The ‘brain-gut axis’ refers to the bi-directional communication between the brain and the digestive system with recent research uncovering a fascinating complexity to this constant and lively communication between the two.
Let's take a look at the varied ways they convey information, and the connection between digestive disturbances and mood disorders such as depression, anxiety and brain fog.
The vagus nerve (cranial nerve X) is part of the parasympathetic (‘rest and digest’) nervous system, and is the opposite of the sympathetic nervous systems ‘flight or fight’ response. ‘Vagus’ is latin for ‘wandering’, and indeed this very long nerve (really two, one on each side of the body) wanders from the brainstem through the thorax, around the abdomen and through the digestive system. The vagus nerve picks up all kinds of information from organs such as the gut, liver, lungs and heart and sends it back to the brain and oversees many bodily functions including digestion, heart rate, immune response and mood! The vagus nerve can be damaged by viral infections, diabetes or abdominal surgery. Depending on where it is damaged, symptoms may include digestive upset, heart rate changes or depression (1).
Embedded within the digestive tract is the enteric nervous system (ENS). This so-called ‘second brain’ consists of over 100 million nerve cells and controls many functions of digestion. This complex neuronal web can actually function on its own, but can also be influenced by the central nervous system of the brain. The ENS works closely with hormones and the immune system to ensure digestion working seamlessly.
The connection between stress and the motility of the gut is known to most of us! It also results in damage to the gastrointestinal tract, systemic inflammation and changes to our brain and mood. Emotional stress triggers hormones that affect the hypothalamic-pituitary-adrenal (HPA) axis and the gut. Stress activates the hormone corticotropin-releasing hormone (CRH) from the hypothalamus, followed by adrenocorticotropic hormone (ACTH) from the pituitary, which then triggers the release of cortisol from the adrenals.
This compromises the integrity of ‘tight junctions’ in the intestinal epithelium, the biochemical and physical barrier to potential pathogens, and leads to a permeable ‘leaky gut’ which pathogens can now invade! This activates a strong response from the immune system, and inflammation of the gut (2). Impairment of the intestinal epithelium produces toxins that can change the microbiome, putting it in a state of imbalance, or dysbiosis.
Stress hormones also affect the ability of the gut to produce hydrochloric acid, bile and enzymes all necessary for good digestion, resulting in abdominal pain and bloating (3).
Collectively known as the gut microbiome are the trillions of bacteria, virus, yeast and fungi cells that inhabit our intestinal tract and they are directly communicating with the brain! We live in a symbiotic relationship with our microbiome: we feed them the food they want and they metabolise nutrients and synthesise vitamins such as vitamin K ( for building bones and blood clotting), B2, B9 and Vitamin B12 (needed for energy and metabolism).
Considering the constant interaction our gastrointestinal system has with the ‘outside world’ it is no surprise that 70% of our immune system is in the gut where it is in intricate interplay with the microbiome and the intestinal epithelial layer. A healthy microbiome full of beneficial bacteria ‘teaches’ the immune system to stay calm, however imbalances in the intestinal microbiome result in an array of bacteria, viruses and yeasts that produce toxins. The immune system responds with chemicals such as cytokines and chemokines that not only result in intestinal inflammation but cross over into the brain and can adversely affect emotion and mood resulting in anxiety, depression and brain fog (4).
Our gut microbes also produce hundreds of chemical messengers called neurotransmitters used by the body to regulate physiological processes but also mental processes such as memory and mood. The bacteria species of Lactobacillus and Bifidobacterium produce acetylcholine and gamma-amino butyrate (GABA) while Streptococcus and Escherichia can synthesise serotonin, dopamine and norepinephrine. These neurotransmitters are involved in the modulation of gastrointestinal motility and secretions and inflammation (5). In the brain, neurotransmitters such as serotonin and dopamine are also produced where they directly affect mood, learning and motivation!
Our gut microbiome is important for the production of short chain fatty acids (SCFA’s) such as butyric acid, propionic acid, and acetic acid from the fermentation of carbohydrates. SCFA are not only used by cells in the intestinal cell wall to keep the gut lining intact but also protect the brain from stress and inflammation (6).
We have long known that there is a strong correlation between our mental health and gut health, and new research is revealing the complexities of this communication between the brain and the gut. In my next post I will discuss how using a holistic approach with nutrition can support the health of not only the gut and its microbiome but have a positive effect on brain health as well.
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
Carabotti, M., Scirocco, A., Maselli, M. A., & Severi, C. (2015). The gut-brain axis: interactions between enteric microbiota, central and enteric nervous systems. Annals of gastroenterology, 28(2), 203–209.
Madison, A., & Kiecolt-Glaser, J. K. (2019). Stress, depression, diet, and the gut microbiota: human-bacteria interactions at the core of psychoneuroimmunology and nutrition. Current opinion in behavioral sciences, 28, 105–110. https://doi.org/10.1016/j.cobeha.2019.01.011
Wiertsema SP, van Bergenhenegouwen J, Garssen J, Knippels LMJ. The Interplay between the Gut Microbiome and the Immune System in the Context of Infectious Diseases throughout Life and the Role of Nutrition in Optimizing Treatment Strategies. Nutrients. 2021 Mar 9;13(3):886. doi: 10.3390/nu13030886. PMID: 33803407; PMCID: PMC8001875.
Bourassa MW, Alim I, Bultman SJ, Ratan RR. Butyrate, neuroepigenetics and the gut microbiome: Can a high fiber diet improve brain health? Neurosci Lett. 2016 Jun 20;625:56-63. doi: 10.1016/j.neulet.2016.02.009. Epub 2016 Feb 8. PMID: 26868600; PMCID: PMC4903954.
Tan J, McKenzie C, Potamitis M, Thorburn AN, Mackay CR, Macia L. The role of short-chain fatty acids in health and disease. Adv Immunol. 2014;121:91-119. doi: 10.1016/B978-0-12-800100-4.00003-9. PMID: 24388214.