Dietary fiber (DF) are edible parts of plants or carbohydrates resisting digestion in the human small intestine, being partially or completely fermented by GM in the large intestine. The Codex Alimentarius defines DF as polymers of edible carbohydrates from natural sources such as cereals, fruits and vegetables, and those from raw materials by physical, enzymatic and chemical mechanisms or synthetic carbohydrate polymers with physiological benefits (24). Moreover, fermentation is influenced by many structural and physicochemical parameters. DF fermentation is associated with lower colonic pH, increased large intestinal bacterial mass, reduction of pathogens, stimulation of antioxidant compound and vitamin production, as well as regulation of epithelial barrier and immune system stimulation, among others.

One main DF classification refers to its solubility, with secondary physiological benefits on viscosity and fermentability, contributing to gel formation in the intestinal tract, and DF metabolized by GM, respectively (25). Insoluble DFs, such as lignin and cellulose, are poorly fermentable by GM, due to their inability to retain water. Soluble DFs, such as pectin, inulin, β-glucans, oligosaccharides, and guar gum are fermentable and constitute the main energy source for GM, and generally, are more fermentable with higher viscosity (26). Fermentation is possible in the presence of keystone species and strains possessing the enzymatic capacity to metabolize specific DF (24).

Interestingly, DF fermentation changes GM diversity, depending on the DF nature (27, 28), allowing production of SCFAs, amines, ammonium, gases, and phenols. Also it stimulates growth and diversity of GM and induces release of energy and water (29). A rich saturated fat and simple carbohydrate diet, but low in DF, is associated with increased risk for obesity, diabetes, cardiovascular disease, and colorectal cancer, conditions having low-grade systemic inflammation (30, 31). Concordantly, high DF intake correlates with reduced incidence of disease and mortality (32–35). DF preserves the gastrointestinal immune barrier (36, 37), with dysfunction triggering autoimmune disease and IBD development (13, 35, 38). These beneficial effects are attributed to fermentation products, mainly SCFAs, interacting with small intestinal immune cells before its degradation by microbial enzymes (39).

The GM and the mucosal immune system is a complex ecosystem, involving mutualistic GM-host relationship, where microorganisms convert diet carbohydrates, proteins, and fats into metabolites, having positive or negative health effects (40). The human digestive system lacks enzymes to digest complex polysaccharides, such as pectins, xylan or celluloses, consequently reaching the colon with their intact structure, and subsequently fermented by GM bacteria (41, 42).

GM has numerous enzymes, 16 families being carbohydrate esterases, 22 polysaccharide lyase, with 130 glycoside hydrolases standing out, allowing DF fermentation, giving GM flexibility to adapt to many substrates (24). Firmicutes and Actinobacteria phyla are the main bacteria responding to DF, although few enzymes initiate substrate degradation (43). The main substrates for bacterial fermentation and SCFA production are inulin, wheat and oat bran, cellulose, guar gum, pectin, and resistant starch; the latter an important source for butyrate production (2). Phylum Bacteroidetes members are major acetate and propionate producers, while phylum Firmicutes produce mostly butyrate (44, 45).

The main human intestinal butyrate-producing bacteria are Faecalibacterium prausnitzii and Clostridium leptum, Ruminococcaceae family members, and Eubacterium rectale and Roseburia spp from the Lachnospiraceae family (44, 45). Additionally, sugar and/or lactate utilizing bacteria such as Eubacterium hallii and Anaerostipes spp. produce butyrate from lactate and acetate in a cross-feeding process (44). However, members of the phyla Fusobacteria, Proteobacteria, Actinobacteria, Spirochaetes and Thermotogae are potential butyrate producers, expressing genes encoding butyrate synthesis enzymes, such as butyryl-CoA dehydrogenase, butyryl-CoA transferase and/or butyrate kinase (46).

Production of other SCFAs is determined by Actinobacteria, such as the Bifidobacterium species, producing lactate during DF fermentation (47). Bifidobacteria metabolizes low molecular weight carbohydrates; Bifidobacterium longum subsp. infantis prefers short-chain fructo-oligosaccharides (48), additionally, a relevant phylum Verrucomicrobia member, the mucin degrader, Akkermansia muciniphila, produces acetate and propionate (45, 49).

Although metabolite production depends on the bacterial species capable of fermenting DF, their consumption is a good way to stimulate metabolite production through modulation of the GM composition.

SCFAs are carboxylic acids produced by DF fermentation, having 1-6 carbon aliphatic tails, with acetate (C2), propionate (C3) and butyrate (C4) being the most abundant (2). Soluble DF rich diets are associated with less inflammation due to higher SCFAs production and stimulation with G-protein-coupled receptors (GPCR), i.e., the primary receptor for SCFA. GPCR inhibits NF-kB activation in immune cells and intestinal epithelial cells (IEC) (50). SCFAs profoundly impacts human health by being an energy for colonocytes, regulating glucose metabolism, fatty acids and cholesterol hepatic biosynthesis, inhibiting pathogen growth and reducing intestinal inflammation (51) and enhancing barrier function both in vitro and ex vivo (52–54) by altering TJs formation ( Figure 1 ) (53, 55). An colonocyte energy source, butyrate is the most relevant SCFAs bacterial metabolite, regulating gene expression and inflammatory (40). Studies evaluating butyrate effect on intestinal permeability, both in vitro and ex vivo, indicate low concentrations of butyrate are beneficial, while excessive luminal concentration could cause mucosal barrier disruption (52, 53). It has been demonstrated that high concentrations of SCFAs induces intestinal mucosa injury in newborn rats, but although maturation dependent, this injury disappears in mature mucosal defense (56). Additional studies (52) show that high butyrate concentrations disrupt assembly of TJs to the intestinal barrier, proposing that barrier function regulation by low butyrate concentrations in Caco-2 cell monolayers facilitates TJs assembly in a dynamic process mediated by AMP-activated protein kinase (AMPK) activation (53).

Both DF and butyrate are important components and metabolites involved in managing inflammatory diseases (57). SCFAs suppress IL-8 secretion and expression (58–61), although, high butyrate doses has the opposite effect (61). Understanding butyrate action in reducing inflammation is important, especially the histone acetylation mechanism (62), requiring more study as butyrate metabolizes to Acetyl-CoA, activating histone acetyltransferase (HAT), affecting cell turnover, structure and function and tumoricity, although with paradoxical effects (63, 64). Butyrate permeate the cell membrane by passive diffusion or is absorbed by apically proton-coupled monocarboxylate transporter 1 (MCT1) and sodium-coupled monocarboxylate transporter 1 (SMCT1) on IECs and/or immune cells (65). MCT1 primarily transports butyrate and is induced by SCFAs, inhibiting HDACs through MCT1 (66).

Cellularly, SCFAs activate signaling pathways through GPCRs: GPR41, GPR43 and GPR109A (expressed in colonic epithelium enteroendocrine cells, polymorphonuclear immune cells and smooth muscle cells) with different affinities ( Figure 1 ). GPR41 and GPR43 have a role in the immune surveillance of the colonic mucosa towards microbial activity (67), whilst GPR109A inhibits NF-kB activation as a tumor suppressor (68). Active absorption of butyrate is reduced in IBD with decreased MCT1 expression in inflamed mucosa of UC patients (69), possibly due to high TNF-α levels (70) or reduced butyrate-producing bacteria, causing faulty oxidization (71, 72), potentially indicating that butyrate reduces IL-8 expression mediated by GPR109A.

GPR41 and GPR43 activate the NLRP3 inflammasome (73), triggering IL-1β and IL-18 secretion (74, 75), and GPR43 regulates immune cells in experimental inflammation (36, 37), with sodium butyrate increasing disease severity in DSS model of germ-free mice (76). Butyrate modulates immune cells, such as macrophages (77), dendritic cells (78) and lymphocytes (77), and inhibits cytokines, such as IL-12p70 and IL-23, polarizing naïve CD4⁺ T cells towards the Th1 and Th17 subtypes (79). Butyrate affects the mucosal immune system through Treg expansion, and inhibits HDAC activity in dendritic cells, resulting in retinaldehyde dehydrogenase 1 (RALDH1) and GPR109A expression (77). RALDH1 modulates naïve T cell differentiation towards Treg producing IL-10 (77, 80, 81), with butyrate also enhancing Foxp3⁺ expression and Treg differentiation, reducing inflammation (80).

GM in IBD patients is characterized by loss of intraindividual diversity, with higher abundance of Proteobacteria and lower of Firmicutes. Interestingly, butyrate-producing bacteria Faecalibacterium prausnitzii, Ruminococcus torques, Roseburia inulinivorans, Blautia faecis, and Clostridium lavalense are reduced in patients (82–85), with lower luminal butyrate concentrations (86) and higher C-reactive protein, reflecting increased inflammation (84). Furthermore, a reduced butyrate-synthesis capacity has been described in IBD patients, with butyryl-CoA:acetate CoA-transferase (BCoAT) gene content inversely associated with disease activity and dysbiosis, being more evident in CD, and may relate to reduced DF intake (87). Alternatively, celiac disease associates with GM alterations, although no SCFA quantity differences or composition exist between celiac patients and healthy patients (88). Interestingly, prolonged administration of oligofructose-enriched inulin in children with celiac disease (under a gluten-free diet), has a moderate effect on GM composition, while SCFA production is stimulated in celiac children (89).

Accordingly, activated NF-kB in UC patients colonic mucosa is reduced by butyrate and SCFAs enemas (100 nM), directly correlating with disease activity (90, 91), with no effect in patients in remission (92). Depending on butyrate physiological concentration, it can be beneficial, with fewer molecules reaching colonocytes, as mucus layer prevents excessive concentrations towards epithelial cells, and increasing MUC2 expression (93). Regarding the above, the fact that butyrate effect is variable, is related to the physiological irrigation of this SCFA. At these or other doses, it is likely that in an in vivo IBD model, fewer butyrate molecules reach the colonocytes than in an in vitro model, since the colonic epithelium is covered by a thick mucus layer preventing passage of excessive concentrations of butyrate into the cell. Consequently, as butyrate concentration gradients run from the crypt-lumen axis and the luminal axis proximal-distal, colonocytes at the crypt base are exposed to an estimated µM concentration range, thus explaining its variable effects on inflammation.

Certain DF have a direct anti-inflammatory effect by directly interacting with pattern recognition receptors (PRRs), mainly Toll-like receptor (TLR) 2 and 4, or Galectin-3 in intestinal immune barrier cells. Accordingly, low-grade methyl esterification pectins (Low-DM) block innate immune receptors, with pectin directly inhibiting pro-inflammatory TLR2-TLR1 pathway (without affecting TLR2-TLR6 tolerogenic pathway in vitro) ( Figure 1 ) and preventing clinical symptoms in a TLR-2 dependent doxorubicin-induced ileitis murine model (65). Additionally, low-DM citric pectin suppresses TLR-induced inflammatory cytokine expression in a mouse model of endotoxin shock (94). Moreover, apple pectin oligo-galactan exhibits a protective efficacy in a murine model of dextran sulfate sodium (DSS)-induced colitis, decreasing LPS-induced TNF-α, and is probably associated to a mechanism comprising TLR4 internalization and redistribution (95). Similarly, resistant starch (RS) also binds predominantly to TLR2 and/or TLR5 in dendritic cells, affecting PRR signaling (96).

Pectin directly interacts with TLR2, inhibiting the proinflammatory signaling pathway, thereby contributing to barrier function and preventing colorectal cancer development (97). Similarly, β2 fructans also regulate mucosal homeostasis through activation of TLR2 (to a lesser degree TLR4, TLR5, TLR7, TLR8 and Nucleotide Binding Oligomerization Domain Containing 2; NOD2) producing IL-10/IL-12 in mononuclear cells (in chain length-dependent manner) (98), and regulating barrier function in T84 cells (99). Wheat bran and peanut fiber also improve intestinal barrier function, changing GM composition and TLR2 expression in pigs (100). Likewise, chicory pectin and intact guar gum DF exhibit an anti-inflammatory effect in a murine model of DSS-induced colitis decreasing proinflammatory expression (101, 102). Additionally, bengkoang DF binds to TLR4, favoring phagocytic activity in J774.1 macrophages (103).

Soluble DF has immunomodulatory effects by influencing human peripheral blood mononuclear cells (PBMCs) functions (independent of SCFA production) (39), and has symbiotic effects with certain Lactobacillus, synergistically stimulating IL-6 and IL-8 production in immune cells (104). Inulin and short-chain fructo-oligosaccharides (scFOS) directly promote, in a GM-independent manner, specific intestinal barrier function in a damage-induced model of Enterohemorrhagic E. coli (105).

All these fibers can be instrumental in prevention or lowering symptoms of chronic inflammatory conditions where the aforementioned gut epithelial processes are involved. Examples are celiac disease which results from oral tolerance to gluten breakdown in subjects carrying HLA-DQ2 or HLA-DQ8 variants (106). High maternal fiber intake during pregnancy is associated with a lower risk for celiac disease in children, while gluten intake associates with a high risk, suggesting that high dietary fiber and low gluten intake during pregnancy is a protective factor for celiac disease (107). The mechanisms are not yet clear but might be associated to the aforementioned protective effects of DF on gut epithelial immune barrier function.

Galectin-3, extracellularly or intracellularly expressed in immune and epithelial cells, plays a fundamental role in health and disease (108), interacting with carbohydrates and intracellular proteins. Galectin-3, acts as a PRR inducing innate responses against pathogens (109), enhanced by pectins (109), through galactose residues or arabinose side chains with rhamnogalacturonan I (RG-I) and rhamnogalacturonan II (RG- II) (110).

Primary bile acids (BAs) synthesized in the liver from cholesterol, conjugate to glycine or taurine before released and concentrated into bile in the gallbladder, and eventually secretion (111). Approximately 95% of BAs are reabsorbed in the ileum and recirculate to the liver, while a smaller percentage reach the colon, and are metabolized by GM or excreted; additionally, GM regulates BAs synthesis and uptake (112). In healthy people, the amount of daily secreted BAs depends on eating habits, with levels fluctuating between 200-600 mg per day (113) BAs metabolism by GM, and its effects on health and disease recently begun to be considered (114, 115), with two BAs being synthesized by the liver, colic and chenodeoxycholic acid. GM-mediated BAs transformations include deconjugation via bile salt hydrolases (BSH) hydrolyzing the amide bond and transforming deconjugated primary BAs to secondary BAs, mainly by 7α-dehydroxylation reactions exerted by a limited number of bacteria (116).

While deconjugation is performed by numerous bacteria, BSH are encoded in different genus species: Lactobacillus, Bifidobacterium, Bacteroides and Clostridium, being more diverse in Firmicutes (117). Conversion of primary to secondary BAs by 7α-hydroxylation is a relevant microbial BAs transformations in humans (118), as cholic acid is transformed to deoxycholic acid (DCA) and chenodeoxycholic acid to lithocholic acid (LCA). 7α-dehydroxylation, is a characteristic of Clostridium (Clostridium hylemonae and C. scindens, the latter important in the production of LCA), as well as Eubacterium (119, 120). Bacterial BAs metabolism plays a fundamental role in systemic and intestinal health, since imbalances in BA-GM crosstalk associates with inflammatory and gastrointestinal disorders (121).

Secondary BAs are potent nuclear receptor ligands, binding to farnesoid X receptor (FXR), vitamin D receptor (VDR) and pregnane X receptor (PXR), additionally acting as endogenous agonists for the microbial G protein-coupled bile acid receptor (TGR5). These receptors play an important role in BAs synthesis, regulation and metabolism (122) and are relevant in different pathological processes ( Figure 1 ). BAs receptors are expressed in IECs, hepatocytes, liver parenchyma, muscles, neurons, among others (123), as well as in immune cells (124–127), responding to endogenous and bacterial antigens. In BAs metabolism by FXR, biliary acidosis inhibition by CYP7A1 inhibition is an important step, occurring via the hepatic bile salt export pump, inducing small heterodimer partner (SHP) expression. SHP inactivates the hepatic homologous receptor-1 (LRH-1) inhibiting CYP7A1 expression (128). Alternatively, FXR activated by BAs induces FGD15/19 to FGFR4 binding, inhibiting CYP7A1 (129).

During inflammation, FXR regulate intestinal immune responses driven by GM, with low-grade chronic inflammatory disorders associated to dysbiosis and altered BAs profiles in humans (130). IBD patients exhibit a low content of bile salt biotransformation genes (mostly from Firmicutes phylum) as well as low secondary and high primary BAs levels compared to healthy subjects (131, 132). Also FXR null mice are unable to reduce intestinal inflammation, promoted by GM-LPS stimulation of NF-kB and inflammatory cell recruitment (133). Additionally, antibiotic-induced GM alterations allow C. difficile spore germination and inhibit BAs-producing bacteria, depleting BAs production (134) and, thus inhibit FXR activation. Additionally, FXR deficiency in a Apc^Min/+ and azoxymethane (AOM)-DSS murine model of colon carcinogenesis (associated with chronic colitis), resulted in premature death and increased tumor progression through Wnt signaling pathway activation in macrophages, neutrophils (135). Moreover, BAs deficiency (due to bile duct obstruction) associates with bacterial overgrowth and translocation, intestinal damage, reverted by BAs or GW4064 (FXR agonist) administration (136). Obeticholic acid (OCA) also a FXR agonist, decreases severity, maintaining epithelial barrier integrity and decreasing inflammatory cytokine production in a DSS or 2,4,6-Trinitrobenzene sulfonic acid (TNBS)-induced colitis murine model (125, 137), whilst isoDCA (secondary BA) induces peripheral Treg cells (138). Therefore, FXR rationally becomes a therapeutic IBD target, although, clinical trials using FXR agonists will be required.

GM activates TGR5 expression, affecting enteroendocrine cells, exhibiting immunomodulatory and anti-inflammatory effects (139), mainly in macrophages and monocytes (139). Importantly, TGR5 regulates energy metabolism and glucose homeostasis: inducing GLP-1 secretion, glucose metabolism, intracellular cAMP levels, transcription of type 2 iodothyronine deiodinase (Dio2) gene (encoding type 2 deiodinase (D2) converting thyroid hormone T4 into triiodothyronine, T3). Through TGR5 activation, BAs inhibit production of pro-inflammatory mediators IL-1, IL-6 and TNF-α, induced by LPS (140). Likewise, BSH-containing bacteria alters TGR5-mediated pro-inflammatory or anti-inflammatory activity, dissociating taurine or glycine from BAs.

TGR5 agonist effect associates with intestinal IL-10 increase (141). BAR501, a selective agonist of TGR5, protected mice from colitis by polarizing intestinal macrophages from M1 to M2 phenotype and reducing intestinal and circulating monocytes/macrophages (141, 142).

In celiac disease, immunologically mediated extraintestinal manifestations in the liver can occur. Both gut and liver, share lymphocyte recruitment and return routes, with gut T lymphocytes contributing to liver and biliary inflammation (143). The gut-liver axis describes a close metabolic and immunological connection between the intestine and the liver, and an imbalance or damage in this axis can trigger innate immune responses that induce liver damage or contribute to its progression (144). It should be noted that in celiac disease, the intestinal permeability, together with chemokine CXCR3, increase entry of food and bacterial antigens, as well as bacterial metabolites via the portal vein, triggering immune responses by activation of PPRs (CD14/TLR4 complex, inflammasome, etc.), generating liver inflammation and metabolism alterations. Alternatively, dysbiosis in this pathology produces an increase in intestinal permeability and mediates inflammatory processes, enhancing celiac disease immunopathology and altering BAs composition, impacting on hepatic FXR and TGR5, inflammation and BA metabolism (145, 146).

The above demonstrates a clear role of GM-BAs-FXR/TGR5 axis in Tregs, monocytes and macrophages regulation, highlighting the potential use of secondary BAs or other natural or synthetic ligands as new therapies in inflammatory pathologies.