For a long time, the gut and brain were considered separate entities. It was thought the gut was responsible for digestion and the brain was responsible for emotions.
Fast forward to the present day and 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
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), 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.
Also, the gut is the only organ in the body with its own complex network of nerves called the enteric nervous system (or ‘ENS’). The ENS is embedded into the gut wall and coordinates gut functions, including motility and secretions. It’s also capable of working independently and why the gut is often labelled the ‘second brain’.
2. The Hormonal System
With help from gut microbes, certain cells that line the gut (called enteroendocrine cells) can produce neurotransmitters (chemical messengers). One key neurotransmitter is serotonin.
Like the ENS, serotonin coordinates gut functions, including motility and secretions. Beyond the gut, 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.
95% of serotonin is produced in the gut. The other 5% in produced in the brain. Both ‘types’ of serotonin are the same – they are just produced in different places.
Since gut-based serotonin cannot cross the blood brain barrier, scientists are still trying to figure out how gut-based serotonin might affect the brain. Emerging studies in mice have shown that increased serotonin in the gut can influence the expression of serotonin receptors in the brain.
3. The Gut Microbiome
Gut microbes can produce thousands of different compounds.
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.
Beyond the gut, SCFAs are thought to play a key role in the gut-brain connection by altering levels of gut hormones (such as glucagon-like peptide-1, peptide YY) and neurotransmitters (e.g. serotonin, dopamine, gamma-aminobutyric acid).
Some SCFAs can enter the bloodstream. 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
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 bloodstream. For example, cytokines are small proteins with anti-inflammatory or pro-inflammatory effects. They can travel to the brain and alter brain function and behaviour.
With that in mind, here are some ways to support the gut-brain connection:
• Focus on fibre
• Get outside
• Manage stress
• Prioritise sleep
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.
