The gut-brain connection

For many years, the gut and brain were viewed as separate systems. It was thought the gut was responsible for digestion and the brain was responsible for emotions.

Today, we know this isn’t the case at all.

In fact, the gut and the brain have a very close relationship and are in constant communication with one another – called the ‘gut-brain connection’.

An unhappy gut will send signals to the brain letting it know, and an unhappy brain will send signals to the gut letting it know. It’s a two-way street. For example, gut symptoms can be caused by emotions (low mood, anxiety, stress) or be the cause of emotions (low mood, anxiety, stress).

So how exactly do they communicate? It’s complex, but they talk using four key channels, including:

1. The Nervous System 
2. The Hormonal System 
3. The Gut Microbiome 
4. The Immune System

Let’s discuss each one in a bit more detail.

1. The Nervous System

A big player of the nervous system involved in the gut-brain connection is the vagus nerve. Running from the gut to the brain, it physically connects the two organs and is sometimes called the ‘information superhighway’.

The vagus nerve contains millions of nerve fibres (the long spindly bits on nerve cells that carry messages), but only 10-20% of nerve fibres run ‘top-down’ (from the brain to the gut). The other 80-90% of nerve fibres run ‘bottom-up’ (from gut to the brain).

In other words, there is more communication going up to the brain from the gut than down to the gut from the brain, suggesting the brain is more of a ‘receiver’ rather than a ‘giver’ of information.

What’s more, the gut is the only organ in the body with its own complex network of nerves, known as the enteric nervous system (or ‘ENS’). Embedded within the gut wall, the ENS regulates gut function – such as motility and secretions - with the support from serotonin, a key neurotransmitter. It can operate independently and why the gut is often labelled the ‘second brain’.

2. The Hormonal System

Cells that line the gut wall - known as enteroendocrine cells – also serve as a key link between the gut and the brain. For example, they produce neurotransmitters, including serotonin.

Outside the gut, serotonin is widely recognised for its influence on regulating how you feel and behave – although scientists are still trying to figure out how gut-based serotonin might affect the brain.

But enteroendocrine cells are not the only producers of neurotransmitters. Gut microbes can also produce neurotransmitters, including dopamine and gamma-aminobutyric acid (GABA).

Dopamine is known for its role in motivation, memory, and attention, whereas GABA is thought to have calming, stress-reducing effects.

3. The Gut Microbiome

Beyond neurotransmitters, gut microbes can produce many other compounds that can impact the brain. Some of these compounds can act on nerves in the ENS and the vagus nerve to transmit signals to the brain. Others may enter the bloodstream and reach the brain that way.

One example is short chain fatty acids (SCFAs). They have several important gut-focused roles including supporting the gut barrier, providing gut cells with energy, and preventing growth of potentially harmful microbes.

But some SCFAs can enter the bloodstream too. Once they’ve travelled to the brain, it’s thought they might cross the blood brain barrier and alter brain structure and function.

4. The Immune System

Around 70% of the immune system is housed along the gut wall in what’s called gut-associated lymphoid tissue (or ‘GALT’ for short).

Because the gut is constantly exposed to a wide range of potentially harmful threats from our food and the environment, this strategic placement keeps immune cells on standby and ready to jump into action, when needed.

Immune cells can pump out chemical messengers, such as cytokines, that enter the bloodstream. These small proteins have anti-inflammatory or pro-inflammatory effects and can travel to the brain where they can alter brain function and behaviour.

Supporting the gut-brain connection

Supporting the gut-brain connection begins with core health habits. For example:

• Eating plenty of fibre feeds your gut microbiome and indirectly benefits brain health.
• Spending time outdoors, especially in blue or green spaces, is linked with a more diverse gut microbiome and mental health benefits.
• Managing chronic stress helps maintain the gut barrier, limits inflammation, and helps prevent gut symptoms.
• Prioritising sleep promotes gut microbiome diversity and enhances immune defence.  

Sources

Crosstalk between intestinal serotonergic system and pattern recognition receptors on the microbiota–gut–brain axis. Frontiers. (2021). Link.

Gut-brain axis: mechanisms and potential therapeutic strategies for ischemic stroke through immune functions. Frontiers. (2023). Link.

Human-derived bifidobacterium dentium modulates the mammalian serotonergic system and gut–brain axis. Cellular and Molecular Gastroenterology and Hepatology. (2020). Link.

Short chain fatty acids: Microbial metabolites for gut-brain axis signalling. Molecular and Cellular Endocrinology. (2022). Link.

The blood–brain barrier. Perspectives in Biology. (2015). Link.

Vagus nerve as modulator of the brain–gut axis in psychiatric and inflammatory disorders. Frontiers in Psychiatry. (2018). Link.