, we provide an overview of the four main routes.
1. The nervous system
The vagus nerve is large and runs from your brain down through your body, branching out to your gut and other organs. It’s a direct link between your brain and your gut.
Roughly 80% of its nerve fibres carry signals from your organs, including your gut, to your brain, while the remaining 20% carry signals the opposite way.
Your gut also has its own nervous system, called the enteric nervous system. This dense mesh of neurons is embedded in the wall of your gut and runs your digestive functions, including things like muscle contractions and the release of digestive enzymes.
This happens without instructions from your brain, since the enteric nervous system is part of the autonomic nervous system, which controls involuntary functions like your heartbeat.
Sometimes people call the gut your ‘second brain’, since it can control digestion. But that's misleading, as your enteric nervous system can't think or feel the way your brain does.
The gut is also where most of your serotonin is made. This neurotransmitter serves important functions in many of your organs.
Around 90% of serotonin is made by specialised cells in the gut wall called enterochromaffin cells. Here, it helps regulate digestion, including how food moves through your gut.
In your brain, serotonin helps regulate mood and emotions. But serotonin made in the gut can't cross the blood-brain barrier, so it can't reach your brain. The serotonin that acts in your brain is made there. So, while it's true that most of your serotonin is in your gut, that doesn't mean it directly affects your mood.
Still, some research shows that serotonin in your gut can activate vagus nerve fibres, sending signals to brain regions involved in mood, stress responses and immune regulation.
2. The hormonal (endocrine) system
Your gut and your brain can also communicate through the body's central stress-response system, the hypothalamic-pituitary-adrenal (HPA) axis.
This means psychological stress can lead to measurable changes in gut function, while signals from your gut can affect how you respond to stress.
The HPA axis isn't the only hormonal route, though. Cells in the gut lining release gut peptide hormones that carry messages to the brain to regulate hunger, appetite and feelings of fullness.
GLP-1 is a well-known gut peptide, and its receptor is the main target of weight management medications like Wegovy and Mounjaro.
3. The gut microbiome and its metabolites
The gut microbiome is the community of trillions of microbes living mainly in your colon. These microbes are essential for your health and support your digestion, metabolism and immune system.
Some ferment fibre from your diet to produce chemical messengers, such as short-chain fatty acids (SCFAs), which support your gut lining and interact with your immune system. The most common SCFAs are acetate, butyrate and propionate.
There's some evidence that SCFAs may also communicate with the brain, directly or indirectly. However, much of this research is in animals, and scientists are still working out whether it applies to humans.
Gut microbes can also produce neurotransmitters or their building blocks, including tryptophan (the precursor for serotonin), GABA and compounds related to dopamine. As with SCFAs, most of this research has been in animals so far.
4. The immune system
Your gut hosts a large concentration of immune cells. Together with your gut microbes and gut lining, they help protect you from infection.
The immune system in your gut interacts with the vagus nerve, enteric nervous system and endocrine system to convey messages to your brain.
Immune cells also produce chemical messengers, such as cytokines, that can be either pro-inflammatory or anti-inflammatory. Cytokines can carry signals to the brain and influence how brain cells function, including the brain’s own immune cells (microglia).
The gut microbiome and your mood
Animal studies point to significant links between the gut microbiome, brain development and behaviour.
For example, anxiety-like behaviour, stress hormone regulation and brain structure differ significantly between mice raised without a gut microbiome and normal mice.
Whether your gut bacteria affect your mood is harder to say, but studies with human volunteers are starting to emerge.
One large study of over 1,000 people found that having certain butyrate-producing bacteria was linked with better self-reported quality of life, and some of these bacteria were consistently depleted in people with depression.
A systematic review and meta-analysis also reported that dysbiosis, an imbalance in your gut microbes, is linked with severe mental health conditions and chronic fatigue.
Stress, digestion and IBS
Stress is a well-established factor in common digestive conditions. So, if stress seems to be playing havoc with your stomach, your gut-brain axis may be involved.
That’s because the HPA axis, which regulates your stress response, can change how sensitive your gut is and how food moves through it.
Disorders of the gut-brain interaction are conditions where gastrointestinal symptoms arise from a mix of disturbed gut motility, heightened sensitivity to normal gut sensations, altered gut microbes and changes in how the central nervous system processes signals.
Symptoms can include abdominal pain, bloating and changes in bowel habits, such as diarrhoea or constipation.
These disorders are very common, with research suggesting that around 40% of people have one. Irritable bowel syndrome (IBS) is the best-known disorder of the gut-brain interaction.
A 2025 systematic review and meta-analysis of 67 trials found that behavioural therapies targeting the gut-brain axis, including cognitive behavioural therapy, psychotherapy and hypnotherapy, showed reported improvement in IBS symptoms. However, the authors noted the evidence quality was low.
Probiotics, diet and ‘psychobiotics’
There are lots of marketing claims about probiotics and diet fixes for mood and mental health, but the evidence is mostly mixed or still emerging. So let’s examine what it actually says.
Can psychobiotics and probiotics improve mood?
Let’s start with the terminology. Probiotics are live microbes that can improve health when you consume them at an adequate dose. Prebiotics, such as fibre, are compounds that feed your gut microbes.
A psychobiotic is a probiotic or prebiotic intended to influence mood or mental health by acting on the gut-brain axis.
There's some evidence from systematic reviews and meta-analyses that psychobiotics can improve symptoms of depression and anxiety. But the trials varied widely in bacterial strains, doses and study duration.
So, if a product makes a claim, it’s important to look for evidence behind its specific strain and dose.
Which foods support the gut-brain axis?
The strongest, most consistent evidence is for overall dietary patterns, rather than individual foods.
A diverse dietary pattern rich in plant foods, fibre, whole grains, nuts, seeds and healthy fats (such as a Mediterranean-style diet) may support your gut microbiome and, in turn, your mood and mental health.
For example, in one small clinical trial, people with depression reported significant improvements after switching to a healthier diet. A 2025 systematic review of 25 clinical trials found that calorie restriction and low-fat dietary advice produced small reductions in depressive symptoms, but the overall evidence quality was low.
Does fibre help mental health?
Some observational research has found that people who eat more fibre are less likely to have depression, but this doesn't prove that fibre itself prevents it.
A study with postmenopausal women found that those with higher fibre intakes had better mental health-related quality of life scores.
But so far, these studies only show associations. How fibre might help mental health isn't clear and is still being actively investigated.
Can improving gut health help with brain fog?
There's no medical definition of ‘brain fog’. It describes cognitive symptoms, like memory and concentration problems, that some people experience during perimenopause and menopause, or with COVID-19, sleep problems or other health conditions.
What causes brain fog isn’t clear, but scientists have suggested that inflammation, gut-brain axis disruption and poor sleep may contribute. Some reports link a poor diet with brain fog, so researchers have speculated that improving diet quality could help.
But more research is needed to investigate any links between diet, gut health and brain fog.
Strong evidence vs weak evidence
If you’re wondering what evidence you can trust, here's a quick summary of what's well-supported and what isn't.
Well-supported insights: The gut-brain axis is a real thing, with plenty of good-quality evidence showing two-way communication between the gut and brain. It involves your nervous system (including the vagus nerve and enteric nervous system), hormonal signalling, your gut microbiome and your immune system. The link between stress and gut symptoms is well-established. For example, IBS is a disorder of gut-brain interaction, and there’s some evidence that therapies targeting the gut-brain axis can ease symptoms.
Emerging evidence: Psychobiotics and dietary changes may help with mood and mental health conditions. But there aren't any definitive studies yet, so it's unclear which probiotic strains, prebiotics or dietary patterns work best. Researchers are also exploring whether supporting gut health can help clear brain fog, but good-quality studies are lacking.
How to support your gut-brain axis
Your gut-brain axis lets different parts of your body communicate. It's important for your health and plays a part in several health conditions.
The best way to support it is to build evidence-based habits for your gut and mental health into your daily life. There's no evidence for 'quick fixes' or gut-brain 'resets'.
Based on the available evidence, habits to support your gut-brain axis include:
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Managing stress: There's a well-established link between stress and gut symptoms. Addressing your stress levels can take many forms, including journaling, therapy and self-care.
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Eating a diverse, fibre-rich diet: While the evidence on diet and mood is still emerging, eating plenty of different plants can support your gut health. Increase your fibre gradually, in line with what your gut tolerates.
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Focusing on sleep and movement: Good sleep and regular physical activity are both linked with a more diverse gut microbiome and better gut health.
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Getting professional care for your health concerns: Work with a healthcare professional who knows your full health picture. Research around probiotics and diet is emerging, but this shouldn’t be a substitute for evidence-based medical care for mental or physical health conditions.
Knowing how your gut and your brain communicate can help you understand how your body works. It can also help you make sense of the potential connection between stress and gut symptoms.
It also means you can look more critically at products that promise to 'fix your gut' or 'fix your mood' – and build habits that support your health.
Sources
Advances in understanding of cognitive symptoms during menopause. The Lancet Obstetrics, Gynaecology, and Women’s Health. (2026). Link.
A prospective analysis of dietary fiber intake and mental health quality of life in the Iowa Women’s Health Study. Maturitas. (2019). Link.
A randomised controlled trial of dietary improvement for adults with major depression (the ‘SMILES’ trial). BMC Medicine. (2017). Link.
Association of dietary fiber and depression symptom: A systematic review and meta-analysis of observational studies. Complementary Therapies in Medicine. (2021). Link.
Brain fog in menopause: A health-care professional’s guide for decision-making and counseling on cognition. Climacteric. (2022). Link.
Dietary fiber intake, depression, and anxiety: A systematic review and meta-analysis of epidemiologic studies. Nutritional Neuroscience. (2023). Link.
Disorders of gut–brain interaction and the Rome V process. Gastroenterology. (2026). Link.
Effects of prebiotics and probiotics on symptoms of depression and anxiety in clinically diagnosed samples: Systematic review and meta-analysis of randomized controlled trials. Nutrition Reviews. (2024). Link.
Efficacy of behavioural therapies for irritable bowel syndrome: A systematic review and network meta-analysis. The Lancet Gastroenterology and Hepatology. (2025). Link.
Feeding gut microbes to nourish the brain: Unravelling the diet–microbiota–gut–brain axis. Nature Metabolism. (2024). Link.
Gut dysbiosis in severe mental illness and chronic fatigue: A novel trans-diagnostic construct? A systematic review and meta-analysis. Molecular Psychiatry. (2021). Link.
Interaction of the vagus nerve and serotonin in the gut–brain axis. International Journal of Molecular Sciences. (2025). Link.
Moderate- to long-term effect of dietary interventions for depression and anxiety: A systematic review and meta-analysis. Annals of Internal Medicine. (2025). Link.
Nutritional approaches to managing brain fog: Insights into neuroinflammation, the gut-brain axis, and sleep. Current Nutrition Reports. (2026). Link.
Nutritional psychiatry: The present state of the evidence. Proceedings of the Nutrition Society. (2017). Link.
Psychobiotics and the manipulation of bacteria–gut–brain signals. Trends in Neurosciences. (2016). Link.
Short-chain fatty-acid-producing bacteria: Key components of the human gut microbiota. Nutrients. (2023). Link.
Stress & the gut-brain axis: Regulation by the microbiome. Neurobiology of Stress. (2017). Link.
The central signaling pathways related to metabolism-regulating hormones of the gut-brain axis: A review. Journal of Translational Medicine. (2025). Link.
The efficacy of probiotics, prebiotics, and synbiotics on anxiety, depression, and sleep: A systematic review and meta-analysis of randomized controlled trials. BMC Psychiatry. (2025). Link.
The gut microbiota-immune-brain axis: Therapeutic implications. Cell Reports Medicine. (2025). Link.
The microbiota-gut-brain axis. Physiological Reviews. (2019). Link.
The neuroactive potential of the human gut microbiota in quality of life and depression. Nature Microbiology. (2019). Link.
The role of short-chain fatty acids in microbiota–gut–brain communication. Nature Reviews Gastroenterology and Hepatology. (2019). Link.
Vagus nerve and gut-brain communication. The Neuroscientist. (2025). Link.
Worldwide prevalence and burden of functional gastrointestinal disorders, results of Rome Foundation global study. Gastroenterology. (2021). Link.
