Gut–brain axis
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The gut–brain axis is the two-way biochemical signaling that takes place between the gastrointestinal tract (GI tract) and the central nervous system (CNS). The term "microbiota–gut–brain axis" highlights the putative role of gut microbiota interacting with brain functions, according to preliminary research. Broadly defined, the gut–brain axis includes the central nervous system, neuroendocrine system, neuroimmune systems, the hypothalamic–pituitary–adrenal axis (HPA axis), sympathetic and parasympathetic arms of the autonomic nervous system, the enteric nervous system, vagus nerve, and the gut microbiota.
Chemicals released by the gut microbiome can influence brain development, starting from birth. A review from 2015 states that the gut microbiome influences the CNS by "regulating brain chemistry and influencing neuro-endocrine systems associated with stress response, anxiety and memory function".
Various factors influence the human gut microbiota composition, accounting for the vast variability found amongst humans. Factors such as diet, medication exposure such as antibiotics, genetics, and environment all affect the framework of the human gut microbiota. These differences across populations may affect how the gut-brain axis is interpreted and studied. The diversity in the gut microbiomes across individuals has led to inconsistencies in this field of research.
The bidirectional communication may involve immune, endocrine, humoral and neural connections between the gastrointestinal tract and the central nervous system. A 2019 review of laboratory research suggests that the gut microbiome may influence brain function by releasing chemical signals, possibly including cytokines, neurotransmitters, neuropeptides, chemokines, endocrine messengers and microbial metabolites, such as "short-chain fatty acids, branched chain amino acids, and peptidoglycans". These chemical signals are then transported to the brain via the blood, neuropod cells, nerves, endocrine cells, where they may impact different metabolic processes.
The first of the brain–gut interactions shown, was the cephalic phase of digestion, in the release of gastric and pancreatic secretions in response to sensory signals, such as the smell and sight of food. This was first demonstrated by Pavlov through Nobel prize winning research in 1904.
As of October 2016, most of the work done on the role of gut microbiota in the gut–brain axis had been conducted in animals, or on characterizing the various neuroactive compounds that gut microbiota can produce.