What Is the Gut–Brain Axis?

What Is the Gut–Brain Axis?

Your gut and brain are in constant communication. Signals travel in both directions through the nervous system, immune system, hormones and substances produced by microorganisms living in the gut.

This network of communication is known as the gut–brain axis. It helps the brain receive information about what is happening in the digestive system, while signals from the brain can also influence how the gut functions.

The gut–brain axis is a complex biological system, and researchers are still learning how all of its different parts work together.

How Do the Gut and Brain Communicate?

There isn't one single pathway connecting the gut and brain. Instead, several systems work together.

The nervous system

The gut contains an extensive network of nerve cells called the enteric nervous system. It helps coordinate processes such as movement and digestion.

The gut also communicates with the brain through nerves, including the vagus nerve, which carries signals in both directions.

This means that communication isn't simply a message travelling from the gut to the brain. The brain can also send signals back to the digestive system.

Immune and hormonal pathways

The immune system is another important part of the conversation.

The digestive tract contains a large proportion of the body's immune tissue. Cells within the gut can respond to what is happening in the intestinal environment and release signalling molecules that influence other parts of the body.

Hormones and other chemical messengers also contribute to communication between the gut and brain. Together, these neural, immune and hormonal pathways allow information about the body's internal environment to travel in both directions.

Where Does the Gut Microbiome Fit In?

Your digestive tract is home to a complex community of microorganisms, including bacteria, fungi and other microbes. Together, these communities make up the gut microbiome.

These microorganisms interact with the food we eat and with the cells lining the digestive tract. Their activity can produce a variety of substances that interact with the gut and other biological systems.

Some of these substances are being investigated as possible signals involved in gut–brain communication.

But this is an area where it is important to separate what we know from what we are still discovering. Researchers have identified many potential mechanisms, but the importance of individual microbes or microbial changes in human health is not yet fully understood.

Diet and microbial activity

What Happens When We Eat?

What we eat can influence the environment in which gut microorganisms live.

Dietary fibre is particularly interesting because some types of fibre are fermented by gut microbes. During this process, microorganisms produce substances known as metabolites, including short-chain fatty acids such as acetate, propionate and butyrate.

These compounds can interact with cells in the gut and immune system, and researchers are investigating how they may contribute to communication between the gut and brain.

This does not mean that one food or nutrient can directly change your mood or brain function. The biology is much more complicated than that. Instead, diet is one of many factors that can influence the gut environment and the wider gut–brain system.

Stress and the gut

What About Stress?

The relationship between stress and the gut works in both directions.

When we experience psychological stress, the nervous and hormonal systems involved in the body's stress response can affect digestion, gut movement and sensitivity.

At the same time, signals arising from the gut can interact with systems involved in the body's response to stress.

Researchers are also studying relationships between the gut microbiome and mental wellbeing. Some findings are interesting and promising, but associations do not necessarily mean that one causes the other. Factors such as diet, medication, lifestyle, sleep and stress can influence both the microbiome and our health, making cause and effect difficult to determine.

What do we actually know?

What Does the Research Actually Show?

We know that communication between the gut and brain is real and involves several interconnected biological pathways. We also know that gut microorganisms interact with their environment and produce substances that can influence cells and biological processes.

What remains less certain is how much individual differences in the microbiome contribute to particular health outcomes in people. Much of the research into detailed mechanisms comes from laboratory and animal studies. Human research is growing, but translating findings from experimental models into meaningful effects in people is not always straightforward.

This is why claims about the microbiome and mental health need to be treated carefully.

Key takeaways

The Gut–Brain Connection in a Nutshell

The gut–brain axis is a two-way communication network linking the digestive system and brain.

It involves:

• The nervous system, including the vagus nerve and enteric nervous system

• Immune and hormonal signalling

• The gut microbiome and substances produced by microbial activity

• Signals influenced by what happens inside the digestive tract

The science is developing rapidly. Some aspects of gut–brain communication are well established, while other areas remain the subject of active research. The most useful way to think about the gut–brain axis is not as a single pathway or a simple explanation for health and wellbeing, but as a complex network of biological communication that researchers are continuing to understand.

Sources and further reading...

1. Ohara TE, Hsiao EY. Microbiota–neuroepithelial signalling across the gut–brain axis. Nature Reviews Microbiology, 2025.

2. Aburto MR, Cryan JF. Gastrointestinal and brain barriers: unlocking gates of communication across the microbiota–gut–brain axis. Nature Reviews Gastroenterology & Hepatology, 2024.

3. Dong TS et al. Advances in Brain-Gut-Microbiome Interactions: A Comprehensive Update on Signaling Mechanisms, Disorders, and Therapeutic Implications. Cell Mol Gastroenterology and Hepatology, 2024.

4. Nature Mental Health. Distinguishing the causative, correlative and bidirectional roles of the gut microbiota in mental health, 2025.