**2. Microbiota metabolites in health and disease**

Exercise or physical activity can greatly affect the composition of the gut microbiota. It improves several metabolic and inflammatory parameters in chronic diseases and has been used as a therapeutic strategy in chronic diseases. In this chapter, we summarize several experimental findings on the possible mechanisms by which physical activity could influence gut microbiota. We also discuss the health benefits of physical activity, probiotic consumption, and microbiota diversity. The modification in the composition and function of the gut microbiota has an impact on intestinal permeability, digestion, metabolism, and immune responses. Many diseases, from digestive to metabolic problems as well as immunological and neuropsychiatric disorders, are linked to the pro-inflammatory state caused by the alternation of gut microbiota balance [13]. The gut microbiome contributes to digestion and promotes food absorption for host energy production. Its fermentation of non-digestible dietary residues leads to metabolites such as short-chain fatty acids (SCFAs, like butyrate, acetate, and propionate), which modulate the host energy balance increasing the availability of nutrients [25]. Fermented SCFAs, secreted into the gut lumen, exceed the epithelial barrier and are released into the bloodstream. They can be used as energy sources by the intestinal microbiota and by the host cells. They could provide nearly 10% of our daily energy requirements [26]. Butyrate is used as an energy source primarily by

### *Regular Physical Activity Influences Gut Microbiota with Positive Health Effects DOI: http://dx.doi.org/10.5772/intechopen.110725*

epithelial cells in the colon. Propionate is involved in liver gluconeogenesis [27]. It also decreases serum cholesterol levels, inhibits fatty-acid synthesis, and may be involved in weight control by stimulating satiety [28]. Acetate is metabolized in muscle tissue and can also cross the blood-brain barrier. It is used as a substrate for liver cholesterol and fatty acid synthesis [29, 30], increases colonic blood flow and oxygen uptake, and enhances ileal motility by affecting ileal contractions [31].

SCFAs can also contribute to shaping the gut environment and colon physiology, participating in different host-signaling mechanisms as well as possessing some anti-inflammatory effects [32–34]. Butyrate regulates the neutrophil function and migration, increases the expression of tight junction proteins in colon epithelia, enhances gut integrity, and activates intraepithelial lymphocytes (IELs), which express cytokines (IFN-γ and keratinocyte growth factor) to protect epithelial cells from injury [35, 36].

Besides producing SCFAs, bacterial species of the gut microbiota synthesize glycan, amino acids, and vitamins (K, B12, biotin, folate, and thiamine) and participate in the digestion of polysaccharides, increasing the amount of glucose in the liver and, therefore, increasing lipogenesis [33, 37–39].

Protective functions of microbiota are performed also through competition with pathogens for nutrients and receptors and the production of antimicrobial molecules and metabolites to avoid colonization by pathogens [40]. Through ligands from commensal bacteria (lipopolysaccharide, LPS), the gut microbiota influences the mucosal immune system development and function [41].

Gastrointestinal mucosa is a complex system acting as a physical barrier that regulates epithelial permeability. The regulation of trans-epithelial permeability allows the absorption of nutrients from the intestinal lumen through the cells lining the gut wall into the blood circulation [42]. Gut bacteria-epithelial cell interactions have been suggested as key contributors to epithelial permeability. Dysregulation of the gut microbiota and disruption of the gut mucosa enable harmful substances to pass through the barrier and can lead to the development of several chronic diseases [13, 43]. Gut dysbiosis, characterized by an imbalance in the composition and activity of gut microbial communities, has been linked to functional and inflammatory disorders [44, 45].
