b'THE ATHLETE MICROBIOTA Early research suggests that the gut microbiota of physically active individuals contains a greater abundance of beneficial bacterial species and greater bacterial diversity 4 . More specifically, lean body mass and VOhave been positively 2maxcorrelated with microbial diversity and metabolic function, and greater populations of bacteria which can modulate mucosal immunity, improve gut barrier function, and synthesise short-chain fatty acids (SCFAs). Other factors common amongst athletes, such as fibre intake, diversity of dietary plant intake, time spent outdoors, and possibly other genetic factors are likely to also contribute to this beneficial microbial community 4 . A single bout of exercise may also transiently modify the intestinal microbiota 4 . Certain metabolites produced during exercise, such as lactate, can be resorbed by the gut and selectively used as an energy source by certain bacterial species 5 . Excessive exercise, however, may have a deleterious effect on the gut. Prolonged, strenuous exercise can perturb gastrointestinal (GI) function and integrity, and may induce unfavourable shifts in the microbiota composition 6 . Recent research has highlighted the important role of the gut microbiota in mediating athlete health and performance via the microbiota-immune, the gut-brain and the gut-lung axes.THE GUT-BRAIN AXISThe gut-brain axis refers to the bi-directional communication MICROBIOTA-IMMUNE AXIS that occurs between the gut and the brain 9 . Most of the Our gut microbiota plays a fundamental role in theinformation (~80%) relayed between these organs is fed from priming and functioning of our immune system 7 . Overthe gut to the brain. There are many lines of communication, half of all immunologically active cells in the body sitincluding neural innervation, inflammatory and immune below the mucous layer of the gut, in the epithelium,responses, enteroendocrine signalling and production of known as gut-associated lymphoid tissue (GALT).microbial metabolites. The most direct connection between GALT processes and responds to antigens from foodthe gut and the brain is a neural innervation via the vagus and our gut microbes in a host-microbe exchangenerve. The gut is also innervated by the enteric nervous based on signalling molecules, but does not launchsystem (ENS), which controls many autonomic functions of an inflammatory response. The microbiota supportsthe gut including contractions, secretions, and blood flow. The the development and function of innate and adaptiveENS is capable of functioning independently of the CNS and is immune cell function, including the balance betweentherefore often termed the second brain. pro-inflammatory and anti-inflammatory cytokines.The health of our GIT and microbiome can affect The tolerogenic interaction between the host andneurocognitive function and vice versa. The influence of the microbiota helps to keep the immune system alert. Asgut-brain axis on cognitive functions and psychological effects, such, we tolerate our commensal bacteria day-to-day,includes altering responses to stress, changing behaviour but when threatened by pathogenic agents, our bodystates and has been used as a target for interventions in launches a coordinated immune response to identifymental health conditions. Mental illnesses and neurological and remove the threat. Those with a suppresseddiseases have been closely related to dysbiosis in the gut immune system are at risk of pathogens overridingmicrobiota. Contrastingly, dietary interventions with effects these defences and contracting an infection moreon the gut microbiota diversity have been related to cognitive often. Conversely, an overactive immune systemand psychological benefits. The interactions in the gut-brain can also cause damage, as is seen for those withaxis can be modulated by lifestyle factors such as diet, stress, autoimmune diseases 8 . sleep and medication. Exercise is associated with greater diversity in the gut microbiota and moderate exercise can reduce stress 4 .2'