GUT BRAIN AXIS Gut-brain axis is a real phenomenon, describing a two-way communication between the central nervous system (CNS) and the enteric nervous system (ENS). "It links emotional and cognitive centres of the brain with intestinal functions, " A key player within this communication system is the gastrointestinal tract, home to trillions of microbes. A person's diet has a significant impact on these microbes. Fibre, for example, is fermented by gut bacteria, which produce metabolites called short-chain fatty acids. These include butyrate, propionate, and acetate. According to a 2020 study, published in the journal Molecular and Cellular Neurosciences, short-chain fatty acids have the power to cross the blood-brain barrier, where they can impact brain structure and function. The immune system is part of the gut-brain axis and is an important signalling cascade from microbes to the brain, " said Foster. An imbalance in gut bacteria — known as dysbiosis — causes the barrier between the gut and the bloodstream to become permeable. This can allow "bad" bacteria to enter the bloodstream, potentially causing inflammation. A 2020 review, published in the journal Frontiers in Immunology, suggests that dysbiosis also alters the blood-brain barrier, contributing to inflammation of the brain matter. Inflammatory pathways have been linked to neuroinflammatory conditions including multiple sclerosis, Alzheimer’s and Parkinson’s diseases, as well as anxiety and depressive-like disorders. The human gut contains nearly 500 million neurons which are connected to the brain through nerves. The vagus nerve is one of the biggest nerves connecting the gastrointestinal tract to the nervous system and plays many important roles in the body. Psychological stress, for example, may have a particularly harmful effect on the vagus nerve, finding it may be involved in the development of gastrointestinal disorders like irritable bowel syndrome and inflammatory bowel disease. The gut and the brain also communicate through chemicals called neurotransmitters. Some of these neurotransmitters are produced in the brain and are involved in regulating emotions, mood and the "fight or flight" response. They can also be produced in the gut, affecting aspects of the digestive. Neurotransmitters including norepinephrine, epinephrine, dopamine, and serotonin are able to regulate and control blood flow and affect bowel movements, nutrient absorption and the composition of the microbiome.