The Gut-Brain Connection: How The Gut and The Brain Talk to Each Other

Updated: a day ago

This is the second article in the Mental Health Nutrition Series. The first article can be found here.


Have you experienced nausea or cramping during an anxious moment? Or felt butterflies in your stomach?


You can attribute that to the gut-brain connection through the body’s nervous system, also known as the gut-brain axis. The neurons in our gut are connected through nerves in our nervous system, including the vagus nerve — which is the longest cranial nerve that sends signals back and forth between the gut and the brain.


Signalling molecules created by gut microbes and cells of your intestine send messages to your brain along the gut-brain axis, influencing your mental and emotional functions. Signals also travel in the opposite direction, such that our mental and emotional states affect our gut function (1-2).

Bidirectional communication network between the CNS, enteric nervous system in the gut and gut microbes.


Why Is Gut Health Important In Mental Health?


Your gastrointestinal (GI) tract does more than move food from one end to the other. It is responsible for absorbing the nutrients your organs — including the brain — need to function properly. To do these important jobs, your gut relies on healthy intestinal cells and beneficial bacteria, which help manufacture vitamins, absorb minerals, and digest food.


The human gut is lined with more than 100 million nerve cells—it’s practically a brain unto itself.


Given the fascinating connections between the gut and brain, it should come as no surprise that the foods we eat ultimately impact our mental health! In fact, our gut is where 75% of our immune system resides (3) and where over 90% of your serotonin (4), the primary neurotransmitter responsible for your mood, is produced.


Gut dysbiosis (imbalances) and inflammation of the gut have been linked to causing several mental illnesses including anxiety and depression, which are prevalent in society today (5).


Meet the Microbiome


Our gut bacteria (microbiome) is a diverse population of microbes that live in the gastrointestinal (GI) tract which plays an important role in the health of your gut, and in other aspects of your physical health, from inflammatory skin disorders to obesity. Researchers now say that this role of promoting good health may extend to include the health of your brain and neurological systems.


The organisms that exist within our gut are responsible for bodily functions including immune health, detoxification, nutrient absorption, and neurotransmitter and vitamin production.


How Does The Gut Bacteria Interact With The Gut-Brain Axis?


The microbes of the gut microbiota interact with the gut-brain axis through the following pathways:


The Vagus Nerve


The vagus nerve is the major pathway that connects the brain to the gut. It runs from the brain, through the lungs, heart, spleen, liver, kidneys, and down to the intestines. It establishes one of the connections between the brain and the gastrointestinal tract and sends information about the state of the inner organs to the brain via afferent fibres (6).


Neuroendocrine (gut hormone) signalling


This is where neurons in the gut release gastrointestinal hormones such as peptide YY, neuropeptide Y (NPY) and cholecystokinin. These neuropeptides then enter the bloodstream and/or directly influence the enteric nervous system (7).


Interference with Serotonin Production


Gut microbiota plays an important role in the production of serotonin as approximately 90% of serotonin (the happy hormone) is produced in the gut (7).


Low-Grade Inflammation = Altered Intestinal Permeability


When the human microbiome is challenged with changes in diet and stress, the physiology of the normal microbiome undergoes change. This change or imbalance in the gut bacteria is called dysbiosis. Dysbiosis promotes gut inflammation and can lead to increased intestinal permeability. Intestinal permeability allows contents such as bacterial metabolites and molecules as well as bacteria themselves to pass through the submucosa (gut lining) and into the systemic circulation (8, 9).


Increased intestinal permeability leads to detrimental effects on the host immune system, which have been demonstrated in diseases such as inflammatory bowel disease (IBD), diabetes, asthma, and psychiatric disorders including depression, anxiety, and autism (10, 11, 12).


Production of Microbial Metabolites


The bacteria in the gut produce gamma-aminobutyric acid (GABA), which is the main inhibitory neurotransmitter in the brain (13) thus reduces the activity of neurons in the brain and central nervous system, which in turn increases relaxation, reduces stress and balances mood. Bacteria also produce short-chain fatty acid (SCFAs), such as butyric acid that stimulates the sympathetic nervous system, serotonin release, and thus influence the memory and learning process in the brain (13).


Conclusion


The bidirectional link between the brain, gut, and microbiome has come to the forefront of the medical research community in the past few years. The growing amount of evidence substantiating this link indicates it will be a valuable area for future medical and nutritional practice, and research. This article demonstrates the importance of a healthy microbiome, particularly the gut microbiota, as dysbiosis and inflammation in the central nervous system have been linked as potential causes of mental illness.


References:


1) Appleton J. (2018). The Gut-Brain Axis: Influence of Microbiota on Mood and Mental Health. Integrative medicine (Encinitas, Calif.), 17(4), 28–32.


2) Cryan JF et al. (2019). The Microbiota-Gut-Brain Axis. Physiol Rev. 1;99(4):1877-2013.


3) Wu, H. J., & Wu, E. (2012). The role of gut microbiota in immune homeostasis and autoimmunity. Gut microbes, 3(1), 4–14.


4) Thomas C. Fung, Helen E. Vuong, Cristopher D. G. Luna et al. (2019). Intestinal serotonin and fluoxetine exposure modulate bacterial colonization in the gut. Nature Microbiology.


5) Clapp, M., Aurora, N., Herrera, L., Bhatia, M., Wilen, E., & Wakefield, S. (2017). Gut microbiota's effect on mental health: The gut-brain axis. Clinics and practice, 7(4), 987.


6) Breit, S., Kupferberg, A., Rogler, G., & Hasler, G. (2018). Vagus Nerve as Modulator of the Brain-Gut Axis in Psychiatric and Inflammatory Disorders. Frontiers in psychiatry, 9, 44. https://doi.org/10.3389/fpsyt.2018.00044


7) 18) Hirokazu Fukui, Xin Xu and Hiroto Miwa. Role of Gut Microbiota-Gut Hormone Axis in the Pathophysiology of Functional Gastrointestinal Disorders. Journal of Neurogastroenterology and Motility 2018; 24(3): 367-386 https://doi.org/10.5056/jnm1807119


8) Zoppi S, Madrigal JL, Pérez-Nievas BG, Marín-Jiménez I, Caso JR, Alou L, García-Bueno B, Colón A, Manzanares J, Gómez-Lus ML, Menchén L, Leza JC. Endogenous cannabinoid system regulates intestinal barrier function in vivo through cannabinoid type 1 receptor activation. Am J Physiol Gastrointest Liver Physiol. 2012 Mar 1; 302(5):G565-71.


9) Kelly, J. R., Kennedy, P. J., Cryan, J. F., Dinan, T. G., Clarke, G., & Hyland, N. P. (2015). Breaking down the barriers: the gut microbiome, intestinal permeability and stress-related psychiatric disorders. Frontiers in cellular neuroscience, 9, 392. https://doi.org/10.3389/fncel.2015.00392


10) Carabotti M, Scirocco A, Maselli MA, Severi C. The gut-brain axis: interactions between enteric microbiota, central and enteric nervous systems. Ann Gastroenterol. 2015 Apr-Jun; 28(2):203-209.


11) Bischoff SC, Barbara G, Buurman W, Ockhuizen T, Schulzke JD, Serino M, Tilg H, Watson A, Wells JM. Intestinal permeability--a new target for disease prevention and therapy. BMC Gastroenterol. 2014 Nov 18; 14():189.


12) Foster JA, McVey Neufeld KA. Gut-brain axis: how the microbiome influences anxiety and depression. Trends Neurosci. 2013 May; 36(5):305-12.


13) Amar Sarkar, Soili M. Lehto, Siobhán Harty, Timothy G. Dinan, John F. Cryan, Philip W.J. Burnet. (2016). Psychobiotics and the Manipulation of Bacteria–Gut–Brain Signals. Volume 39, issue 11, p763-781.




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