How smart is your gut? The gut-brain axis explained

Author: Dr Alena Pribyl

30 October 2019 Education Latest Science
Gut health brain health

Did you know the microbes in your gut may be influencing your brain?

Over the last decade we’ve been learning about how the community of microorganisms that live in our gut, called the gut microbiome, can influence our health. One of the most intriguing aspects of this exciting new research is how the microorganisms in our gut may be influencing our brain.

Gut-brain communication

Scientists have known for a long while that our brain communicates with our gut and vice versa. One of the main routes for this communication is a long nerve that runs the length of our body and connects our bodily organs (including our gut) directly to our brain1. This is called the vagus nerve and acts as a superhighway for the brain to send signals to the gut and for the gut to send signals to the brain. When we feel stressed or anxious, we often end up with an upset stomach because of the signals our brain has sent to our gut. Likewise, the vagus nerve senses what is happening in our gut and sends signals to our brain. This results in the brain generating a response to the signals, such as stopping us from eating because we are full1.

Until recently, scientists believed this gut-brain communication only involved signals derived from our body. However, over the last decade increasing evidence has come to light that the microorganisms in our gut are also involved in this communication between the gut and the brain.

gut microbiome test

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Our gut microbes talk to our brain

We now know that substances produced by our gut bacteria can influence how our gut talks to our brain. For example, when our gut bacteria produce the beneficial short chain fatty acids (SCFAs) butyrate and propionate, these SCFAs stimulate our gut cells to produce appetite supressing hormones2 and serotonin3, both of which can activate the vagal nerve endings in our gut to send signals to our brain4. There is also evidence that some species of gut bacteria can directly produce the neurotransmitters dopamine, noradrenaline, serotonin, and GABA5, although we are still in the early days of understanding how or if these bacterially-produced chemicals influence the human body.

As research progresses in this area, scientists are discovering more connections between the gut microbiome and our brain health.

Research in mouse models has indicated that the gut microbiome is likely involved in brain development, social behaviour, cognition and neuroinflammation6–8. In humans, studies have observed differences in the gut microbiome of healthy people compared to people with neurodegenerative diseases9 and mental disorders. When scientists have transplanted the stool of humans with major depressive disorder into mice, the mice started demonstrating depression-like behaviours11.

Diet can influence gut-brain communication

Intriguingly, research is also indicating that the primary substances produced by bacteria when we eat fibre, the SCFAs butyrate, propionate and acetate, are likely playing an important role in mediating gut-brain communication12.  For example, mice that were treated with SCFAs prior to being subjected to long-term stress had decreased depressive/anxious behaviour and an improved intestinal barrier compared to mice who did not receive SCFAs13. Another study demonstrated that a high-fibre diet led to increased production of SCFAs and reduced neuroinflammation in ageing mice14. These effects on the brain still need to be confirmed in humans, but we do know that SCFAs play many important roles in the gut as well, such as maintaining the intestinal cell barrier, suppressing inflammation, regulating appetite, and promoting the production of serotonin15.

A good strategy for improving both brain and gut health is to make sure your diet has plenty of diverse sources of fibre such as wholegrains, fruits, vegetables, legumes, nuts and seeds.

Although we are only at the start of our journey in deciphering how the gut microbiome is influencing our brain, the early findings indicate this is an area that holds a lot of promise for the development of future ways to treat neurodegenerative diseases and mental disorders.

Taking a stool sample to understand important functions of the gut microbiome.

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