**2. Dietary fibers and their metabolites in molecular function**

Dietary habits, dietary patterns, and lifestyles determine the presence of different microbial species [5]. In addition to modulating the gut microbiota composition, dietary fibers directly influence biological processes and homeostasis via the metabolites that are a result of microbial fermentation of nutrients, such as short-chain unsaturated fats (SCFAs) [6]. The gut microbiota is vital for the metabolization of DFs, such as nondigestible carbohydrates (NDCs), proteins, and peptides, which have escaped digestion by host enzymes in the upper gut and have been absorbed in the lower digestive tract [7], which are known to have beneficial effects by behaving as signaling molecules via different pathways. Acetate is the most abundant SCFA produced, and it is used by many gut commensals to produce propionate and butyrate in a growth-promoting cross-feeding process. In addition, SCFA has been shown to regulate metabolic activity. Acetate affects the metabolic pathway via the G proteincoupled receptor (GPCR) and free fatty acid receptor 2 (FFAR2/GPR43), while butyrate and propionate transactivate peroxisome proliferator-activated receptors (PPAR/ NR1C3) and regulate Angptl4 in colonic cells. FFAR2 regulates insulin-induced lipid accumulation in adipocytes and inflammation, while peptide tyrosine-tyrosine and glucagon-like peptide 1 control appetite. Microbiota-dependent NDCs regulate glucose homeostasis, gut integrity, and hormones via GPCR, NF-kB, and AMPK.

Dietary fibers are metabolized by the microbiota in the cecum and colon resulting in the formation of major products, such as acetate (C2), propionate (C3), and butyrate (C4) SCFAs. Acetate is a major SCFA metabolite produced from pyruvate. Propionate (C3) is created when succinate is converted to methylmalonyl-CoA through the succinate pathway. Through the classical pathway, butyrate is produced by the condensation of two molecules of acetyl-CoA and their subsequent reduction to butyryl-CoA. Butyrate is then converted to butyrate by phosphorus butyrylase and butyrate kinase [8].
