Fig. 3

Gut microbiota effects on mood disturbance, fatigue, insomnia and risk of depression during exercise. The putative mechanisms by which bacteria connects with the brain and influence behavior during exercise include bacterial subproducts that gain access to the brain via the bloodstream and the area postrema, via cytokine release from mucosal immune cells, via the release of gut hormones such as 5-hydroxytryptamine (5-HT) from enteroendocrine cells, or via afferent neural pathways, including the vagus nerve. Stress during intense period of training and competitions can influence the microbial composition of the gut through the release of stress hormones or sympathetic neurotransmitters that influence gut physiology and alter the habitat of the microbiota (reviewed by Mach [23]). Alternatively, host stress hormones such as noradrenaline might influence bacterial gene expression or signaling between bacteria, and this might change the microbial composition and activity of the microbiota.