The functional roles of short chain fatty acids as postbiotics in human gut: future perspectives

Abstract

The significance of gut microbiome and their metabolites (postbiotics) on human health could be a promising approach to treat various diseases that includes inflammatory bowel diseases, colon cancer, and many neurological disorders. Probiotics with potential mental health benefits (psychobiotics) can alter the gut-brain axis via immunological, humoral, neuronal, and metabolic pathways. Recently, probiotic bacteria like Lactobacillus and Bifidobacterium have been demonstrated for SCFAs production, which play a crucial role in a variety of diseases. These acids could enhance the production of mucins, antimicrobial proteins (bacteriocins and peptides), cytokines (Interleukin 10 and 18) and neurotransmitters (serotonin) in the intestine to main the gut microbiota, intestinal barrier system and other immune functions. In this review, we discuss about two mechanisms such as (i) SCFAs mediated intestinal barrier system, and (ii) SCFAs mediated gut-brain axis to elucidate the therapeutic options for the treatment/prevention of various diseases.

Introduction

Probiotics are living organisms that exert health benefits in humans (Hotel and Cordoba, 2001). The primary carbon flux from the diet to the host microbiota is represented by short chain fatty acids viz. butyrate, acetate and propionate. To sustain the host’s normal health and defend against various diseases, intestinal equilibrium must be preserved in the gut. Numerous studies are currently being conducted on the relationship between diet and significance of beneficial bacteria for human welfare (Markowiak-Kopec and Slizewska, 2020). Probiotics can directly interact with the gut epithelial and immune cells and they can release active metabolites like short-chain fatty acids (SCFAs), which have a number of anti-inflammatory and cytoprotective effects (Markowiak and Sliżewska, 2017). They are long-term, incapacitating gastrointestinal (GI) disorders that result in a variety of clinical symptoms like abdominal discomfort, diarrhoea, and rectal bleeding.

Individuals showed similar microbial metabolic pathways while having different gut microbial compositions, highlighting the significance of functional genes. Understanding the inter-individual variety of the gut microbiota requires the use of functional metabolomic methods. For example, dysbiosis, which has been linked to chronic conditions like obesity, diabetes, and inflammatory bowel disease, may be a key therapeutic target. Metabolites generated from beneficial bacteria (postbiotics) such as short-chain fatty acids (SCFAs), exopolysaccharides (EPS), vitamins, teichoic acids, bacteriocins, enzymes and peptides offer significant potential in the treatment of gut dysbiosis by directly altered the gut microbiome by increasing the EPS secretion and also prevent pathogen colonization by producing antimicrobial compounds like proteins and peptides. The immunomodulation may also regulated by SCFAs synthesis by promoting IL-10 and IL-18 functions. These characteristic features of postbiotics could improve the host health.

Subepithelial myofibroblasts are a key source of prostaglandins (PG), which regulating the activity of intestinal epithelial cells. SCFAs activate E1 and E2 prostaglandins production that enhance mucin secretion and provide muco-protection against infections. Antigen presenting cells (APCs), white blood cells (neutrophil and monocytes) and endocrine cells have SCFAs receptors like FFAR2 and FFAR3 (Free fatty acid receptors) which regulates/promotes IL-10, IL18 and other antimicrobial compounds to eliminate pathogens from the gastrointestinal tract. In addition, SCFAs induces the stimulation of 5-HT (3) receptors on the vagal sensory fibres which transform signals from the gut into brain axis. These findings of SCFAs provide new perspectives of psychobiotics.

Probiotics have been a fascinating therapeutic option for IBDs for over a decade. A dynamic and intricate gut microbiota is continuously exposed to the intestine. Interaction between commensal bacteria and immune cells are being disrupted in IBDs. The function and diversity of the gut microbiota are altered (dysbiosis) by dysregulated mucosal immune response and it serve as physiopathology’s cornerstone (Abraham and Medzhitov, 2011). Inflammatory cells emit different compounds which invade the intestinal mucosa in IBDs. These compounds induce inflammation in the gut, which causes illness and tissue damage (Toumi et al., 2021).

Other than IBD, obesity and type 2 diabetes also characterised by a decreased number of certain microbes as well as their metabolites leads to leaky gut, and impaired metabolism balance (Cani, 2014). It has been shown that daily consumption of SCFAs and glucose as a metabolic fuel has significant impact in humans. These acids, especially acetic acid, butyric acid, and propionic acid, must be present in the human body for the host to be healthy (Inoue et al., 2014). However, the necessary substrates like prebiotics must be available in the intestine to increase the fermentation processes in order to produce SCFAs. These acids are necessary for the body to maintain immunological and gastrointestinal homeostasis. In addition, SCFAs can also act as a mediator for gut-brain axis (Dalile et al., 2019). SCFAs regulate anti-inflammatory activity through receptors and enzymes mediated pathways viz. G-coupled protein receptors and inhibition of histone deacetylases. The antagonistic activity of SCFAs has been linked to mucin synthesis. Mucus layer provide energy to colon cells as well as commensal bacteria. It also plays crucial role in disease management which caused by Helicobacter pylori, Salmonella, Rotavirus. MUC2 gene expression was influenced by SCFAs and exert antagonistic activity against various pathogens through various aspects viz. mucin synthesis, maintenance of commensal bacteria, regulation of immune cells. In this review, we discussed about SCFAs mediated (i) gut barrier function including immune responses and (ii) gut-brain axis which is linked to pathogenesis of several diseases.

The role of probiotics, prebiotics and postbiotics

Probiotics enhance the intestinal epithelial barrier system through a number of processes viz. pathogen defence through food competition, synthesis of antibacterial agents, activation of immunoglobulin and mucin secretion (Omenetti and Pizarro, 2015). Commensal bacteria affect the host health by various aspects, for example it often has a major impact on brain functions including motivation and mood (Sommer and Bäckhed, 2013). Probiotics have significant positive effects on gut flora and overall health, either employed alone or in combination with prebiotics. The most well-known prebiotics are Fructo-Oligosaccharides (FOS), that support the growth and activity of Lactobacilli and Bifidobacteria. Thus, probiotics and prebiotics (Synbiotics) combined food products have a significant effect on host health. The orally administered probiotic cocktail containing Lactobacillus sp. and Bifidobacterium sp. and Streptococcus sp. demonstrated for decreased remission rate in colitis patients when traditional medication failed to response (Bibiloni et al., 2005). The use of synbiotics considered as a beneficial method that tackles intestinal dysbiosis and the dysregulated immune response. Especially, probiotics either alone or with prebiotics enhance the production of beneficial metabolites (postbiotics) in humans. Postbiotics don’t contain live bacteria; therefore they have a lower risk of adverse reactions (Żółkiewicz et al., 2020). Postbiotics viz. EPS, SCFAs, vitamins, phenols, enzymes and peptides exert synergetic effects in humans. Moreover, postbiotics are stored easily at 25 °C without losing their biological activity for longer period (Salminen et al., 2021). Moreover, it has been demonstrated that butyrate can inhibit bacterial invasion and translocation from the intestine to the circulation by down regulating the expression of pathogenicity genes in Salmonella strains. Therefore, SCFAs decrease the virulence of Salmonella and can reduce the severity of infections. Similar antagonistic activity was observed for enterohemorrhagic Escherichia coli (EHEC) strains. Table 1 shows SCFAs produced by probiotics.

Table 1.

Recent reports on SCFAs produced by probiotics

SCFAs	Probiotic strains	Health benefits	Reference
Acetate	Bifidobacterium bifidum	Induced cytotoxicity and apoptosis in Caco-2 cells	Lim et al. (2023)
Butyrate	Saccharomyces boulardii, Lactobacillus rhamnosus, Lactobacillus acidophilus, and Bifidobacterium breve	Reduce intestinal inflammation	Di Martino et al. (2023)
	Clostridium butyricum	Anti-inflammatory activity and expression of tight junction proteins	Choi et al. (2023)
	Lactobacillus sp. and Enterococcus faecium	Anti-inflammatory activity by producing IL-10 and IL-6	Moens et al. (2019)
Acetate and butyrate	Adlercreutzia and Bifidobacterium animalis	Prevent the incidence of colitis-related lung injury	Nan et al. (2023)
	Lachnospiraceae NK4A136	maintain gut system	Ma et al. (2020)
Propionate and butyrate	Lactobacillus rhamnosus BFE5264	Cholesterol lowering effects	Park et al. (2018)

Health benefits of SCFAs

The gut microbiota ferment dietary fibers to produce SCFAs viz. acetic, propionic, and butyric acids. The commensal bacteria Faecalibacterium prausnitzii and Eubacterium rectale are the primary producers of butyrate and it has been demonstrated for anti-inflammatory properties (Auger et al., 2022). Butyrate provides vigour to colonocytes due to its role in the restoration of gut epithelium and its capacity to modify gene express which cause inflammatory bowel disease (IBD) and ulcerative colition by ineffectively blocking histone deacetylases. Additionally, it possesses immuno suppression qualities (Żółkiewicz et al., 2020). Also, its demonstrated to encourage tolerance towards food by up regulating interleukins (IL-1,18,33,25), Caspase, Mitogen activated kinase, toll like receptors and down regulating several cytokines like interferons, interleukin 10, transforming growth factor β. Furthermore, butyrate inhibits the function of the NF-B1 and its signalling system within the cells, which has immunosuppressive effects in humans (Lee et al., 2017). It has been linked to gastrointestinal functions such as hypertension, biological rhythms, innate and adaptive immune regulation (Pluznick, 2017; Tahara et al., 2018). It has roles in the host’s insulin sensitivity and appetite management, which may help in the treatment of metabolic disorders and the prevention of obesity and diabetes (Shimizu et al., 2019). Moreover, SCFAs provide building blocks for the production of endogenous glucose and lipids (Park et al., 2018; Van de Wouw et al., 2017).

These acids can affect the energy levels by inducing transmembrane receptors especially, GPCRs and production of glucagon like GLP-1. Acetate induced GLP-1 was linked to insulin sensitivity and reduced body fat (Aoki et al., 2017). The increased acetate level induces the release of GLP-1 and pancreatic peptide YY which ultimately reduce the blood sugar level. Therefore, SCFAs producing bacteria may enhance the host glucose metabolism which is crucial for diabetic patients (Chen et al., 2023). Supplementation of fiber rich food along with acetic acid could modulate the GI flora to inhibit the development of hypertension which leads to prevention of cardiac infraction in animals (Marques et al., 2017). Propionate serves as a major substrate for gluconeogenesis in the liver. It also regulates the metabolism of carbohydrates as well as blocking the cholesterol production pathway. Similar to butyrate, propionate exhibits an in vivo anti-inflammatory activity (Park et al., 2018). However, SCFAs production varies among individuals based on their diet and genetic backgrounds. Health benefits of SCFAs illustrated in Fig. 1.

Fig. 1.

Health benefits of SCFAs

The effect of SCFAs on various signalling pathways

SCFAs mediated functions mainly, regulated by two signalling pathway viz. receptors mediated signalling pathway (Fig. 2) and enzyme (histone deacetylases) mediated signalling pathway. Ionised SCFA activates transmembrane receptors (GPR41) and fatty acid binding proteins like FFAR, and GPR109a, which then transmit signals to the nucleus via extracellular activity (Brown et al., 2003; Nohr et al., 2015). Numerous studies revealed the strong relationship between metabolism and immunological responses of SCFAs (de Vos et al., 2022; Yonezawa et al., 2013). It has been shown that SCFAs alter the metabolism of innate immune cells such macrophages, monocytes, and neutrophils as well as block HDACs and reduce transcription factors (NF-B) involved in the immune response (Ratajczak et al., 2019; Zheng et al., 2017). Most of the cells viz. epithelial cells, lipocytes, WBCs, enteroendocrine L cells and nerve cells involved in autonomic and somatic nervous system express fatty acid specific G coupled protein receptors on their surface (Tan et al., 2017). Histone deacetylases (HDACs) are involved in the epigenetic regulation of gene expression and can be inhibited intracellularly. Although all cells naturally express the HDAC family of enzymes, SCFA only seems to inhibit diabetes 1 and 2. Therefore, fatty acids may occupy the active sites on the receptor proteins in human cell lines especially butyric acid is the most effective histone deacetylases inhibitor (Davie et al., 2003; Sekhavat et al., 2007; Waldecker et al., 2008). SCFAs may activate GPR41 and GPR43, to imparting MPAK signalling and induce the synthesis of cytokines (Kim et al., 2013). SCFAs are highly effective stimulators of Tregs (scurfin) to induce anti-inflammatory effects and regulate autoimmunity by activating GPR109A in phagocytes (Singh et al., 2014). In LPS induced monocytes, butyric acid and propionic acid decrease the production of nitric oxide synthase and tumor necrosis factor (Vinolo et al., 2011). Additionally, colon cancer cells exposed to butyrate can undergo TLR4-mediated phosphorylation of MAPK (Xiao et al., 2018). Butyrate and propionate administration can upregulate IL-10 expression in lipopolysaccharide induced monocytes and macrophages through histone deacetylase inhibition. It can also suppress the release of TNF and the activity of NF-B (Chang et al., 2014; Masui et al., 2013). Additionally, SCFAs have a significant impact on the immune system not only in the gut but also in the liver, the lungs, and the central nervous system (Dragano et al., 2017).

Fig. 2.

SCFAs mediated signalling pathways

SCFAs mediated intestinal barrier function

Probiotic strains are frequently used to enhance the host immunity by maintaining commensal bacteria in the GI. Inflammation of the digestive system that occurs repeatedly and persistently is the hallmark of IBD, which mostly includes ulcer, colon cancer. One of the histological signs of colitis is a persistent superficial mucous membrane irritation that spreads entire colon. In UC, inflammation only affects the mucosa not the muscles (Štofilová et al., 2022). Prostaglandins are differently regulated by SCFAs which stimulate the MUC2 gene expression in GI epithelial cells. UC patients have defect in MUC2 gene expression which leads to leaky gut. Environmental variables in conjunction with human genetics, an impaired gut microbiome and immune system are crucial components in the development of UC. However, the pathophysiology of the disease is still unknown (Chang, 2020). Therefore, the management of UC requires the creation of novel medications and the identification of complementary approaches that target microbial dysbiosis. Probiotics are used for the treatment of gut dysbiosis to alter commensal microbes, enhance the protective role of intestine, and balance the immune system (Selvamani et al., 2022).

Probiotics produce metabolites like SCFA that lower the pH of GI and make it hostile to pathogens in addition to competing with pathogenic bacteria for nutrition and adhesion sites (Todorov et al., 2020). Additionally, their independent treatment can result in modifications in gut microbiota linked to enhanced SCFA synthesis, as well as the inhibition of inflammation (Davani-Davari et al., 2019). Probiotics interact with immune cells and the intestinal epithelium, demonstrating an immunomodulatory effect. The innate immune system and their receptors like TLR present on epithelial as well as APCs which detect the presence of probiotics (Price et al., 2018). By preventing the synthesis of NF-kB by triggering immuno regulation of regulatory T cells in the intestine, probiotics can reduce inflammatory processes in UC (Llewellyn & Foey, 2017). The anti-inflammatory properties of probiotics are primarily seen as an up regulation of tight junction proteins, up regulation of mucin synthesis and interleukin-10 production (Din et al., 2020). The summary of SCFAs mediated intestinal barrier function was illustrated in Fig. 3. SCFAs especially, butyrate has an inhibitory effect on HDAC activity by influencing the cell growth, cell division, inflammation and assisting in maintaining intestinal homeostasis and preventing cancer (Chriett et al., 2019). HDACs influence the development of myeloid cells and the inflammatory response, which is regulated via toll like receptors and expression of viperin protein (Shakespear et al., 2011). Additionally, the treatment of HDACs inhibitors in experimental murine colitis lowers disease severity and prevents the production of interleukin-6, interferons, tumour necrosis factor (Yoon et al., 2022). These results highlight the significance of butyrate as an HDAC inhibitor and are encouraging in the search for alternatives to current therapies for IBD.

Fig. 3.

SCFAs mediated intestinal barrier function

Goblet cells continuously manufacture mucins, however SCFAs can control this process by influencing the expression of the mucin gene. In addition to a study, Muc2 production in the colon was greatly boosted when butyrate served as the only carbon source. Muc2 may be dependent on GPR109A receptor in goblet cells (Wu et al., 2022). Enhanced Muc2 gene expression by butyrate, and acetate can effectively protect the GI from pathogens in animals. Some investigations have demonstrated that SCFAs are involved in the transcription of MUC-2 by HDAC at the promotor region (Panebianco et al., 2022). These findings suggest that SCFAs control GI permeability primarily through raising Muc2 levels, which strengthens the host’s defences against invading infections (Liu et al., 2021). Also, down regulation of MCT1 reduced SCFAs absorption which cause inflammatory bowel disease (IBD) and ulcerative colitis in humans. Thus, postbiotics are essential for establishing stable populations of gut microbiota in order to maintaining intestinal homeostasis and epithelial barrier function. Therefore, SCFAs either alone or along with probiotics could be a promising therapeutic option for treating various GI diseases.

SCFAs mediated gut-brain axis

SCFAs have pleiotropic effects on epigenetic regulation, neuro-inflammation modulation, blood-brain barrier maintenance, regulation of brain metabolism, and a plaque development (Yadav et al., 2022). Furthermore, it has been shown that gut dysbiosis directly affects numerous pathways which are related to synaptic plasticity, neuronal growth and repair, memory, and learning processes. Aging and lifestyle elements like diet, exercise, sleep, and stress have an impact on the risk of AD through gut dysbiosis (Tarawneh and Penhos, 2022). Through their effects on cognitive function and the pathophysiology of neurodegenerative diseases (NDD), SCFAs have a wide range of therapeutic advantages. In addition, alterations in the gut microbiota, such as a rise in the number of pathogens and opportunistic bacteria, are linked to NDDs. By controlling inflammation and oxidative stress, SCFAs support healthy mitochondrial function and promote the maturation of microglia, which inhibits the onset of NDD and cognitive decline (Yadav et al., 2022). According to Ramirez et al. (2017), persistent stress poses a significant risk for the emergence of neuropsychiatric diseases, while Cruz-Pereira et al. (2000) found that the gut mediated brain axis was crucial in the interaction between stress and the brain.

Recent clinical studies have demonstrated the potential properties of SCFAs to modify the primary neuro-endocrine system responsiveness in brain (Dalile et al., 2020). SCFAs regulate the inflammatory response, neuronal apoptosis, oxidative stress, the blood brain barrier’s (BBB) integrity, and other processes in central nervous system (CNS) diseases (Dong and Cui, 2022). SCFAs have been linked to microglial maturation and astrocyte gene expression in humans (Erny et al., 2017; Spichak et al., 2021). As the BBB prevents viruses and unwelcome poisons from entering the brain and helps to maintain brain homeostasis, it must be taken into account when analysing the direct impact of short chain fatty acids on the cerebrum. Butyrate administered mice exhibits decreased permeability and increased TJP expression that ultimately improved BBB integrity (Braniste et al., 2014). Both the peripheral and central nervous systems consist of neuronally expressed SCFA receptors. It has been demonstrated that SCFAs can trigger the mid-colon to produce serotonin (5-HT), a neurotransmitter. Acetic acid was found to alter the serotonin action through lowering the expression of serotonin receptor like 5-HT3B (Bhattrarai et al., 2017; Dalile et al., 2020). Increased levels of SCFAs have been linked to dietary fibers and amino acids, which are necessary for the production of serotonins in the GI to maintain healthy mind (Singh et al., 2017). Table 2 summarises the effect of SCFA in various CNS diseases. According to recent studies, SCFAs may block the ghrelinR which improve the glycemic control in humans (Silva et al., 2020; Torres-Fuentes et al., 2019). Leptin and insulin levels also influenced by SCFAs. Additionally, it has been demonstrated that butyrate can decrease appetite by decreasing stimulation of a group of neuropeptide Y expressing neurons in the hypothalamus (Li et al., 2018). In neurons expressing neuropeptide Y, FFAR activation suppresses or exigenic hypothalamic activity that associated with hunger and biological rhythms. Interaction of SCFAs with gut GLP-1 and Peptide YY send signals to the cerebrum through vagus nerve/blood flow. This interaction has an impact on mood, memory, and learning (O’Riordan et al., 2022). Figure 4 shows SCFAs mediated brain functions.

Table 2.

Beneficial effects of SCFAs in CNS system

SCFAs	Beneficial effects	References
Acetate	Altered expression levels of anorexigenic neuropeptides, glutamate, GABA, and glutamine in the hypothalamus	Fung et al. (2017)
Butyrate	Pro-inflammatory mediators are downregulated, while anti-inflammatory mediators are upregulated, via epigenetically controlling the microglia response	Patnala et al. (2017)
	Histone crotonylation in the brain	Fellows et al. (2018)
	Non-inflammatory and inflammatory conditions led to the reversible extension of microglial processes via Akt activation	Wang et al. (2018)
Propionate	High-dose propionate administered to mouse lungs, simulating antibiotic exposure, changed SCFA levels leading to a reduced immunological containment of Staphylococcus aureus pneumonia	Tian et al. (2019)
	Reduced expression of CD41 in hCMEC/D3 cells reduced oxidative stress and inflammation and impacted the trans localization of NFE2L2, Nrf2 implicated in oxidative stress	Hoyles et al. (2018)
Acetate & propionate	Short chain fatty acid induced stimulation of insulin secretion and reduction of apoptosis in mouse	Pingitore et al. (2019)
Acetate, Propionate & butyrate	Leptin levels have been decreased and body weight/fat accumulation to be suppressed when SCFA levels are elevated.	Gabriel and Fantuzzi (2019)

Fig. 4.

SCFAs mediated brain function

SCFAs prevented the production of harmful oligomers from amyloid peptides which is responsible for progression of Alzheimer’s disease (AD). The decreased SCFA levels in several diseases suggest that SCFA supplementation is a promising therapeutic option for many diseases. Patnala et al. (2017) reported that butyric acid has the ability to prevent neuronal damage by reducing microglial cells, lowering proinflammatory marker levels, and elevating anti-inflammatory marker levels. The butyrate producing C. butyricum prevented inflammation caused by microglia in Tg2576 mice (Sun et al., 2020). Similarly, acetic acid prevents inflammation in Tg2576 mice by activation of free fatty acid receptors and the regulation of MAPK signalling cascade (Liu et al., 2020). Hence, SCFAs have the potential to regulate neuronal development and excitability by directly modulating the serotonins levels and neurotrophic factors in enteric and central nervous system. Additionally, it has been demonstrated for altered HPA axis, which controls respond to stress. Thus, SCFAs may improve the mental health via gut-brain axis.

Future perspectives of SCFAs

SCFAs can alter the pathophysiology of the cancer environment via significant signalling systems like GPRCs and FFAR3. The creation of novel targeted colonic delivery techniques in conjunction with postbiotic supplements may be the best options for achieving optimal targeted SCFA delivery for treating various gut related diseases. To establish the correlation and comprehend the potential contribution of prebiotics or diets that increase SCFA in the body to unravelling mechanistic linkages, more research is required. Better eating habits should be promoted since they improve mental health. However, advanced study on brain physiology, behaviour, and function is required, and it should be supported in order to develop therapeutic interventions for brain diseases. More investigation is required to pinpoint the precise method through which gut microbiota/probiotics controls the brain function in humans. Also, it can be incorporated into various functional food products to generate next generation of probiotics (psychobiotics) in humans.

Declarations

Conflict of interest

The authors declare no conflicts of interest in this work.