*Immune System, Gut Microbiota and Diet: An Interesting and Emerging Trialogue DOI: http://dx.doi.org/10.5772/intechopen.104121*

from glycocholic to taurocholic acid, compromising barrier integrity and resulting in increased abundance of *Bilophila wadsworthia*, a bile-tolerant pathobiont able to trigger a Th-1 immune response [56]. Carbohydrate intake affects bile acids metabolism, in particular, the role of soluble and insoluble dietary fiber in binding bile acids is well-documented in several studies. In a recent randomized cross-over clinical study, the consumption of a diet rich in whole grains, legumes, vegetables, and fruits was compared with a refined grain diet (high glycemic load) for the effect on circulating bile acids. The results showed a significant increase in the concentrations of specific bile acid ligands of FXR and TGR5 associated to a reduction of insulin resistance [57]. However, the role of the diet on bile acids composition and health is still partially known and needs to be confirmed and expanded in order to translate these findings into clinical settings. Lastly, tryptophan metabolites represent another important class of bacterial metabolites. Tryptophan is an essential amino acid and an important precursor of both microbial and host metabolites. Tryptophan can follow three different metabolic pathways, leading to the formation of serotonin, quinurenine, or indole and its derivatives, which represent the ligands of the aryl hydrocarbon receptor (AhR). In particular, the main microbial metabolite is indole, although the metabolic processes and pathways are complex and multiple. Indole derivatives are considered key mediators of intestinal homeostasis, as they act on epithelial renewal and barrier integrity through the activation of AhRs, which are expressed on many immune cell types, such as IELs, Th17 cells, ILCs, macrophages, dendritic cells, and neutrophils. The main effect is the production of IL-22 by ILC3, which in turn regulates metabolism by improving insulin sensitivity, modulating lipid metabolism in adipose tissue and liver, and promoting intestinal barrier integrity. Indole metabolites may also promote Th17/Treg reprogramming [4, 51].
