Gut Microbial Metabolite Butyrate and its Therapeutic Role in Inflammatory Bowel Disease

1. Introduction

Inflammatory bowel disease (IBD) is characterized by chronic intestinal inflammation in the gastrointestinal (GI) tract that includes Crohn’s disease (CD) and ulcerative colitis (UC). Although both CD and UC present with chronic inflammation, they differ in many aspects such as location, distribution, and depth of inflammation, and complications and rectal involvement (Table 1). The exact cause of IBD is still not well understood, but the pathogenesis is interlinked with genetic factors, abnormal immune reactivity, microbiota dysbiosis, diet, and environmental factors being involved. The dynamic balance between commensal microflora and host defensive responses in the intestine plays a key role in the initiation and chronic progression of IBD [1]. Disturbed immune function and epithelial barrier integrity are the major features of IBD.

While the pathogenic mechanisms of CD and UC remain unknown, IBD is not curable. Current therapies for IBD, including corticosteroids, immunomodulators, and biologics, are designed to induce remission [2]. However, patients’ response to the treatments decreases over time, and relapses occur frequently. Moreover, the side effects of these treatments are significant, and sometimes intolerable to patients. It is important to identify novel therapeutic targets and discover effective and safe treatments for IBD patients. Short chain fatty acids (SCFAs) are the most abundant microbial metabolites in the intestine and provide 60-70% of energy needs for colonocytes [3]. Specifically, butyrate is the major fuel source for the epithelial cells and has gained more attention than any other SCFA as it regulates intestinal homeostasis and maintains epithelial integrity. A reduced number of butyrate-producing bacteria and lowered butyrate concentration have been found in IBD [4,5]. As butyrate is shown to modulate immune function and intestinal barrier function, it is considered a therapeutic target in the treatment of IBD. In this review, we discuss the production and metabolism of butyrate and the therapeutic implications of butyrate in IBD.

2. Gut microbiota and metabolites

The human gastrointestinal tract harbors a complex and diverse microbial population termed gut microbiota. The gut microbiota comprises trillions of microbes including archaea, bacteria, fungi, and viruses. Many bacteria, particularly anaerobic bacteria, colonize the intestinal tract in a symbiotic relationship, which plays a critical role in maintaining the intestinal homeostasis of the host. The high-throughput DNA sequencing technology has enhanced our understanding of gut microbiota without the need for microbial culturing. More than 1,000 bacterial species colonized in the human gastrointestinal tract especially in the colon. Most of these bacterial species belong to two major phyla: Firmicutes and Bacteroidetes [6].

The gut microbiota produces a wide range of metabolites including short chain fatty acids (SCFAs), polyamines, vitamins, tryptophan-derived metabolites, and secondary bile acids using exogenous undigested dietary substrates and endogenous compounds [7]. These metabolites can be classified into three types: (1) Metabolites produced by microbial transformation of dietary components or drugs such as compound K; (2) Metabolites produced from host secretions that are modified by gut microbiota such as secondary bile acids; (3) Metabolites synthesized by gut microbiota from diet components such as SCFAs [8]. The microbial metabolites can be both beneficial and toxic to the host (Figure 1).

Primary bile acids cholic acid and chenodeoxycholic acids are synthesized from cholesterol and conjugated to glycine or taurine in the liver then stored in the gallbladder and released into intestine to facilitate dietary fats emulsification, digestion, and absorption in the small intestine. The remaining bile acids are absorbed in the terminal ileum and reached to the liver through enterohepatic circulation [9]. The escaped bile salts during enterohepatic circulation become substrates for gut microbial metabolism including deconjugation, oxidation, epimerization and dehydroxylation. The bacteria genera including Clostridium, Bifidobacterium, Bacteroides, Listeria and Lactobacillus are involved in the deconjugation of bile acids. Bacteroides, Eggerthella, Escherichia, Clostridium, Ruminococcus and Peptostreptococcus are involved in oxidation and epimerization [10]. The intestinal bacteria Clostridium and Eubacterium genera transforms cholic acid and chenodeoxycholic acid into deoxycholic acid and lithocholic acid, respectively by dihydroxylation using hydroxysteroid dehydrogenase enzymes [10,11,12]. Undigested dietary proteins enter the colon and serve as substrate for gut microbial metabolism. Tryptophan is an essential amino acid consumed from diet. Undigested or escaped tryptophan is fermented by colonic bacteria, producing various metabolites, indole, indoleacetic acid, indole-3-lactate, indole-3-propionate through direct tryptophan transformation pathway [13]. Indole-producing bacteria, such as Acinetobacter oleivorans, Vibrio cholera, Escherichia coli, Pseudomonas chlororaphis, and Synbiobacterium thermophilus, produce indole from tryptophan [14].

SCFAs including acetic, propionic, and butyric acids are a group of carboxylic acids that consist lesser than six carbon atoms. SCFAs are derived from fermentation of non-digestible carbohydrates in the proximal colon and by proteolytic fermentation in the distal colon. SCFAs can be formed from fermentable carbohydrates through the glycolytic pathway and the pentose phosphate pathways by microbial fermentation [15]. Butyrate is mainly produced from species of Firmicutes phylum including Roseburia species, Faecalibacterium prausnitzii, Eubacterium rectale, whereas acetate and propionates are produced from the species of Bacteroidetes phylum [16,17]. The production of SCFAs in the intestine is substrate dependent. About 300 to 600 mmol of SCFAs are produced in the human intestine per day and only a small amount of SCFAs (~ 10 mmol) are excreted through fecal excretion. The remaining SCFAs are rapidly absorbed by the host epithelial cells via passive diffusion or active transport [18,19].

3. Butyrate production, absorption, and metabolism

Butyrate, a four-carbon SCFA, is one of the major energy sources for intestinal epithelial cells. Gut microbiota produces butyrate from acetyl-CoA, lysine, glutarate, or succinate pathways in the colon [20]. Various bacterial species in the human intestine generate enzymes that can synthesize butyrate from complex fermentable substrates. The predominant butyrogenic bacterial species including Faecalibacterium prausnitzii, Clostridium spp., Eubacterium spp., and Roseburia spp. are from two clusters (Clostridium clusters IV and XIVa) in Firmicutes phylum and the Clostridiales order [21,22]. Most of luminal butyrate is synthesized from non-digestible carbohydrates via acetyl-CoA pathway (Figure 2). In first step, non-digestible carbohydrates are catabolized into pyruvate through pentose phosphate pathway or Embden-meyerhof-parnas pathway. Pyruvate can be converted into acetyl-CoA, which is further breakdown into butyryl-CoA. In the final step, butyryl-CoA can be converted into butyrate by butyryl-CoA: acetyl-CoA transferase or phosphorylated to butyryl-phosphate through phospho-transbutyrylase and then subsequently converted to butyrate through butyrate kinase [23,24,25]. Acetate is required to produce butyrate via butyryl-CoA: acetyl-CoA transferase through cross-feeding microbial reactions. Butyrate is produced by cross-feeding interactions between acetate producing Bifidobacterium spp. and acetate utilizing Faecalibacterium prausnitzii [22]. Moreover, the metabolite cross-feeding within microbial community plays a key role in maintaining the diversity of gut microbial ecosystem [26]. In succinate pathway, butyrogenic bacteria convert succinate to crotonyl-CoA, which is subsequently converted into butyrate. Crotonyl-CoA is the common butyrate precursor in L-lysine and glutarate pathways (Figure 2).

Gut microbiome-derived butyrate is taken up rapidly by colonocytes through passive non-ionic diffusion or active carrier-mediated transport [27]. Ionized form of butyrate is transported across the apical surface of intestinal epithelial cells through active transport mediated by H⁺-monocarboxylate transporter-1 (MCT1) and Na⁺-coupled monocarboxylate transporter-1 (SMCT1). Solute carrier family 5 member 8 (SLC5A8) is one of the major SMCT1 transporters of butyrate across the colonocytes [28]. The gene expression levels of SLC5A8 are abundant in the apical membrane of the colon and ileum. On the basolateral membrane, butyrate is transported through carrier-mediated bicarbonate exchange system [29]. Butyrate predominantly presents in the anionic form in the colon due to colonic luminal pH conditions. Thus, it requires carrier-mediated transportation for the cellular entry.

The absorbed butyrate is metabolized in the intestinal epithelial cells, liver cells, and other tissues and cells [30]. In the epithelial cells, butyrate is transformed into acetyl-CoA, and enters tricarboxylic acid (TCA) cycle in the mitochondria to produce ATP, which is consumed by the colon epithelial cells. The portion of butyrate which is not utilized by epithelial cells can reach to the liver via portal circulation, where it is metabolized into acetyl-CoA and becomes a substrate for fatty acids, cholesterol, and ketone bodies by hepatocytes [21,31]. The plasma concentration of butyrate is very low compared to colonic levels, only 2% of butyrate enters systemic circulation, being utilized by other tissues and cells [31]. The remaining SCFAs including butyrate are excreted through the lungs and urine.

4. Role and mechanisms of butyrate in the regulation of barrier function and immune response

The single layer of intestinal epithelium serves as a barrier between the host and its external environment that controls the interaction between luminal contents and the internal milieu of the body. The intestinal epithelial monolayer contains several types of specialized cells: (1) enterocytes, for absorption of nutrients; (2) goblet cells, producing secretory and gel forming mucins which are glycosylated proteins that form polymeric nets called mucus layer, a physical barrier between intestinal bacteria and epithelial cells; (3) enteroendocrine cells, secreting various hormones regulating digestive function; (4) Paneth cells, residing at crypt base and secreting antimicrobial peptides such as lysozyme, defensins, and cryptidins; (5) microfold cells (M cells), sampling antigens from lumen to sub-epithelium; and (6) tuft cells, for chemosensing function in the epithelium [32,33]. These epithelial cells are connected by intercellular desmosomes, tight junctions (TJs), and adherent junctions (AJs), which create a physical barrier for luminal contents of the gut and regulate epithelial permeability. TJs are a complex network formed by transmembrane proteins such as claudins, occludin, tricellulin, and junctional adhesion molecules and cytosolic scaffold proteins such as zonulae occludens (ZO) and cingulin [34,35]. Both TJs and AJs are connected to the actin cytoskeleton and form an apical junction complex. On the basal side, epithelial cells are connected by hemidesmosomes.

The intestinal epithelium lies between the commensal organisms in the gut lumen and the immune cells in lamina propria. The complex immune interactions between commensal microflora, the epithelial layer, and the sub-epithelial immune cells maintain homeostasis under normal conditions. Lamina propria contains the gut-associated lymphoid tissue (GALT) which is comprised of Peyer’s patches, a group of lymphoid follicles containing several immune cells, such as specialized M cells, dendritic cells, T cells, B cells, intraepithelial lymphocytes, and macrophages [36]. The dendritic cells (DCs) from lamina propria sample the luminal food and microbial antigens by extending their dendrites between epithelial cells and transport to antigen-presenting cells (APCs) in GALT [37,38]. Upon activation, GALT performs effector immune functions by activating immune cells to produce specific cytokines from T cells and immunoglobulins from B cells. Antigens in the gut lumen can be taken up by specialized M cells and delivered to DCs for effector functions in the Peyer’s patches [39]. Intestinal epithelial cells themselves can also act as dynamic sensors by pattern recognition molecule receptors (PRRs) such as toll-like receptors (TLRs) and nucleotide-binding oligomerization domain (NOD) like receptors (NLRs) to sense pathogen-associated molecular patterns.

Gut microflora and their metabolites play a major role in maintaining epithelial barrier function and immune homeostasis. Among the microbial metabolites, butyrate involves a number of signaling pathways in the gut immune cells and epithelial cells for restoration of impaired colonic barrier function and gut homeostasis (Figure 3). Pathophysiology of IBD involves both epithelial barrier dysfunction and abnormal immune cell activation. Changes in TJs structure, down-regulation of claudin proteins, and up-regulation of pore-forming claudin-2 were observed in both CD and UC conditions [34]. Since 2007, butyrate was found to enhance the intestinal barrier function by facilitating tight junction assembly via activation of AMPK, Akt, and other signaling pathways in a dose-dependent manner as shown in studies with transepithelial electrical resistance (TEER) and fluorescein isothiocyanate-dextran (FITC-dextran) permeability assays in in vitro settings [40,41,42]. Marinelli et al. [43] demonstrated that butyrate regulates the epithelial barrier function by acting as a signaling molecule for cell-surface G-protein-coupled receptors (GPRs) and nuclear factors (NFs). Indeed, butyrate was found to induce T cell-independent IgA secretion in the colon via activation of GPR41 (free fatty acid receptor 3, FFAR3) and GPR109A (hydrocarboxylic acid receptor 2, HCAR2), and inhibition of histone deacetylase (HDAC) to restore epithelial barrier function under inflammatory conditions [44]. Studies also explored the effect of butyrate on claudins expression. Zheng et al. [45] reported that butyrate promotes epithelial barrier function through interleukin-10 receptor α-subunit (IL-10RA) dependent repression of claudin-2 TJ protein. Wang et al. [46] demonstrated that butyrate treatment improved epithelial barrier function via up-regulation of claudin-1 transcription by

facilitating the interaction between specific motifs in the claudin-1 promoter region and SP1 transcription factor. Moreover, butyrate enhances mucin secretion and protects epithelial cells by inducing MUC2 gene expression via AP-1 and acetylation/methylation of histones at the MUC2 promoter in intestinal epithelial goblet cells [47]. Hypoxia-inducible factor 1 (HIF-1) - dependent mechanism may also contribute to butyrate-enhanced epithelial barrier function [48].

An inappropriate immune response to antigens derived from intestinal components is a key feature in IBD, leading to imbalance of inflammatory cytokines, tissue damage, and disease progression [49,50]. Increased phagocytic activity of macrophages and cytokines’ secretion (for example, IL-1, IL-6, IL-17, and TNF) has been found in IBD patients [51]. T lymphocytes (T-cells) play a crucial role in maintaining immune homeostasis by regulating innate and adaptive immune responses. Upon specific antigen stimulation, naïve CD4⁺ T-cells differentiate into effector T helper (Th) cells including Th1, Th2, T regulatory (Treg), and Th17 cells [52]. Each Th type secret specific cytokines to perform protective or pathogenic roles. Treg cells have immunosuppressive properties that help to maintain immune homeostasis by secreting anti-inflammatory cytokines including IL-10 [53]. IBD is associated with dysregulated T-cell immune responses such as increased Th1, Th2, Th17 cells function and decreased Treg cells function [54]. Th17 produce inflammatory cytokines such as IL-17A, IL-17F, and IL-21 which are involved in the pathogenesis of IBD. Gut microbial metabolite butyrate regulates differentiation and proliferation of T cells (Figure 4). Butyrate administration enhanced Treg cells function and suppressed IL-17 levels as well as Th17 cells in the peripheral blood and colon tissues of TNBS-induced colitis rats compared to control group [53]. Zimmerman et al. [55] have demonstrated that butyrate inhibits proliferation of both CD4⁺ and CD8⁺ T cells in a dose-dependent manner and it induces apoptosis in T cells through the Fas-mediated apoptosis pathway. Butyrate facilitates Treg cell differentiation by increasing histone H3 acetylation at the promoter and CNS3 region of the FOX3 gene locus [56]. Chen et al. [57] found that butyrate enhanced Th1 differentiation by promoting IFN-γ levels and T-bet expression in healthy condition, but inhibited Th1 differentiation through IL-10 production and T-bet expression in colonic inflammation. In addition, butyrate has shown to regulate inflammatory response by influencing NF-κB activity. NF-κB is a transcription factor involved in the regulation of various inflammatory mediators and cytokines expression including TNF-α and IL-6 [58]. Butyrate is shown to reduce inflammatory response by suppressing NF-κB activity. Several studies have demonstrated the ability of butyrate to reduce NF-κB activity in human colon cell lines and in lamina propria mononuclear cells isolated from CD patients [59,60,61]. Butyrate activates transmembrane GPRs and nuclear receptors such as aryl hydrocarbon receptor (AhR) in the intestinal epithelial cells. AhR is a ligand-activated transcription factor that resides in cytosol in activated form, and translocates to nucleus up on activation thereby regulating AhR-dependent gene expression [62,63]. SCFAs including butyrate are shown to enhance AhR ligand interations in mouse and human colon cells [43,64].

5. Therapeutic implications of butyrate for IBD

IBD is characterized with aberrant immune response and barrier dysfunction and associated with reduced number of butyrate-producing bacteria in the gut. As butyrate was found to not only provide energy to colonic epithelial cells but also help maintain intestinal integrity and modulate immune responses [42], numerous studies have investigated the role of various forms of butyrate in reducing gut inflammation [65,66,67]. Many of the studies have demonstrated the efficacy of oral butyrate supplements, butyrate enema, butyrogenic diet and bacterial supplements in the treatment of IBD.

5.1. Butyrate supplements

5.1. a. Oral administration

Dysbiosis of gut microbiota leads to decreased butyrate synthesis and impaired butyrate metabolism as observed in IBD [68]. Although a low concentration of butyric acid is commonly present in our daily regular diet, it may not be sufficient to restore the epithelial function in inflammation in the colon. Many studies have investigated the therapeutic potential of butyrate oral supplements in gut inflammation in both preclinical studies and clinical trials. Table 2 summarizes these results (Table 2). Butyrate has been shown to reduce gut inflammation and ameliorate symptoms in a dose dependent manner. Butyrate at 20 mg/kg/day or lower doses was found to have no significant effect, while at 100 mg/kg it was effective against inflammation in mice [67,69]. Lee et al. [67] reported that the oral supplementation of sodium butyrate at 100 mg/kg of body weight daily decreased colitis scores, prevented body weight loss, and induced histone H3 acetylation in colonic mucosa in mouse models of acute and chronic colitis. Moreover, butyrate treatment restored the microbial community diversity and reduced microbiota dysbiosis in gut inflammation [70].

As orally supplemented butyrate is rapidly absorbed in the duodenum, majority of the orally administered butyrate would not reach the colon. Moreover, the clinical application of oral butyrate is limited due to its unpleasant taste and odor. To address these issues, some studies used colon targeted formulations and encapsulated butyrate to test if butyrate in such formulations has better effects in IBD patients especially UC patients [66,71,72]. Sabatino et al. [71] demonstrated that enteric-coated butyrate tablets administration effectively reduced ileocaecal inflammation and maintained clinical remission in Crohn’s disease patients. Lipophilic microencapsulated-sodium-butyrate treatment showed enrichment of butyrogenic colonic bacteria in IBD patients [66]. Wang et al. [73] developed butyrate micelles so that butyrate is released in the lower gastrointestinal tract. They found that butyrate micelles significantly improved intestinal barrier function and reduced disease severity in DSS-induced colitis and CD45RB^hiT-cell transfer colitis in mice.

Table 2. Impact of oral butyrate supplements on IBD.

	Treatment name	Concentration	Colitis model	Effects	Authors
Mice	Sodium butyrate	0.5% of sodium butyrate	DSS-induced colitis	Decreased mucosal inflammation	Vieira et al. [74]
Mice	Butyrate-releasing polysaccharide derivative	200 mg/kg	DSS-induced colitis	Reduced disease activity index, rebalanced gut microbiota and reversed the imbalance between pro- and anti-inflammatory cytokines	Zha et al. [65]
Mice	Balatable butyrate-releasing derivative, N-(1-carbamoyl-2-phenylethyl) butyramide (FBA)	42.5 mg/kg	DSS-induced colitis	Reduced disease activity index	Simeoli et al. [75]
Mice	Sodium butyrate	200 mM	Citrobacter rodentium infection model	Prevented mice from weight loss and suppressed intestinal inflammation	Zhou et al. [76]
Mice	Sodium butyrate	200 mM	DSS-induced colitis	Suppressed intestinal inflammation and lowered pathology scores	Zhou et al. [76]
Mice	Sodium butyrate	5 g/L	TNBS induced colitis	Decreased disease activity index and suppressed inflammation	Chen et al. [77]
Mice	Sodium butyrate	100 mg/kg/day	DSS-induced acute colitis Piroxicam-induced chronic colitis	Decreased colitis scores and prevented weight loss	Lee et al. [67]
Mice	Sodium butyrate		DSS-induced colitis	Decreased disease activity index, and restored the balance of gut microbial communities	Dou et al. [70]
Mice	Sodium butyrate150 mM	150 mM sodium butyrate	DSS-induced colitis	No significant difference in histologic scores	Lee et al. [78]
Human	Enteric-coated tablets	4 g/day	Crohn's disease	Induced clinical improvement and reduced disease activity index	Sabatino et al. [71]
Human	Sodium butyrate tablets	4 g/day	Crohn's disease	Induced clinical improvement or remission	Di Sabatino et al. [79]
Human	Microencapsulated-sodium-butyrate	1800 mg/day	IBD-both CD and UC	Increases the growth of bacteria able to produce SCFA with potential anti-inflammatory action	Faccin et al. [66]
Human	Microencapsulated sodium butyrate	1000 mg/day	UC in clinical remission	Helped to maintain clinical remission	Vernero et al. [72]
Human	Sodium butyrate	150 mg/twice a day	IBD-both CD and UC	No significant effects in newly diagnosed children and adolescents	Pietrzak et al. [80]

Table 2. Impact of oral butyrate supplements on IBD.

	Treatment name	Concentration	Colitis model	Effects	Authors
Mice	Sodium butyrate	0.5% of sodium butyrate	DSS-induced colitis	Decreased mucosal inflammation	Vieira et al. [74]
Mice	Butyrate-releasing polysaccharide derivative	200 mg/kg	DSS-induced colitis	Reduced disease activity index, rebalanced gut microbiota and reversed the imbalance between pro- and anti-inflammatory cytokines	Zha et al. [65]
Mice	Balatable butyrate-releasing derivative, N-(1-carbamoyl-2-phenylethyl) butyramide (FBA)	42.5 mg/kg	DSS-induced colitis	Reduced disease activity index	Simeoli et al. [75]
Mice	Sodium butyrate	200 mM	Citrobacter rodentium infection model	Prevented mice from weight loss and suppressed intestinal inflammation	Zhou et al. [76]
Mice	Sodium butyrate	200 mM	DSS-induced colitis	Suppressed intestinal inflammation and lowered pathology scores	Zhou et al. [76]
Mice	Sodium butyrate	5 g/L	TNBS induced colitis	Decreased disease activity index and suppressed inflammation	Chen et al. [77]
Mice	Sodium butyrate	100 mg/kg/day	DSS-induced acute colitis Piroxicam-induced chronic colitis	Decreased colitis scores and prevented weight loss	Lee et al. [67]
Mice	Sodium butyrate		DSS-induced colitis	Decreased disease activity index, and restored the balance of gut microbial communities	Dou et al. [70]
Mice	Sodium butyrate150 mM	150 mM sodium butyrate	DSS-induced colitis	No significant difference in histologic scores	Lee et al. [78]
Human	Enteric-coated tablets	4 g/day	Crohn's disease	Induced clinical improvement and reduced disease activity index	Sabatino et al. [71]
Human	Sodium butyrate tablets	4 g/day	Crohn's disease	Induced clinical improvement or remission	Di Sabatino et al. [79]
Human	Microencapsulated-sodium-butyrate	1800 mg/day	IBD-both CD and UC	Increases the growth of bacteria able to produce SCFA with potential anti-inflammatory action	Faccin et al. [66]
Human	Microencapsulated sodium butyrate	1000 mg/day	UC in clinical remission	Helped to maintain clinical remission	Vernero et al. [72]
Human	Sodium butyrate	150 mg/twice a day	IBD-both CD and UC	No significant effects in newly diagnosed children and adolescents	Pietrzak et al. [80]

5.1. b. Butyrate enemas

Treatment with butyrate enemas had mixed effects in preclinical and clinical studies as summarized in Table 3 (Table 3). Butyrate enema showed inhibition of NF-κB activation in lamina propria macrophages of UC patients, and it also reduced disease activity [81]. Segain et al. [59] observed reduction of TNF-α induced NF-κB in colon tissues in butyrate enema treated colitis rats. However, some clinical studies found that butyrate enema did not show any significant improvement on UC patients in remission and in patients with left-sided UC [82,83].

5.2. Butyrogenic diets

As IBD is associated with decreased butyrate-producing bacteria and butyrate production in the colon, many investigators have tested if intake of butyrate producing fermentable dietary fibers could be beneficial for IBD. Fernandez-Banares et al. [88] observed increased concentrations of fecal butyrate after the intake of fiber rich plantago ovata seeds in UC patients. Moreover, plantago ovata seed supplementation showed effectiveness in maintaining UC remission. Further studies confirmed that butyrogenic diet supplementations attenuated colonic inflammation by regulation of gut microbial balance, increased production of SCFAs, upregulation of anti-inflammatory cytokines and Treg cells, and reduced the mucosal damage (Table 4). Fiber rich diet such as oat bran and germinated barley foodstuff have shown positive effects in IBD, especially in reducing risk of relapse while maintaining prolonged remission in UC patients [89,90]. It was shown that β-glucan derived from oats and barley ameliorates colitis through the regulation of tight junction proteins and inhibition of pro-inflammatory factors by increased SCFAs production via gut microbial fermentation [91,92]. IBD patients showed good tolerability to dietary fiber intake, particularly during clinical remission stage [90,93]. Despite the beneficial effects of fiber, IBD patients are advised to reduce fiber consumption during disease exacerbation period. Thus, the long term effects of high fiber intake in active CD remain uncertain due to limited clinical data [94,95].

5.3. Combination therapies

A combination therapy is a treatment modality that combines two or more therapeutic agents. It is found in most of the studies that combination therapies with butyrate and other agents are more effective than single therapy in the treatment of IBD or colitis models. Please see Table 5 for a summary of the outcomes of the studies (Table 5). Combinations of butyrate with other SCFAs, prebiotics, and probiotics have been investigated. Mixture of butyrate, Pistacia atlantica, and Lactobacillus casei or butyrate, Lactobacillus casei, and L-carnitin showed synergistic effects than single agent on TNBS-induced rat colitis model [101,102]. Combination of SCFAs, mainly acetate, propionate, and butyrate, showed increased effects against colitis [78]. However, treatment of SCFAs rectal enema (sodium acetate, propionate, and butyrate) did not improve histological and clinical state of left-sided UC [103]. Co-administration of sodium butyrate and mesalazine improved the efficacy of oral mesalazine in UC patients [104].