Written by Amos Darius ‘25
Edited by Helen Chow ‘26
Introduction
The saying, “always trust your gut” may be true in more ways than one. Growing evidence suggests that having a healthy gut microbiota may improve one’s mental health. The gut microbiota refers to the array of microorganisms such as fungi, viruses and bacteria that are present in a person's gastrointestinal tract [1]. The gut microbiota aids in many bodily functions such as fermentation and digestion of food to harness energy, protection against pathogens or disease producing organisms, assistance in immune response, and production of vitamins [1,2].. According to new research, disruption in gastrointestinal bacteria can be linked to asthma, obesity, eczema, cancer, diabetes, and even heart disease [1]. A significant aspect in the importance of the gut microbiota is the gut-brain axis or connection, which is the biochemical signaling that goes back and forth between the gastrointestinal tract and central nervous system [3]. It is this connection that draws the gut microbiota into the realm of mental health as its influence goes beyond hunger satiation, food sensitivities, and metabolism regulation and also impacts behavior, stress levels, pain sensitivity, and cognitive function [3]. A balanced diet is important for the health of the gut microbiota as certain foods or diets may positively or negatively impact the microbiome, thus potentially impacting mental health. Many claim that certain diets such as the keto diet may be especially helpful for improving mental health.
Anatomical Rundown
For the anatomy of the gut brain-axis, the key aspects are the enteric nervous system and vagus nerve. The enteric nervous system is part of one's autonomic nervous system. Often referred to as the “second brain” due to the large number of neurons, this neural network helps with digestive function and also collects and processes information about the gastrointestinal tract without sending it to the brain [3]. When it comes time to send information to the brain, something that’s done constantly at speeds from 70 to 120 m/s, it is done through the vagus nerve which connects the enteric nervous system to the organ. As the longest of the 12 cranial nerves, the vagus nerve helps with reflexive actions such as coughing and sneezing, in addition to digestion [4].
Anxiety and Depression
One of the most prominent mental health areas that the gut microbiota has been shown to have ties to is anxiety and depression. According to a research study, a sample of people with anxiety were found to have a lower Simpon’s diversity, which is a measure of species richness and evenness [5]. A pattern of less microbiota diversity and evenness in people with anxiety has been found in multiple studies. In one study mice with more anxiety had notably less levels of the bacteria Firmicutes. Another study found that patients with inflammatory bowel diseases that also had anxiety were seen to have less bacterial diversity and less Prevotella_9 and Lachnospira than those without anxiety [5]. Similar results were found with patients with depression in one study in which people who had depressive episodes had less alpha diversity [5]. It is known that having less species diversity in the gut puts one more at risk for diseases that have to do with the digestive system such as celiac disease, inflammatory bowel disease, and irritable bowel syndrome. However these connections to anxiety and depression also signify that the gut’s connection to mental health requires further exploration.
Ketogenic Diet and Bipolar Disorder
Like anxiety and depression, bipolar disorder may also have a connection to the gut microbiota. However with BPD, more attention has been placed on how certain foods impact the gut microbiome and how that in turn may affect mental health. A man named Iain Cambell had bipolar disorder and started the ketogenic or “keto” diet to combat weight gain from psychiatric medications [6]. The keto diet consists of high fat and low carbs, raising one's ketone levels. A few weeks after the diet Cambell experienced a sense of mental clarity and happiness that he claims he hadn’t had since childhood [6]. Many people online shared similar results from the diet, and it also sparked interest in medical professions. An observational study by British researchers found that in people with bipolar disorder, 85.5% had mood stabilization after starting the ketogenic diet and 41.2% had fewer depressive episodes [7]. While the origins of the keto diet are thought to be tied to treatment of epilepsy dating back a hundred years, people are still unsure about its connection to other mental health issues including bipolar disorder. Studies like the ones performed by the British researchers are impactful; however, there is still a lack of rigorous clinical evidence and difficulty due to the multifactorial nature of BPD when accounting for genetics. Some hypothesize the keto diet may strengthen the gut microbiome due to its anti-inflammatory effects. Others hypothesize that the links to both the gut microbiota and ketones are due to their roles in energy production and that perhaps problems with one’s mitochondria may be the cause of many mental health issues [6]. However, these theories have yet to be proven.
Conclusion
Connections have also been made between the gut microbiota and several other mental disorders such as schizophrenia, autism spectrum disorder, post traumatic stress disorder, attention deficit hyperactivity disorder, and anorexia [5]. Even Alzheimer's disease, which is a neurological disorder, has been shown to have a connection to the metabolic tract as it’s believed that anti-inflammatory foods and bacteria can help promote brain health [8]. As for possible treatment to mental health diapers through the metabolic tract, the ketogenic diet, although promising, is still not researched enough to be considered a medically approved treatment for mental disorders. The diet also has some health risks such as possibly leading to low blood sugar, low blood pressure, kidney stones, and increased heart disease risk [9]. Research on the connection between the gut and mental health continues through the growing field of metabolic psychiatry developed by Dr. Shebani Seth, a psychiatrist at Stanford University [6]. As research continues to be undertaken, we have learned that a healthy gut also has many other benefits outside of mental health such as strengthening the immune system and improving sleep quality [10]. Therefore, doctors recommend a diet of fiber rich foods such as vegetables and nuts and limiting processed foods.
References
MacGill, M. (2023, February 15). What are the gut microbiota and human microbiome?https://www.medicalnewstoday.com/articles/307998#the-gut-microbiota
Hou, K., Wu, Z., Chen, X. Y., Wang, J., Zhang, D., Xiao, C., Zhu, D., Koya, J., Wei, L., Li, J., & Chen, Z. (2022, April 23). Microbiota in health and diseases. Signal Transduction and Targeted Therapy. https://doi.org/10.1038/s41392-022-00974-4
Professional, C. C. M. (n.d.). The Gut-Brain Connection. Cleveland Clinic. https://my.clevelandclinic.org/health/body/the-gut-brain-connection
Seladi-Schulman, J. (2023, February 14). What is the Vagus Nerve? Healthline. https://www.healthline.com/human-body-maps/vagus-nerve#anatomy-and-function
Xiong, R.-G., Li, J., Cheng, J., Zhou, D.-D., Wu, S.-X., Huang, S.-Y., Saimaiti, A., Yang, Z.-J., Gan, R.-Y., & Li, H.-B. (2023, July 23). The role of gut microbiota in anxiety, depression, and other mental disorders as well as the protective effects of dietary components. Nutrients. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10384867/
Stone, W. (2024, January 27). Patients say keto helps with their mental illness. science is racing to understand why. NPR. https://www.npr.org/sections/health-shots/2024/01/27/1227062470/keto-ketogenic-diet-mental-illness-bipolar-depression#:~:text=All%20Things%20Considered-,Keto%20diet’s%20new%20frontier%3A%20Bipolar%20disorder%2C%20depression%20and%20other%20mental,And%20research%20is%20taking%20off
Image Citation: Needham Gastroenterology Associates. (2024, April 9). Tilting on the Gut-Brain axis. https://www.needhamgastro.com/articles/tilting-on-the-gut-brain-axis
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