The gut and the brain have a very intimate relationship and are in constant communication with one another – called the ‘gut-brain axis’. 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 the result of our emotions (low mood, anxiety, stress) or the cause of our emotions (low mood, anxiety, stress).
But how exactly do they communicate? It’s complex, but they talk using four key channels, including:
- The Nervous System
- The Hormonal System
- The Gut Microbiome
- The Immune System
1. The Nervous System
One big player of the nervous system is the vagus nerve. This nerve runs all the way from the gut to the brain 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). What’s interesting is that the number of nerve fibres running ‘top-down’ (brain to the gut) accounts for 10-20%.
On the other hand, the nerve fibres running ‘bottom-up’ (gut to the brain) accounts for 80-90%.
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, (called the Enteric Nervous System or ‘ENS’) that work independently (hence why the gut is often labelled the ‘second brain’).
The ENS is embedded into the gut wall and coordinates gut functions including motility and secretions.
2. The Hormonal System
Some cells that line the gut can produce neurotransmitters (chemical messengers). One key neurotransmitter is serotonin.
Serotonin has a reputation for regulating how we feel and behave. In fact, it’s considered to be a natural mood stabiliser when functioning as it should.
Interestingly, 95% of serotonin is produced in the gut (5% in the brain). Both ‘types’ of serotonin are the same – they are just produced in different places.
Serotonin in the gut can act on nerves on the ENS and the vagus nerve to transmit signals to the brain.
3. The Gut Microbiome
We know the gut microbiome is a hub of activity, with the ability to release thousands of different chemicals.
Some of these chemicals can act on nerves in the ENS and the vagus nerve to transmit signals to the brain. Others may enter the blood circulation and reach the brain that way.
Short chain fatty acids (SCFAs) are key chemicals produced by your microbes – they have several important gut focused roles including supporting the gut barrier, providing gut cells with energy, and preventing growth of potentially harmful microbes.
Beyond this, SFCAs are thought to play a key role in the gut-brain axis by altering levels of gut hormones (e.g. glucagon-like peptide-1, peptide YY) and neurotransmitters (e.g. serotonin, dopamine, gamma-aminobutyric acid).
It’s possible that SCFAs may cross the blood brain barrier (which serves as the main ‘gatekeeper’ of the brain) to alter brain function. In fact, SCFAs have been implicated in the development of brain disorders.
4. The Immune System
The majority of our immune system (70%) sits within the gut wall (called gut-associated lymphoid tissue or ‘GALT’ for short).
This is because the gut is constantly exposed to a wide range of potentially harmful threats from our food and the environment. These immune cells are primed ready to jump into action, if needed.
Immune cells can pump out chemical messengers which can enter the blood stream and make their way to the brain. For example, cytokines (small proteins) can act on nerves in the ENS and vagus nerve. They can also enter the bloodstream, reach the brain and affect its behaviour that way.
Keeping your gut-brain happy
Focus on nourishing your gut microbes with the things they enjoy. Include a wide variety of plant-based foods such as wholegrains, fruit, vegetables (fresh and frozen count), legumes, nuts, seeds, herbs and spices.
But it’s not just what you eat.
Getting outside in nature by going for a country walk, keeping active such as going for a swim at your local leisure centre or doing a YouTube workout session, de-stressing with some relaxing activities such as reading a book or listening to a podcast, and prioritising sleep, are all ways to support gut and brain health.