## 8 Probiotics and Butyrate  
The human body contains a huge amount (10\({}^{14}\)) of microorganisms that all together constitute the microbiota. Above all the gastrointestinal tract is the most colonized, but every region of our body in contact with the external environment is actually rich of microorganisms. In humans the composition of microbiota strongly changes among individuals, depending on their health, age and local biogeography [112]. Independently on their composition, the importance of the intestinal microbiota resides in its numerous metabolic functions that lead to the production of nutrients, such as vitamins (e.g., folic acid, vitamin B12), amino acids and short-chain fatty acids (SCFA). Therefore, the intestinal microbiota is involved in physiological, metabolic, and nutritional processes with a further role in the development of the intestinal mucosal immunity [113]. As expected, the microbiota is sensitive to external and internal changes, whereby it undergoes relevant modifications (dysbiosis) in response to a high-fat diet [114], micronutrient deficiency [115], chronic inflammation such as the inflammatory bowel disease (IBD) [116], and in obesity [117]. Based on this background, the modulation of microbiota results an effective vehicle for improving and maintaining health. For this purpose, the use of probiotics and prebiotics, whose mixture constitutes the synbiotics, is suggested. According to the World Health Organization (WHO) criteria, a probiotic is \"a live organism, which ensures a benefit to the host when provided in adequate quantities\" [118]. The primary microorganisms classified as probiotics were various lactic acid-producing lactobacillus strains and a number of bifidobacteria strains; in addition, today several multistrain probiotic preparations are used. Over the last years the mechanisms of action of probiotics have been thoroughly investigated, whereby their roles have been defined, such as the regulation of the mucosal and systemic immune response of the host, the modification of the microbiota composition and the role in keeping the barrier functions of the intestinal epithelium. Moreover, probiotics are able to contrast the pathogen-induced epithelial damage, by reducing and preventing the adhesion of bacterial pathogen [119], and inducing the host to produce antimicrobial compounds [120]. In the last years the effect of probiotics against various diseases has been widely evaluated with promising results. In fact, in patients affected by obesity and hypertension the probiotic treatment reduced the blood pressure and the body mass index after only three weeks [121]; in addition, the probiotic mixture VSL#3 (_Bifidobacterium breve, Bifidobacterium infants, Bifidobacteriumiumosum, Lactobacillus acidophilus,Lactobacillus bulgaricus, Lactobacillus casei, Lactobacillus planatarum, and Streptococcus salivarus subspecies thermophilus) improved the health of overweight patients after a daily treatment for 6 weeks with \(112.5\times 10^{9}\) CFU of probiotic by enhancing the insulin sensitivity, changing the intestinal microbiota composition and limiting the concentration of lipids and inflammatory markers [122]. Several lines of evidence demonstrated the significant effect of probiotics on remission of patients after surgery due to [PERSON]'s disease and of patients with ulcerative colitis [123]. Apart from the promising direct effects of probiotics on intestinal microorganisms, additional effects can be found by analyzing the produced metabolites and their functions. In response to high fiber consumption, SCFA are naturally produced via fermentation in the colon, mainly as butyrate, propionate, and acetate. Among the SCFA, the butyrate has received a particular attention, due to its beneficial properties both in physiological and pathological conditions. Indeed, in the intestine it modulates cell proliferation and controls the intestinal barrier function, even protecting from the inflammatory disease [124].