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What is the Gut Brain Axis?

Contributed by Cecilia Jennings

What is the Gut Brain Axis?

Ever gone with your ‘gut feeling’? Well, with good reason; it is backed by science!

What is the Gut-Brain Axis?

The gut-brain axis speaks of the bi-directional (two-way) communication between the enteric nervous system of the gastrointestinal tract (aka the gut) and the central nervous system (aka the brain) (1).

Anatomically, the two are linked by the vagus nerve. But beyond just an anatomical link, this communication extends to have widespread physiological (physical) and psychological (mental) effects throughout the body and taps into everyday functions such as:

Endocrine (Hormone) System

Hunger and Satiety (Fullness) Cues. Our gut hormones (PYY, GLP-1 & grehlin) influence appetite through modulating “appetite centres” in the brain (2). Being in tune with hunger and fullness cues help to regulate eating behaviours, through intuitive eating (3). Good gut health will ensure these hormones are able to function optimally. We explore the different types of hunger on our blog.

Cognitive Function & Mental Health

Intestinal microbiota (gut bacteria) influence the relationship between the gut and the brain, with research finding that fluctuating levels of bacteria are correlated with changes in emotion regulation and mental state (4). In other words, what we eat can impact our mood and emotions!

Nervous System

Our brain can influence our bowel movements! Ever felt nervous or stressed and needed to use the loo? Had butterflies in your stomach? That's the brain and gut having a conversation via the vagus nerve. Stress and anxiety can impact the ability of the gut to function optimally (5). More and more people are experiencing digestive issues and Irritable Bowel Syndrome (IBS) symptoms such as diarrhoea and bloating from stress and anxiety (not from the food they eat which is what people first think of).

Immune System

70% of the immune system resides in the gut (6) and the microbiota metabolise (break down) certain foods to produce short-chain fatty acids, which support healthy function and immune defences (7).

Increasingly, the term ‘microbiota-gut-brain’ axis is being used, which recognises the microbiota (the trillions of microorganisms within our gastrointestinal tract) as being a key mediator and regulator of gut-brain communication.

This two-way communication is such that the brain can influence intestinal activity and immune cell function, whilst an increasing body of evidence finds that the gut can impact our mood, cognition and mental health!

Gut Health, Cognitive Function & Mental Health

The microbiota produce approximately 90% of serotonin, the happy hormone, used by the body. Dysbiosis, which is an imbalance in the ratio of good:bad gut bacteria, has been identified as a likely cause of - or contributing factor to - mental illness, such as anxiety and depression (8). Dysbiosis leads to increased intestinal permeability, which permit bacteria and inflammatory cytokines (molecules) to escape into the body’s circulation (9). It is thought that these inflammatory cytokines travel to the brain where they cause harmful effects, although this mechanism of action is not yet fully understood (10).

Nonetheless, in a recent study, levels of inflammation in participants with depression were significantly higher as compared to those without depression (11). Therefore, it seems that optimal gut health – that is, a healthy ratio of good:bad gut bacteria – can certainly support neurological function with regards to cognitions (thoughts) and mental wellbeing.

Can diet be a tool to improve our Mental Health?

The SMILES trial recruited participants with major depression and randomly assigned them to either receive 3-month dietary support from a dietitian, who guided the individual to consume a Mediterranean diet or to receive counselling support for 3 months (12). The Mediterranean diet is rich in wholegrains, beans, pulses and lentils, nuts and seeds, fruits and vegetables, healthy fats and fish. It also advises limiting intake of processed foods. An impressive 32.3% of the dietary intervention group achieved full remission of their clinical depression, compared to 8% in the counselling group!

Depression rates have also been seen to decrease upon probiotic supplementation (13,14). That said, probiotics are certainly not a cure-all solution; whilst they offer a targeted dose of live ‘good’ bacteria which may be of use to specifically replenish low levels of bacteria after a course of antibiotic treatment, for example, equally important and beneficial is consuming a wide variety of natural ‘good’ bacteria through fermented foods in the diet.

Although the SMILES trial did not address whether this improvement in mood works through improved gut health, a convincing body of research is growing in support of the Mediterranean-style diet in activating improvements in gut health (15,16).

Five Ways to Improve Gut Health and support your Mental Health

1. Nutrition

  • Variety is key! Aim to include 30 different plants per week for a range of micronutrients and fibre. This includes fruit, vegetables, nuts, seeds, herbs, spices, pulses, wholegrains, beans and legumes

  • Include various sources of prebiotic foods: the indigestible fibres which specifically feed your gut bacteria. Think onions, garlic and asparagus

  • Include various sources of probiotic foods. These contain live ‘good’ bacteria. Experiment with kefir, kimchi, kombucha and sauerkraut

  • Consider taking a probiotic supplement upon advice of a Dietitian or Registered Nutritionist

2. Stay hydrated - aim to drink between 1.5 – 2 litres of water daily. NB. increase this up to 2.5 litres when in the heat

3. Daily movement such as walking, yoga, dancing, barre, cycling

4. Prioritise good quality sleep, aiming for between 7-9 hours. Limit blue light exposure two hours before bed

5. Minimise stress through breathing exercises, meditation, journaling

It is now well-established that supporting our gut health improves overall bodily health and functioning, through the communication of the gut-brain axis. As regards our mental health, eating well certainly will not go amiss, and a compelling body of research is indeed growing to showcase the role of good nutrition in assisting cognitive function… Good food, good mood. You can read more in our Good Food, Good Mood blog.

We would love to support you at the TC Nutrition virtual clinic with private nutrition consultations. We specialise in helping clients heal their relationship with food and find balance with eating. Get in touch to book your free 15 minute discovery call to see how we can support you.

Cecilia Jennings

Guest Contributor

Follow Cecilia on instagram @_cecilias_table for more nutrition tips and recipe inspiration.



[1] Appleton J. The Gut-Brain Axis: Influence of Microbiota on Mood and Mental Health. Integr Med (Encinitas) [Internet]. 2018 Aug [cited 2021 Jun 2];17(4):28–32. Available from:

[2] Sun L-J, Li J-N, Nie Y-Z. Gut hormones in microbiota-gut-brain cross-talk. Chin Med J (Engl) [Internet]. 2020 Apr 5 [cited 2021 Jun 6];133(7):826–33. Available from:

[3] Moran GW, Thapaliya G. The Gut–Brain Axis and Its Role in Controlling Eating Behavior in Intestinal Inflammation. Nutrients [Internet]. 2021 Mar [cited 2021 Jun 2];13(3):981. Available from:

[4] Carabotti M, Scirocco A, Maselli MA, Severi C. The gut-brain axis: interactions between enteric microbiota, central and enteric nervous systems. Ann Gastroenterol [Internet]. 2015 [cited 2021 Jun 2];28(2):203–9. Available from:

[5] Mayer EA, Savidge T, Shulman RJ. Brain-gut microbiome interactions and functional bowel disorders. Gastroenterology. 2014 May;146(6):1500–12.

[6] Wu H-J, Wu E. The role of gut microbiota in immune homeostasis and autoimmunity. Gut Microbes [Internet]. 2012 Jan 1 [cited 2021 Jun 2];3(1):4–14. Available from:

[7] Silva YP, Bernardi A, Frozza RL. The Role of Short-Chain Fatty Acids From Gut Microbiota in Gut-Brain Communication. Front Endocrinol [Internet]. 2020 [cited 2020 Dec 31];11. Available from:

[8] Clapp M, Aurora N, Herrera L, Bhatia M, Wilen E, Wakefield S. Gut microbiota’s effect on mental health: The gut-brain axis. Clin Pract [Internet]. 2017 Sep 15 [cited 2021 Jun 2];7(4). Available from:

[9] Foster JA, McVey Neufeld K-A. Gut–brain axis: how the microbiome influences anxiety and depression. Trends in Neurosciences [Internet]. 2013 May 1 [cited 2021 Jun 2];36(5):305–12. Available from:

[10] Capuco A, Urits I, Hasoon J, Chun R, Gerald B, Wang JK, et al. Current Perspectives on Gut Microbiome Dysbiosis and Depression. Adv Ther [Internet]. 2020 [cited 2021 Jun 2];37(4):1328–46. Available from:

[11] Pitharouli MC, Hagenaars SP, Glanville KP, Coleman JRI, Hotopf M, Lewis CM, et al. Elevated C-Reactive Protein in Patients With Depression, Independent of Genetic, Health, and Psychosocial Factors: Results From the UK Biobank. AJP [Internet]. 2021 May 14 [cited 2021 Jun 6];appi.ajp.2020.20060947. Available from:

[12] Jacka FN, O’Neil A, Opie R, Itsiopoulos C, Cotton S, Mohebbi M, et al. A randomised controlled trial of dietary improvement for adults with major depression (the ‘SMILES’ trial). BMC Medicine [Internet]. 2017 Jan 30 [cited 2021 Jun 25];15(1):23. Available from:

[13] Steenbergen L, Sellaro R, van Hemert S, Bosch JA, Colzato LS. A randomized controlled trial to test the effect of multispecies probiotics on cognitive reactivity to sad mood. Brain Behav Immun. 2015 Aug;48:258–64.

[14] Akkasheh G, Kashani-Poor Z, Tajabadi-Ebrahimi M, Jafari P, Akbari H, Taghizadeh M, et al. Clinical and metabolic response to probiotic administration in patients with major depressive disorder: A randomized, double-blind, placebo-controlled trial. Nutrition. 2016 Mar;32(3):315–20.

[15] Nagpal R, Shively CA, Register TC, Craft S, Yadav H. Gut microbiome-Mediterranean diet interactions in improving host health. F1000Res [Internet]. 2019 May 21 [cited 2021 Jun 6];8. Available from:

[16] Ghosh TS, Rampelli S, Jeffery IB, Santoro A, Neto M, Capri M, et al. Mediterranean diet intervention alters the gut microbiome in older people reducing frailty and improving health status: the NU-AGE 1-year dietary intervention across five European countries. Gut [Internet]. 2020 Jul 1 [cited 2020 Dec 30];69(7):1218–28. Available from:


Talia Cecchele Nutrition is a team of registered dietitians that specialise in eating disorder recovery, disordered eating, digestive issues and sports nutrition. We aim to bring balance back to nutrition, help you to break free from food rules and find food freedom. We offer virtual consultations and group programs so whether you are based in London, the United Kingdom or around the world we would love to support you. To enquire about a private consultation please fill out a contact form.


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