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By: Lexi Rosmarin

We typically think of mental health as being just that – the intricacies of the mind and brain working to control our thoughts and feelings. However, our mental health consists of a multitude of hormones, neurons, and chemicals produced throughout our bodies. One particularly important contributor to the production of chemicals that are integral to our mental health is the gut. The gut, or gastrointestinal tract, is not only important in digestion and absorption of vitamins; its microbiome also produces 95% of the serotonin in the body, which is the chemical that most closely controls our mood.[1] 

Serotonin is one of the most important neurochemicals responsible for our mental health. SSRIs—Selective Serotonin Reuptake Inhibitors—are a common medication used to treat mood disorders, such as anxiety and depression. This medication works by rebalancing the amount of serotonin our body produces.[2] Given the importance of serotonin for our mental health, the gut must be healthy in order to produce the right amounts of serotonin for our bodies. In the other direction, chemicals produced in the brain may also affect the gut, possibly causing issues like Irritable Bowel Syndrome (IBS).[3] 

It is important to understand the role that the microbiome of the gut—the good and bad bacteria that make up our intestine—plays in the health of multiple bodily systems. The microbiome of the gut is developed in infancy and early childhood. The amount of bacteria that an infant is exposed to during birth as well as the nutrition of a young child contribute to the microbiome of their gut into adulthood. Diet impacts the microbiome during all periods of life, but the bacteria that an infant is exposed to during birth is critical to the development of a healthy gut and healthy immune system.[4]

The gut affects the brain and the brain affects the gut. These organs interact through the brain-gut axis that allows communication through neurons and neurochemicals. Researchers find that an issue in the brain, gut, or anywhere in between can cause difficulty to the efficiency and efficacy of the pathway to communicate and allow the chemicals it produces to do their job.[5] The essential mechanism of the gut-brain axis is two-way communication between the microbiome in the gut and the Central Nervous System (CNS) in the brain. This happens through multiple pathways in the body, including the immune, autonomic, and neuroendocrine pathways.[6] 

Photo Credit: Halacious, Unsplash (https://unsplash.com/photos/OgvqXGL7XO4)

These pathways make up the “axis” part of the gut-brain axis. Any disturbance in the microbiome of the gut, any of these pathways, or in the CNS in the brain can affect the ability of neurochemicals and neurotransmitters to communicate their messages properly. For example, if the gut does not contain a healthy microbiome, there may be dysregulation in the production of important chemicals, such as serotonin. This may lead to psychiatric conditions that are thought to stem from an imbalance of serotonin, like anxiety and depression.[7]

The gut microbiome in adulthood is influenced by many factors, including personal health and the environment.[8] When the gut microbiome undergoes changes—which can happen through diet, chronic stress, immune system shifts, food, and hormone imbalances—it does not produce chemicals properly, possibly leading to physical issues like IBS in addition to mental issues.[9] IBS is thought of as a stress-related disease; moreover, imbalances in the gut microbiome are thought to be the main contributors to the symptoms caused by IBS.[10] Since stress can cause dysfunction of the important pathways related to the gut-brain axis, researchers are investigating the potential causality of the gut-brain axis in its connection between stress and the gut microbiome in causing IBS symptoms.[11]

While most current studies on the gut-brain access are on animals, they provide compelling evidence that similar processes happen in humans. One of the first but most influential studies into the mind-gut connection finds that giving mice a probiotic reduces behaviors related to anxiety and depression, leading to evidence that a healthier gut microbiome may lead to better mental health.[12] Limited human trials with probiotics yield similar results; in as many as 10 clinical trials, the introduction of multiple specifically selected probiotics leads to a decrease in depressive symptoms and at times decrease in anxiety symptoms.[13] Additionally, studies show that manipulating the gut microbiome through the use of probiotics and/or antibiotics leads to a decrease in IBS symptoms.[14] This demonstrates the impact that gut healthiness, and potentially the gut-brain axis, may have on mental health and physical health.

The gut-brain access holds an important avenue for future research regarding a wide range of disorders and treatments. If research finds this axis to be a prominent contributor to mental health issues, it may be the new target of therapies to ease mental health issues in addition to SSRIs. This may be appealing to people because, for one, SSRIs often have many side effects and can be nonspecific to the disorder they are treating.[15] Additionally, those who are drawn to homeopathic methods of treating mental health disorders may be interested in therapies that target this axis because newer possible treatments, like specific probiotics, are potentially more specific and less invasive through natural methods of healing the gut microbiome with hopes of rebalancing levels of serotonin. 

Also, given the typical lack of treatment for IBS, discoveries regarding the role of the gut microbiome and the gut-mind axis will improve many patients’ lives. Overall, the gut-brain axis is an exciting research avenue into the pathology of mental illnesses and other disorders. The possibility of new therapeutic pathways is a step towards improved treatment of mental and physical health issues.

References:

[1] Carpenter, S. (2012, September). That gut feeling. Monitor on Psychology, 43(8). http://www.apa.org/monitor/2012/09/gut-feeling

[2] Ferguson J. M. (2001). SSRI Antidepressant Medications: Adverse Effects and Tolerability. Primary care companion to the Journal of clinical psychiatry, 3(1), 22–27. https://doi.org/10.4088/pcc.v03n0105

[3] ​​Martin, C. R., Osadchiy, V., Kalani, A., & Mayer, E. A. (2018). The Brain-Gut-Microbiome Axis. Cellular and molecular gastroenterology and hepatology, 6(2), 133–148. https://doi.org/10.1016/j.jcmgh.2018.04.003

[4] 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. https://doi.org/10.4081/cp.2017.987

[5] Martin, C. R., Osadchiy, V., Kalani, A., & Mayer, E. A. (2018). The Brain-Gut-Microbiome Axis. Cellular and molecular gastroenterology and hepatology, 6(2), 133–148. https://doi.org/10.1016/j.jcmgh.2018.04.003

[6] Riedera, R., Wisniewski, P. J., Alderman, B. L., & Campbell, S. C. (2017). Microbes and Mental Health: A Review. Brain, Behavior, and Immunity, 66, 9-17. https://doi.org/10.1016/j.bbi.2017.01.016

[7] Riedera, R., Wisniewski, P. J., Alderman, B. L., & Campbell, S. C. (2017). Microbes and Mental Health: A Review. Brain, Behavior, and Immunity, 66, 9-17. https://doi.org/10.1016/j.bbi.2017.01.016

[8] 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. https://doi.org/10.4081/cp.2017.987

[9] 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. https://doi.org/10.4081/cp.2017.987

[10] Kennedy, P. J., Cryan, J. F., Dinan, T. G., & Clarke, G. (2014). Irritable bowel syndrome: a microbiome-gut-brain axis disorder?. World journal of gastroenterology, 20(39), 14105–14125. https://doi.org/10.3748/wjg.v20.i39.14105

[11] Kennedy, P. J., Cryan, J. F., Dinan, T. G., & Clarke, G. (2014). Irritable bowel syndrome: a microbiome-gut-brain axis disorder?. World journal of gastroenterology, 20(39), 14105–14125. https://doi.org/10.3748/wjg.v20.i39.14105

[12] Bray, N. (2019). The microbiota-gut-brain axis. Nature. https://www.nature.com/articles/d42859-019-00021-3

[13] Appleton J. (2018). The Gut-Brain Axis: Influence of Microbiota on Mood and Mental Health. Integrative medicine (Encinitas, Calif.), 17(4), 28–32. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6469458/ 

[14] Kennedy, P. J., Cryan, J. F., Dinan, T. G., & Clarke, G. (2014). Irritable bowel syndrome: a microbiome-gut-brain axis disorder?. World journal of gastroenterology, 20(39), 14105–14125. https://doi.org/10.3748/wjg.v20.i39.14105

[15] Ferguson J. M. (2001). SSRI Antidepressant Medications: Adverse Effects and Tolerability. Primary care companion to the Journal of clinical psychiatry, 3(1), 22–27. https://doi.org/10.4088/pcc.v03n0105

Photo credit for featured image:
Kira auf der Heide, https://unsplash.com/photos/_Zd6COnH5E8