I know you’re probably tired of hearing it but it would be doing a disservice to not mention the significant impact stress has on the gut and the gut has on stress. Have you ever been so excited and had literal butterflies in your stomach or so nervous that you felt nauseous having to run to the bathroom? It’s not a coincidence, your gut is directly connected to your brain via a nerve called the vagus nerve [4,8].
The vagus nerve (VN) is part of a system called the autonomic nervous system. The functions of the autonomic nervous system occur automatically without any thought or effort on your end such as breathing, your heart beating, brain functioning and gut motility. For example, when you exercise your heart rate rises and when you are done your vagus nerve is the communicator of your autonomic nervous system that brings your heart rate back to resting without any conscious effort on your end. The connection between the gut and the brain via the vagus nerve is bi-directional meaning your gut can send messages to your brain and your brain can send messages to your gut [4,8,11]. The exact mechanism by how these messages are sent is not fully understood but, for simplicity, it has been determined the bacteria present in the gut are responsible for the initiation and types of messages sent [7]. When the brain receives these messages, it sorts them to the areas responsible for action, most notably in response to stress and inflammation the messaging is transferred to a region of your brain called the hypothalamus [10]. The primary function of the hypothalamus is to maintain homeostasis throughout your body by controlling appetite, temperature regulation, energy requirements, memory and stress among many other functions [11]. In doing so, it is part of an axis called the HPA axis, or the hypothalamic-pituitary-adrenal axis. An axis indicates these three areas are in constant communication and feedback with one another. A primary function of the pituitary gland is to secrete Thyroid Stimulating Hormone (TSH) which controls the function of the thyroid gland as well as secretion of Luteinizing Hormone (LH) and Follicle Stimulating Hormone (FSH), two hormones essential for ovulation and regulation of progesterone and estrogen. The pituitary gland also stimulates the adrenal gland which primarily responds to stressors by secreting glucocorticoids such as cortisol, the stress hormone, but is also involved in the production of androgens such as testosterone [12]. Figure 2 represents the pathways of the HPA axis from the hypothalamus to the adrenal gland as well as the branching stimuli from the pituitary gland to the ovaries. Repeated exposure to stressful circumstances for the body leads to dysregulation of this axis. Stressors are not always obvious such as work and home life but also, as mentioned previously, leaking of unwanted particles through the intestinal wall can stimulate a stress response and over time can contribute to further dysregulation of this axis. Furthermore, a stress response can be initiated by inflammatory messaging directly from the bacteria present in the gut leading to suboptimal production of thyroid hormone, reproductive hormones and chronic exposure to cortisol which influences a myriad of chronic metabolic and endocrine conditions by its impact on glucose regulation, immune function and fat metabolism [4,10,12]. Your body has mechanisms in place to attempt to regulate this axis despite stressors but after years of attempting to maintain homeostasis eventually your body will reach a place of fatigue and this is typically when clinical symptoms will begin to appear.
Are you still a little skeptical of how much power these bacteria hold over your health?
Hundreds of studies have looked at the mechanism and role by which the bacteria in your gut can manifest chronic conditions and stress. One in particular looked specifically at infants in the first month of life when gut bacteria are still developing and found significant positive correlations between ‘good’ gut bacteria and an increase in the ability of the body to react to an increase in cortisol levels or stress [12]. Additionally, clinical trials ingesting pathogens and probiotics into an individual were found to induce activation of the central nervous system proving the direct effect of bacteria on the brain [7]. Further studies have performed Fecal Microbiota Transplants (FMT) to show when the fecal matter of an individual with chronic disease, in particular hypothyroidism, anxiety, and depression, was replaced with fecal matter containing known good gut bacteria, the chronic diseases resolved or significantly reduced the need for medication [1,4,5,7,10].
Rest assured, you probably do not need a Fecal Microbiota Transplant to cure or re-develop your gut bacteria; there are many other interventions to implement before reaching such drastic measures. But remember, your gut microbiome has been developing since before you were born so only implementing a probiotic to your regimen is not going to be the golden ticket for your health. Sometimes it is necessary to dig deeper and work with a functional nutritionist who can provide functional lab testing such as the GI-MAP. GI-MAP can gain insight to your gut bacteria by identifying pathogens, viruses, parasites or other ‘bad’ bacteria growth that may be interfering with your ‘good’ bacteria and your ability to regulate homeostasis in your body.
The takeaway is this: nothing in your body functions independently. Every single cell, organ and hormone present is in constant communication with another in an attempt to maintain homeostasis and create a safe space for your body to function optimally. With your brain and gut being at the center of all of this communication, it would be impossible to resolve any hormone concerns without first addressing the health of your second brain: your gut.
GLOSSARY:
Adrenal Gland: Two glands located on top of the kidneys that produce hormones such as Testosterone and Cortisol that help regulate blood pressure, immune system, metabolism and response to stress
Autonomic Nervous System: A system in your body that regulates involuntary processes such as heart rate, blood pressure, respiration, digestion and sexual arousal
Dysbiosis: A state of being of the presence of more ‘bad’ bacteria than ‘good’ bacteria
Gut Microbiome: The environment of the gut that hosts trillions of bacteria responsible for digestion, absorption of nutrients and immune function
Homeostasis: A state of equilibrium and balance in the body between physiological processes
HPA Axis: The hypothalamic-pituitary-adrenal axis is the linkage of these three areas of the body that secrete hormones to provide direct communication with each other whether to stimulate or suppress
Hypothalamus: A region of the brain responsible for maintaining homeostasis by temperature regulation, appetite control, energy requirements, memory and stress
Permeability: The state or quality of the membrane that allows liquids or gasses to pass through it
Pituitary:A gland located in the brain responsible for regulation of hormone secretions such as Thyroid Stimulating Hormone, Luteinizing Hormone, Follicle Stimulating Hormone and Adrenocorticotropic Hormone which stimulates the Adrenal Gland
Postbiotics: The end product of prebiotics and probiotics; created by digestion and contribute to gut health
Prebiotics: Compounds in food, mostly fiber sources from carbohydrates, that provide the gut microbiome with fuel for growth and development
Probiotics: Live microorganisms in the gut that can improve or restore gut flora: ‘good’ bacteria
Short-chain fatty acids: Postbiotics that send messaging to other areas of the body involved in hormone production and play a vital role in intestinal wall protection and integrity
Vagus Nerve: The main nerve in the parasympathetic nervous system that controls involuntary functions such as digestion, heart rate and immune system
References
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2. Xu F, Fu Y, Sun T-yu, et al. The interplay between host genetics and the gut microbiome reveals common and distinct microbiome features for complex human diseases. Microbiome. 2020;8(1). doi:10.1186/s40168-020-00923-9
3. Lingaiah S, Arffman RK, Morin-Papunen L, Tapanainen JS, Piltonen T. Markers of gastrointestinal permeability and dysbiosis in premenopausal women with PCOS: A case–control study. BMJ Open. 2021;11(7). doi:10.1136/bmjopen-2020-045324
4. Breit S, Kupferberg A, Rogler G, Hasler G. Vagus nerve as modulator of the brain–gut axis in psychiatric and inflammatory disorders. Frontiers in Psychiatry. 2018;9. doi:10.3389/fpsyt.2018.00044
5. Groot H, Van De Vegte YJ, Verweij N, Lipsic E, Karper JC, Van Der Harst P. Human genetic determinants of the gut microbiome and their associations with health and disease: A phenome-wide association study. European Heart Journal. 2020;41(Supplement_2). doi:10.1093/ehjci/ehaa946.3642
6. Nie X, Xie R, Tuo B. Effects of estrogen on the gastrointestinal tract. Digestive Diseases and Sciences. 2018;63(3):583-596. doi:10.1007/s10620-018-4939-1
7. Silva YP, Bernardi A, Frozza RL. The role of short-chain fatty acids from gut microbiota in gut-brain communication. Frontiers in Endocrinology. 2020;11. doi:10.3389/fendo.2020.00025
8. Baker JM, Al-Nakkash L, Herbst-Kralovetz MM. Estrogen–gut microbiome axis: Physiological and clinical implications. Maturitas. 2017;103:45-53. doi:10.1016/j.maturitas.2017.06.025 https://www.frontiersin.org/files/Articles/508738/fendo-11-00025-HTML/image_m/fendo-11-00025-g001.jpg
9. Jiang Y, Greenwood-Van Meerveld B, Johnson AC, Travagli RA. Role of estrogen and stress on the brain-gut axis. American Journal of Physiology-Gastrointestinal and Liver Physiology. 2019;317(2). doi:10.1152/ajpgi.00144.2019
10. Marcondes Ávila PR, Fiorot M, Michels M, et al. Effects of microbiota transplantation and the role of the vagus nerve in gut–brain axis in animals subjected to chronic mild stress. Journal of Affective Disorders. 2020;277:410-416. doi:10.1016/j.jad.2020.08.013
11. Anatomy and function of the hypothalamus. IntechOpen. https://www.intechopen.com/chapters/63258. Accessed August 14, 2022.
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