Common sense tells us that the mind and gut have a close relationship. Your mind knows when it is time to eat, when you are full, when you have abdominal cramps, and when you need to visit a bathroom. This is all made possible by the enteric nervous system, a vast, semi-autonomous web of nerve cells that line the digestive system and communicate with your brain. While the vagus nerve may be the direct connection between the mind and gut that allows them to talk to each other, it is the diverse ecosystem of bacteria in your gut that do much of the talking.
The study of how the microbiome affects the brain began by accident. Psychiatric researchers noticed that individuals with psychological and neurological conditions had a significantly higher likelihood of gastrointestinal issues than the average person. This led to a number of clinical studies which determined that between 50 and 90% of individuals with irritable bowel syndrome also have a mental health condition, including anxiety, depression, and cognitive decline. However, these findings beg the question, do brain disorders and psychiatric issues cause GI problems, or is it the other way around?
Examples of mind to gut transmission are widespread, including the well-documented phenomenon of a "nervous stomach," in which a sudden, overwhelming stressful situation can lead to crippling stomach cramps and an immediate urge to move the bowels. However, it was not until 2011 that evidence of gut to mind transmission emerged. In a highly influential study, scientists identified for the first time that induced gastrointestinal irritation in healthy rats led to long-lasting depression and anxiety-like behaviors, as well as heightened sensitivity to stress. Since this breakthrough discovery, interest in the gut-brain axis and how the microbiome factors into the equation has exploded, and the findings are clear. According stated by Glenn J. Treisman, MD, of the Johns Hopkins School of Medicine, "The take-home message in everything we study is the arrow goes both ways... The microbiome affects your gut, which affects your brain. The brain affects your gut, which affects your microbiome."
A scientific review published in 2020 found conclusive evidence that gut bacteria release metabolites and other molecules that can trigger inflammation in the central nervous system, directly linking the gut biome with brain disorders including depression, anxiety, Alzheimer's disease, and Parkinson's disease. Moreover, the authors singled out the gut-blood barrier and changes to its permeability as one of the biggest contributors to neurological dysfunction. It is for this reason that maintaining a healthy gut microbiome is so essential, not just for your digestive system but also your mental health.
