Formation of short chain fatty acids by the gut microbiota and their impact on human metabolism

Understanding the Holobiont: How Microbial Metabolites Affect Human Health and Shape the Immune System

The human gastrointestinal tract is populated by a diverse, highly mutualistic microbial flora, which is known as the microbiome. Disruptions to the microbiome have been shown to be associated with severe pathologies of the host, including metabolic disease, cancer, and inflammatory bowel disease. Mood and behavior are also susceptible to alterations in the gut microbiota. A particularly striking example of the symbiotic effects of the microbiome is the immune system, whose cells depend critically on a diverse array of microbial metabolites for normal development and behavior. This includes metabolites that are produced by bacteria from dietary components, metabolites that are produced by the host and biochemically modified by gut bacteria, and metabolites that are synthesized de novo by gut microbes. In this review, we highlight the role of the intestinal microbiome in human metabolic and inflammatory diseases and focus in particular on the molecular mechanisms that govern the gut-immune axis.

Significance of Microbiota in Obesity and Metabolic Diseases and the Modulatory Potential by Medicinal Plant and Food Ingredients

Metabolic syndrome is a cluster of three or more metabolic disorders including insulin resistance, obesity, and hyperlipidemia. Obesity has become the epidemic of the twenty-first century with more than 1.6 billion overweight adults. Due to the strong connection between obesity and type 2 diabetes, obesity has received wide attention with subsequent coining of the term "diabesity." Recent studies have identified unique contributions of the immensely diverse gut microbiota in the pathogenesis of obesity and diabetes. Several mechanisms have been proposed including altered glucose and fatty acid metabolism, hepatic fatty acid storage, and modulation of glucagon-like peptide (GLP)-1. Importantly, the relationship between unhealthy diet and a modified gut microbiota composition observed in diabetic or obese subjects has been recognized. Similarly, the role of diet rich in polyphenols and plant polysaccharides in modulating gut bacteria and its impact on diabetes and obesity have been the subject of investigation by several research groups. Gut microbiota are also responsible for the extensive metabolism of polyphenols thus modulating their biological activities. The aim of this review is to shed light on the composition of gut microbes, their health importance and how they can contribute to diseases as well as their modulation by polyphenols and polysaccharides to control obesity and diabetes. In addition, the role of microbiota in improving the oral bioavailability of polyphenols and hence in shaping their antidiabetic and antiobesity activities will be discussed.

Distinct patterns in the gut microbiota after surgical or medical therapy in obese patients

Bariatric surgery is highly successful in improving health compared to conventional dietary treatments. It has been suggested that the gut microbiota is a relevant factor in weight loss after bariatric surgery. Considering that bariatric procedures cause different rearrangements of the digestive tract, they probably have different effects on the gut microbiota. In this study, we compared the impact of medical treatment, sleeve gastrectomy and Roux-en-Y gastric bypass on the gut microbiota from obese subjects. Anthropometric and clinical parameters were registered before, 6 and 12 months after treatment. Fecal samples were collected and microbiota composition was studied before and six months post treatment using 16S rRNA gene sequencing and qPCR. In comparison to dietary treatment, changes in intestinal microbiota were more pronounced in patients subjected to surgery, observing a bloom in Proteobacteria. Interestingly, Bacteroidetes abundance was largely different after six months of each surgical procedure. Furthermore, changes in weight and BMI, or glucose metabolism, correlated positively with changes in these two phyla in these surgical procedures. These results indicate that distinct surgical procedures alter the gut microbiota differently, and changes in gut microbiota might contribute to health improvement. This study contributes to our understanding of the impact of weight loss surgery on the gut microbiota, and could be used to replicate this effect using targeted therapies.

Microbial Interactions and Interventions in Colorectal Cancer

Recently, several lines of evidence that indicate a strong link between the development of colorectal cancer (CRC) and aspects of the gut microbiota have become apparent. However, it remains unclear how changes in the gut microbiota might influence carcinogenesis or how regional organization of the gut might influence the microbiota. In this review, we discuss several leading theories that connect gut microbial dysbiosis with CRC and set this against a backdrop of what is known about proximal-distal gut physiology and the pathways of CRC development and progression. Finally, we discuss the potential for gut microbial modulation therapies, for example, probiotics, antibiotics, and others, to target and improve gut microbial dysbiosis as a strategy for the prevention or treatment of CRC.

Introduction to the human gut microbiota

The human gastrointestinal (GI) tract harbours a complex and dynamic population of microorganisms, the gut microbiota, which exert a marked influence on the host during homeostasis and disease. Multiple factors contribute to the establishment of the human gut microbiota during infancy. Diet is considered as one of the main drivers in shaping the gut microbiota across the life time. Intestinal bacteria play a crucial role in maintaining immune and metabolic homeostasis and protecting against pathogens. Altered gut bacterial composition (dysbiosis) has been associated with the pathogenesis of many inflammatory diseases and infections. The interpretation of these studies relies on a better understanding of inter-individual variations, heterogeneity of bacterial communities along and across the GI tract, functional redundancy and the need to distinguish cause from effect in states of dysbiosis. This review summarises our current understanding of the development and composition of the human GI microbiota, and its impact on gut integrity and host health, underlying the need for mechanistic studies focusing on host-microbe interactions.

Metabolites: messengers between the microbiota and the immune system

In this review, Levy et al. highlight examples of how microbiota-modulated metabolites control the development, differentiation, and activity of the immune system and classify them into functional categories that illustrate the spectrum of ways by which microbial metabolites influence host physiology.

The mammalian intestine harbors one of the largest microbial densities on Earth, necessitating the implementation of control mechanisms by which the host evaluates the state of microbial colonization and reacts to deviations from homeostasis. While microbial recognition by the innate immune system has been firmly established as an efficient means by which the host evaluates microbial presence, recent work has uncovered a central role for bacterial metabolites in the orchestration of the host immune response. In this review, we highlight examples of how microbiota-modulated metabolites control the development, differentiation, and activity of the immune system and classify them into functional categories that illustrate the spectrum of ways by which microbial metabolites influence host physiology. A comprehensive understanding of how microbiota-derived metabolites shape the human immune system is critical for the rational design of therapies for microbiota-driven diseases.

The ecological community of commensal, symbiotic, and pathogenic gastrointestinal microorganisms - an appraisal

The human gastrointestinal tract is inhabited by a vast population of bacteria, numbering ~100 trillion. These microorganisms have been shown to play a significant role in digestion, metabolism, and the immune system. The aim of this study was to review and discuss how the human body interacts with its gut microbiome and in turn the effects that the microorganisms have on its host, overall resulting in a true mutualistic relationship.

Microbiome and colorectal cancer: Unraveling host-microbiota interactions in colitis-associated colorectal cancer development

Dysbiosis of gut microbiota occurs in many human chronic immune-mediated diseases, such as inflammatory bowel disease (IBD) and colitis-associated colorectal cancer (CAC). Reciprocally, uncontrolled immune responses, that may or may not be induced by dysbiosis, are central to the development of IBD and CAC. There has been a surge of interest in investigating the relationship between microbiota, inflammation and CAC. In this review, we discuss recent findings related to gut microbiota and chronic immune-mediated diseases, such as IBD and CAC. Moreover, the molecular mechanisms underlying the roles of chronic inflammation in CAC are examined. Finally, we discuss the development of novel microbiota-based therapeutics for IBD and colorectal cancer.

Gamma-Aminobutyric Acid Increases the Production of Short-Chain Fatty Acids and Decreases pH Values in Mouse Colon

Gamma-Aminobutyric acid (GABA) could regulate physiological functions in the gastrointestinal tract. The present study aimed to investigate the effect of GABA on colon health in mice. The female Kunming mice were given GABA at doses of 5, 10, 20 and 40 mg/kg/d for 14 days. Afterwards, the short-chain fatty acids (SCFAs) concentrations, pH values, colon index, colon length and weight of colonic and cecal contents were determined to evaluate the effects of GABA on colon health. The results showed that intake of GABA could increase the concentrations of acetate, propionate, butyrate and total SCFAs in colonic and cecal contents, as well as the weight of colonic and cecal contents. The colon index and length of the 40 mg/kg/d GABA-treated group were significantly higher than those of the control group (p < 0.05). In addition, decrease of pH values in colonic and cecal contents was also observed. These results suggest that GABA may improve colon health.

Fecal Short-Chain Fatty Acid Variations by Breastfeeding Status in Infants at 4 Months: Differences in Relative versus Absolute Concentrations

Our gut microbiota provide a number of important functions, one of which is the metabolism of dietary fiber and other macronutrients that are undigested by the host. The main products of this fermentation process are short-chain fatty acids (SCFAs) and other intermediate metabolites including lactate and succinate. Production of these metabolites is dependent on diet and gut microbiota composition. There is increasing evidence for the role of SCFAs in host physiology and metabolic processes as well as chronic inflammatory conditions such as allergic disease and obesity. We aimed to investigate differences in fecal SCFAs and intermediate metabolites in 163 infants at 3-5 months of age according to breastfeeding status. Compared to no exposure to human milk at time of fecal sample collection, exclusive breastfeeding was associated with lower absolute concentrations of total SCFAs, acetate, butyrate, propionate, valerate, isobutyrate, and isovalerate, yet higher concentrations of lactate. Further, the relative proportion of acetate was higher with exclusive breastfeeding. Compared to non-breastfed infants, those exclusively breastfed were four times more likely (aOR 4.50, 95% CI 1.58-12.82) to have a higher proportion of acetate relative to other SCFAs in their gut. This association was independent of birth mode, intrapartum antibiotics, infant sex, age, recruitment site, and maternal BMI or socioeconomic status. Our study confirms that breastfeeding strongly influences the composition of fecal microbial metabolites in infancy.

Treatment of reactive hypoglycemia with the macrobiotic Ma-pi 2 diet as assessed by continuous glucose monitoring: The MAHYP randomized crossover trial

BACKGROUND AND AIMS

Nutritional therapy is recommended for management of reactive hypoglycemia (RH), a condition characterized by hypoglycemia that occurs within four hours after a meal. The macrobiotic Ma-Pi 2 diet improves glycemic control in subjects with type 2 diabetes. We explored the effect of this diet on outcomes in non-diabetic individuals with RH.

MATERIALS AND METHODS

Twelve subjects with RH were randomized to the Ma-Pi 2 diet for three days and a control diet for three days in a randomized crossover design. Subjects received snacks on two days out of each three-day period only, and were monitored using continuous glucose monitoring. The 24-h period was divided into daytime (08:00-22:30h [subdivided into 'daytime without snacks' and 'daytime with snacks']) and night-time (22:31-07:59h). The effects of the two diets on the number of RH events (blood glucose <70mg/dL [3.9mmol/L]) and the percentage distribution of glucose readings within each of 16 glycemic intervals from <40mg/dL (2.2mmol/L) to >180mg/dL (4.4mmol/L) were determined.

RESULTS

There were significantly fewer RH events on the Ma-Pi 2 diet than the control diet during daytime without snacks (-2.5 events; 95% CI: -7.5, 0.0; P=0.022) and daytime with snacks (-4.25 events; 95% CI: -7.5; -2.0; P=0.013) but no difference at night. The percentage of glucose readings in the interval 71-80mg/dL (3.9-4.4mmol/L) was significantly higher on the control diet during daytime with and without snacks (P=0.03 for both), while the percentage of glucose readings in the interval 91-100mg/dL (5.1-5.6mmol/L) was significantly higher on the Ma-Pi 2 diet during daytime without snacks (P=0.02).

CONCLUSIONS

The macrobiotic Ma-Pi 2 diet reduced blood glucose excursions during the day, thereby facilitating glycemic control in subjects with RH. The Ma-Pi 2 diet represents an effective nutritional tool for management of RH.

Regulation of intestinal permeability: The role of proteases

The gastrointestinal barrier is - with approximately 400 m² - the human body’s largest surface separating the external environment from the internal milieu. This barrier serves a dual function: permitting the absorption of nutrients, water and electrolytes on the one hand, while limiting host contact with noxious luminal antigens on the other hand. To maintain this selective barrier, junction protein complexes seal the intercellular space between adjacent epithelial cells and regulate the paracellular transport. Increased intestinal permeability is associated with and suggested as a player in the pathophysiology of various gastrointestinal and extra-intestinal diseases such as inflammatory bowel disease, celiac disease and type 1 diabetes. The gastrointestinal tract is exposed to high levels of endogenous and exogenous proteases, both in the lumen and in the mucosa. There is increasing evidence to suggest that a dysregulation of the protease/antiprotease balance in the gut contributes to epithelial damage and increased permeability. Excessive proteolysis leads to direct cleavage of intercellular junction proteins, or to opening of the junction proteins via activation of protease activated receptors. In addition, proteases regulate the activity and availability of cytokines and growth factors, which are also known modulators of intestinal permeability. This review aims at outlining the mechanisms by which proteases alter the intestinal permeability. More knowledge on the role of proteases in mucosal homeostasis and gastrointestinal barrier function will definitely contribute to the identification of new therapeutic targets for permeability-related diseases.

Nonalcoholic Fatty Liver Disease, the Gut Microbiome, and Diet

Nonalcoholic fatty liver disease (NAFLD) is the most common liver disorder in the world, yet the pathogenesis of the disease is not well elucidated. Due to the close anatomic and functional association between the intestinal lumen and the liver through the portal system, it is speculated that the gut microbiome may play a pivotal role in the pathogenesis of NAFLD. Furthermore, diet, which can modulate the gut microbiome and several metabolic pathways involved in NAFLD development, shows a potential tripartite relation between the gut, diet, and the liver. In this review, we summarize the current evidence that supports the association between NAFLD, the gut microbiome, and the role of diet.

Diet and the Microbiome

The gut microbiota provides a range of ecologic, metabolic, and immunomodulatory functions relevant to health and well-being. The gut microbiota not only responds quickly to changes in diet, but this dynamic equilibrium may be managed to prevent and/or treat acute and chronic diseases. This article provides a working definition of the term "microbiome" and uses two examples of dietary interventions for the treatment of large bowel conditions to emphasize the links between diet and microbiome. There remains a need to develop a better functional understanding of the microbiota, if its management for clinical utility is to be fully realized.

Comparative study on the in vitro effects of Pseudomonas aeruginosa and seaweed alginates on human gut microbiota

Alginates pertain to organic polysaccharides that have been extensively used in food- and medicine-related industries. The present study obtained alginates from an alginate overproducing Pseudomonas aeruginosa PAO1 mutant by screening transposon mutagenesis libraries. The interaction between bacterial and seaweed alginates and gut microbiota were further studied by using an in vitro batch fermentation system. Thin-layer chromatography (TLC) analysis indicated that both bacterial and seaweed alginates can be completely degraded by fecal bacteria isolated from study volunteers, indicating that a minor structural difference between bacterial and seaweed alginates (O-acetylation and lack of G-G blocks) didn't affect the digestion of alginates by human microbiota. Although, the digestion of bacterial and seaweed alginates was attributed to different Bacteroides xylanisolvens strains, they harbored similar alginate lyase genes. Genus Bacteroides with alginate-degrading capability were enriched in growth medium containing bacterial or seaweed alginates after in vitro fermentation. Short-chain fatty acid (SCFA) production in both bacterial and seaweed alginates was also comparable, but was significantly higher than the same medium using starch. In summary, the present study has isolated an alginate-overproducing P. aeruginosa mutant strain. Both seaweed and bacterial alginates were degraded by human gut microbiota, and their regulatory function on gut microbiota was similar.

Probiotic mixture VSL#3 reduces colonic inflammation and improves intestinal barrier function in Muc2 mucin-deficient mice

MUC2 mucin is the major glycoprotein in colonic mucus that separates intestinal microbiota from underlying host cells and serves as a food source for some eubacteria. MUC2 deficiency results in impaired epithelial barrier function, imbalance in gut microbiota, and spontaneous colitis. Probiotics have been shown to have a protective effect against colitis. In this study we used Muc2 mucin-deficient (Muc2^-/-) and Muc2^+/+ littermates to test whether the probiotic mixture VSL#3 requires an intact mucin barrier to exert its beneficial effect. VSL#3 alone reduced basal colonic proinflammatory cytokine levels and improved epithelial barrier function in Muc2^-/- animals. Similarly, in dextran sulfate sodium-induced colitis, VSL#3 dampened the proinflammatory chemokines KC, monocyte chemoattractant protein-1, and macrophage inflammatory protein-2 and upregulated the tissue regeneration growth factors transforming growth factor-β, fibroblast growth factor-1, and vascular endothelial growth factor-A, which accelerated resolution of colitis symptoms in Muc2^-/- animals. Importantly, improved colonic health in VSL#3-treated animals was associated with attenuated reactive oxygen species production by peritoneal macrophages, restoration of antimicrobial peptide gene expression in the small intestine, and increased abundance of bacterial commensals in the gut. The beneficial effects of VSL#3 in Muc2^-/- animals were mediated by acetate, an important short-chain fatty acid produced by gut bacteria. These studies provide evidence for the first time that VSL#3 can enhance epithelial barrier function by dampening the proinflammatory cytokine and chemokine response, accelerating restitution, and altering commensal microbiota in the absence of a functional mucus barrier.

NEW & NOTEWORTHY

It is unclear whether probiotics require an intact mucin barrier to first colonize and/or exert their protective functions. In this study we used mucin-deficient (Muc2^-/-) mice to interrogate if the multispecies probiotic mixture VSL#3 could enhance epithelial barrier function. In the absence of a mucus bilayer, VSL#3 dampened proinflammatory and chemokine production, accelerated restitution, and markedly improved gut permeability mediated by the short-chain fatty acid acetate in the colon.

The infant gut bacterial microbiota and risk of pediatric asthma and allergic diseases

Among the many areas being revolutionized by the recent introduction of culture-independent microbial identification techniques is investigation of the relationship between close contact with large animals, antibiotics, breast feeding, mode of birth, and other exposures during infancy as related to a reduced risk of asthma and allergic disease. These exposures were originally clustered under the "Hygiene Hypothesis" which has evolved into the "Microbiota Hypothesis". This review begins by summarizing epidemiologic studies suggesting that the common feature of these allergy risk-related exposures is their influence on the founding and early development of a child's gut microbiota. Next, studies using culture-independent techniques are presented showing that children who have experienced the exposures of interest have altered gut microbiota. Finally, selected mouse and human studies are presented which begin to corroborate the protective exposures identified in epidemiologic studies by elucidating mechanisms through which microbes can alter immune development and function. These microbially driven immune alterations demonstrate that microbial exposures in many cases could alter the risk of subsequent allergic disease and asthma. Hopefully, a better understanding of how microbes influence allergic disease will lead to safe and effective methods for reducing the prevalence of all forms of allergic disease.

Formation of propionate and butyrate by the human colonic microbiota

The human gut microbiota ferments dietary non-digestible carbohydrates into short-chain fatty acids (SCFA). These microbial products are utilized by the host and propionate and butyrate in particular exert a range of health-promoting functions. Here an overview of the metabolic pathways utilized by gut microbes to produce these two SCFA from dietary carbohydrates and from amino acids resulting from protein breakdown is provided. This overview emphasizes the important role played by cross-feeding of intermediary metabolites (in particular lactate, succinate and 1,2-propanediol) between different gut bacteria. The ecophysiology, including growth requirements and responses to environmental factors, of major propionate and butyrate producing bacteria are discussed in relation to dietary modulation of these metabolites. A detailed understanding of SCFA metabolism by the gut microbiota is necessary to underpin effective strategies to optimize SCFA supply to the host.

Fecal microbiota transplantation in inflammatory bowel disease: the quest for the holy grail

Inflammatory bowel disease (IBD) is due to an aberrant immune response toward luminal antigens, probably commensal bacteria, in genetically susceptible subjects and is also influenced by environmental factors. An imbalanced intestinal microbiota known as "dysbiosis," characterized by an increased proportion of pro-inflammatory microorganisms and a decreased proportion of anti-inflammatory microorganisms, has been repeatedly observed in IBD and is now recognized as a key factor in the gut inflammatory process. Fecal microbiota transplantation (FMT) has gained interest as a novel treatment option in IBD. The goal of FMT in IBD is not only to correct the dysbiosis, but also to restore a normal dialog between the host immune system and the microbiota. Data are still scarce, but the results of the first studies suggest that FMT could be a promising therapy in IBD. More studies are needed to define the best indications, optimal timing, frequency, mode of delivery, and the optimal donor for each patient.

Branched short-chain fatty acids modulate glucose and lipid metabolism in primary adipocytes

Short-chain fatty acids (SCFAs), e.g. acetic acid, propionic acid and butyric acid, generated through colonic fermentation of dietary fibers, have been shown to reach the systemic circulation at micromolar concentrations. Moreover, SCFAs have been conferred anti-obesity properties in both animal models and human subjects. Branched SCFAs (BSCFAs), e.g., isobutyric and isovaleric acid, are generated by fermentation of branched amino acids, generated from undigested protein reaching colon. However, BSCFAs have been sparsely investigated when referring to effects on energy metabolism. Here we primarily investigate the effects of isobutyric acid and isovaleric acid on glucose and lipid metabolism in primary rat and human adipocytes. BSCFAs inhibited both cAMP-mediated lipolysis and insulin-stimulated de novo lipogenesis at 10 mM, whereas isobutyric acid potentiated insulin-stimulated glucose uptake by all concentrations (1, 3 and 10 mM) in rat adipocytes. For human adipocytes, only SCFAs inhibited lipolysis at 10 mM. In both in vitro models, BSCFAs and SCFAs reduced phosphorylation of hormone sensitive lipase, a rate limiting enzyme in lipolysis. In addition, BSCFAs and SCFAs, in contrast to insulin, inhibited lipolysis in the presence of wortmannin, a phosphatidylinositide 3-kinase inhibitor and OPC3911, a phosphodiesterase 3 inhibitor in rat adipocytes. Furthermore, BSCFAs and SCFAs reduced insulin-mediated phosphorylation of protein kinase B. To conclude, BSCFAs have effects on adipocyte lipid and glucose metabolism that can contribute to improved insulin sensitivity in individuals with disturbed metabolism.

Randomised clinical study: inulin short-chain fatty acid esters for targeted delivery of short-chain fatty acids to the human colon

BACKGROUND

Short-chain fatty acids (SCFA) produced through fermentation of nondigestible carbohydrates by the gut microbiota are associated with positive metabolic effects. However, well-controlled trials are limited in humans.

AIMS

To develop a methodology to deliver SCFA directly to the colon, and to optimise colonic propionate delivery in humans, to determine its role in appetite regulation and food intake.

METHODS

Inulin SCFA esters were developed and tested as site-specific delivery vehicles for SCFA to the proximal colon. Inulin propionate esters containing 0-61 wt% (IPE-0-IPE-61) propionate were assessed in vitro using batch faecal fermentations. In a randomised, controlled, crossover study, with inulin as control, ad libitum food intake (kcal) was compared after 7 days on IPE-27 or IPE-54 (10 g/day all treatments). Propionate release was determined using (13) C-labelled IPE variants.

RESULTS

In vitro, IPE-27-IPE-54 wt% propionate resulted in a sevenfold increase in propionate production compared with inulin (P < 0.05). In vivo, IPE-27 led to greater (13) C recovery in breath CO2 than IPE-54 (64.9 vs. 24.9%, P = 0.001). IPE-27 also led to a reduction in energy intake during the ad libitum test meal compared with both inulin (439.5 vs. 703.9 kcal, P = 0.025) and IPE-54 (439.5 vs. 659.3 kcal, P = 0.025), whereas IPE-54 was not significantly different from inulin control.

CONCLUSIONS

IPE-27 significantly reduced food intake suggesting colonic propionate plays a role in appetite regulation. Inulin short-chain fatty acid esters provide a novel tool for probing the diet-gut microbiome-host metabolism axis in humans.

α-Galactosides present in lupin flour affect several metabolic parameters in Wistar rats

The aim of this study is to evaluate the influence of α-galactosides present in a lupin diet on metabolic parameters in growing Wistar rats.