The Gut-Brain Axis
This post was co-written with The Mission Dietitian intern, Eimear Sutton.
The link between the gut and the brain has been hinted at for decades, through phrases such as ‘gut feeling’, ‘gut instinct’, and ‘butterflies in my stomach’. Despite this, it’s only in more recent years that scientists have begun to understand this link and how it works.
What is the gut-brain axis?
In its simplest form, the gut-brain axis is the bi-directional communication between the enteric nervous system (ENS), which is found in the lining of the gastrointestinal (GI) tract (aka ‘the gut’), and the central nervous system (CNS) (‘the brain’). A ‘bi-directional communication’ is a two-way communication, meaning that the gut communicates to the brain, and vice versa.
The complex communication between the gut and the brain is essential for our everyday functioning. For example, it helps us to recognise when we’re hungry. The gut-brain axis also plays a key role in monitoring the function of our GI tract in response to physiological and behavioural triggers. For example, when we feel stressed it influences our gut’s ability to function at its best.
The gut-brain axis is often described as a neuro-immuno-endocrine pathway, which means it involves the nervous, immune, and endocrine systems:
Nervous system
The main way that the gut and brain communicate is through the parasympathetic and sympathetic nervous systems, via the vagus nerve1.
Parasympathetic nervous system (PNS):
Called our “rest and digest” nervous system
Switched on when calm and relaxed
Sympathetic nervous system (SNS):
Called our “fight or flight” nervous system
Switched on in the presence of a stressor e.g. running from a wild animal
These neural systems play an important role in monitoring gut function and bowel movement. For example, when we're feeling stressed our PNS down-regulates, which reduces our guts’ ability to function. Meanwhile, our SNS up-regulates in an attempt to get us ready to fight or flight (blood moves away from the gut and to our peripheral muscles so that we can run away). This helps to explain why some people may find it more difficult to poop when they’re feeling stressed; their fight or flight system is overtaking their rest and digest system. Similarly, some people report more frequent or looser bowel movements when stressed.
Endocrine system
Our endocrine system is an intricate and closely controlled system of glands that produce hormones. Many of these hormones are secreted by glands in the GI tract and transported in the blood to the brain, and vice versa.
One of the main hormones secreted in the gut and transported to the brain is ‘Peptide YY’. This hormone is released after we’ve eaten to tell us that we’re full and we should stop eating2.
Immune system
Around 70% of our entire immune system is found in the GI tract3. It’s clear from this that gut health is an incredibly important part of immune health, but these immune cells also appear to play a role in the gut-brain axis.
On top of this, the gut microbiota is an essential aspect of the gut’s immune system, so is another key part of this puzzle.
Microbiota-gut-brain axis
The gut microbiota (GM) is:
A diverse array of microbes found in the large intestine
Mainly bacteria
Also yeasts, viruses, protozoa, archaea, and parasites
The GM has more recently surfaced as a potential part of the gut-brain axis, now referred to as the microbiota-gut-brain axis.
How does it work?
Research on this topic is fairly new and so the way the microbiota communicates with the brain isn’t entirely understood. However, one of the ways is through neurotransmitters and active metabolites produced by the GM, which communicate directly with the brain through the vagus nerve4.
For example, when we eat prebiotic fibre foods such as onions, artichokes, leeks, mango, beans or cashews, the GM ferment these foods and produce beneficial short-chain fatty acids, e.g., propionate, butyrate, and acetate.
Evidence from human and animal studies have shown that the GM can influence brain functioning and behaviour, and that life stressors can influence the GM1. For example, one study on mice found that short-term stress impacted the proportions and diversity of the main bacteria species in the GM5.
There have also been studies showing connections between GM composition and mental health, such as depression and anxiety6:
One study found that patients with major depressive disorder had significantly higher levels of specific bacterial strains in their intestines compared to healthy patients7.
Another study showed that transferring faeces from a depressed patient to a healthy rodent can induce depression-like behaviour8.
Whilst results still aren’t conclusive in humans and we haven’t proven that one of these directly causes the other, this communication could play a key role in GI conditions, like IBS, where gut symptoms are often accompanied by psychological symptoms, like stress or anxiety9.
So can we use diet to improve gut and mental health?
The SMILES trial10 is the first of its kind to investigate this question:
The SMILES trial was a 12 week randomised control trial that recruited people with major clinical depression. These people were then randomly assigned to one of two groups:
To get social support/counselling for 3 months
To get dietary support for 3 months. This consisted of seeing a clinical dietitian who worked with people to help them make gradual positive changes to their diet by increasing their intake of fruits & vegetables, wholegrains, beans, pulses & lentils, nuts & seeds, fish and healthy fats. Whilst also reducing their intake of processed and unhealthy foods.
The Results:
In the dietary group, 32.3% of people achieved full remission of their depression (meaning they were no longer depressed), compared to just 8% in the social support group. Also, the degree of dietary changes correlated with the degree of improvement in depression – meaning the more they changed their diet, the more they improved!
The results of this study are extremely influential for future research and clinical practice as they provide strong scientific evidence for the effect of diet on mental health, and the importance of the gut-brain axis!
Take home messages
The gut-brain axis is a complex bi-directional communication between the gastrointestinal tract, gut microbiota, and central nervous system.
Short-term exposure to stressors can alter the gut function and the gut microbiota community, while changes to microbiota diversity can influence brain function and behaviour.
We can utilise the connection between the brain and the gut to support both the management and treatment of mental disorders (such as depression) and functional gut disorders (such as IBS).
To best look after our gut health and mental health through diet, we should aim to eat a wide diversity of plant based foods. Not sure where to start? Check out Kaitlin’s ‘28 Day Challenge to Boost Gut Health’ and ‘Wellness Weeks’ over on her Instagram.
References
Wang HX and Wang YP. (2016) Gut Microbiota-brain Axis. Chin Med J; 5;129(19):2373-80. DOI: 10.4103/0366-6999.190667
Romijn JA, Corssmit EP, Havekes LM and Pijl H. (2008) Gut-brain axis. Curr Opin Clin Nutr Metab Care; 11(4):518-21. DOI: 10.1097/MCO.0b013e328302c9b0
Vighi G, Marcucci F, Sensi L, Di Cara G and Frati F. (2008) Allergy and the gastrointestinal system. Clin Exp Immunol; 153 Suppl 1:3-6. DOI: 10.1111/j.1365-2249.2008.03713.x
Farzi A, Fröhlich EE and Holzer P. (2018) Gut Microbiota and the Neuroendocrine System. Neurotherapeutics; 15(1):5-22. DOI: 10.1007/s13311-017-0600-5
Galley JD et al. (2014) Exposure to a social stressor disrupts the community structure of the colonic mucosa-associated microbiota. BMC Microbiol; 15;14:189. DOI: 10.1186/1471-2180-14-189
Arneth BM. (2018) Gut-brain axis biochemical signalling from the gastrointestinal tract to the central nervous system: gut dysbiosis and altered brain function. Postgrad Med J;94(1114):446-452. DOI: 10.1136/postgradmedj-2017-135424
Jiang H et al. (2015) Altered fecal microbiota composition in patients with major depressive disorder. Brain Behav Immun;48:186-94. DOI: 10.1016/j.bbi.2015.03.016.
Zheng P et al. (2016) Gut microbiome remodeling induces depressive-like behaviors through a pathway mediated by the host’s metabolism. Mol Psychiatry; 21, 786–796. DOI: 10.1038/mp.2016.44
Clapp M et al. (2017) Gut Microbiota’s Effect on Mental Health: The Gut-Brain Axis. Clin. Pract; 7, 131-136. DOI: 10.4081/cp.2017.987
Jacka FN et al. (2017) A randomised controlled trial of dietary improvement for adults with major depression (the ‘SMILES’ trial). BMC Med; 15, 23. DOI: 10.1186/s12916-017-0791-y