Nonalcoholic fatty liver disease (NAFLD) is a common, multifactorial, and poorly understood liver disease whose incidence is globally rising. During the past decade, several lines of evidence suggest that dysbiosis of intestinal microbiome represents an important factor contributing to NAFLD occurrence and its progression into NASH. The mechanisms that associate dysbiosis with NAFLD include changes in microbiota-derived mediators, deregulation of the gut endothelial barrier, translocation of mediators of dysbiosis, and hepatic inflammation. Changes in short chain fatty acids, bile acids, bacterial components, choline, and ethanol are the result of altered intestinal microbiota. We perform a narrative review of the previously published evidence and discuss the use of gut microbiota-derived mediators as potential markers in NAFLD.
Nonalcoholic fatty liver disease (NAFLD) comprises a variety of diseases extending from simple steatosis (SS), nonalcoholic steatohepatitis (NASH), fibrosis, and cirrhosis, with a growing prevalence worldwide, reaching around 30% of global population [
Precise histological diagnosis, including disease stages (SS and NASH), is commonly based on liver biopsy [
Previous evidence has linked intestinal microbiota dysbiosis with obesity, insulin resistance, metabolic syndrome, and NAFLD [
In order to give a broad overview of primary literature published on this topic, we have used narrative review as literature search strategy in the present article. In this sense, this narrative review will discuss NAFLD and (a) gut microbiome dysbiosis and (b) main gut microbiota-derived mediators. Their potential use as biomarkers for evaluating the status of NAFLD will also be briefly discussed.
Intestinal microbiome is composed mainly of bacteria, virus, and fungi, with several functions, such as host nutrition, bone mineralization, immune system regulation, xenobiotics metabolism, proliferation of intestinal cells, and protection against pathogens [
Dysbiosis may adversely impact metabolism and immune responses, favoring NAFLD and NASH. Because of gut dysbiosis, there is an elevated production of toxic bacterial components and metabolic mediators, which consequently accumulate in the intestine. In addition, an increase in intestinal permeability and further disruption of the epithelial barrier lead to the efflux of these gut microbiota-derived mediators [
Instead of the evidence relating disruption of the gut-barrier and hepatic diseases [
Several previous studies in clinical settings have associated intestinal dysbiosis with the occurrence of NAFLD [
In the present section we will focus on the main gut microbiota-derived mediators related to NAFLD: short-chain fatty acids, bile acids, bacterial components, endogenous ethanol, and choline deficiency. Also, we have performed a summarizing table (Table
Gut microbiota-derived mediators in human NAFLD.
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SHORT CHAIN FATTY ACIDS | Adults, obesity, NAFLD (SS or NASH) | Blood | Higher abundances of enzymes associated with lactate, acetate, and formate in mild/moderate NAFLD. Higher abundances of enzymes for butyrate, D-lactate, propionate, and succinate in advanced fibrosis | [ |
Children, obesity, NAFLD | Fecal specimens | Lower acetate, formate, valerate in NAFLD | [ | |
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BILE ACIDS | Adults, NASH | Blood,Urine | More hydrophobic bile acid profile | [ |
Adults NAFLD, NASH | Liver | Elevated deoxycholic, chenodeoxycholic, and cholic acids | [ | |
Adults, NAFLD, NASH | Blood | Higher glycocholate, taurocholate, glycochenodeoxycholate in NAFLD | [ | |
Adults, NASH | Fecal specimens | Higher primary to secondary BA ratio in NASH | [ | |
Children NAFLD | Blood | Higher CDCA, unconjugated primary BAs (CDCA + cholic acid), lower DCA, TDCA, GDCA, total DCA, GLCA and total lithocholic acid in NASH | [ | |
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TLR | Adults, NASH | Blood | Higher TLR-4/MD-2 expression on CD14 positive cells in NASH | [ |
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ENDOGENOUS ETHANOL | Children, obesity, NASH | Blood | Elevated blood-ethanol concentration in NASH | [ |
Children, NAFLD | Blood | Higher ethanol levels in NAFLD | [ | |
Children, obesity, fatty liver | Blood | Higher ethanol levels in NAFLD | [ | |
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CHOLINE, TMA, TMAO | Children, adolescents, adults | Blood | Decreased choline intake in postmenopausal NAFLD women with fibrosis | [ |
Adults | Blood | Association of TMAO level and presence/ severity of NAFLD | [ | |
Adults | Blood | Higher free choline levels in NASH | [ |
Short-chain fatty acids (SCFAs), such as acid acetic, acid propionic, and acid butyric, are molecules with seven carbon atoms or less, mainly produced by the fermentation of indigestible carbohydrate by gut microbiota [
In general, changes in the microbiota result in increased production of SCFA in the intestine with an increased transport of monosaccharides to the liver, promoting hepatic lipogenesis and steatosis [
Experimental studies have demonstrated that these SCFAs can remodel regulatory T cell expansion and enhance neutrophil chemotaxis, modulating inflammation in mice models [
However, other previously published studies point in another direction and have reported that SCFAs could be beneficial in the progression of NAFLD; for example, butyrate activates AMP-activated protein kinase (AMPK) in the liver [
Furthermore, clinical studies have demonstrated SCFA enrichment in fecal samples of children and adults with NAFLD [
The close relation between microbiota dysbiosis and SFCAs production—as part of carbohydrate bacterial fermentation—with the results of previous experimental and clinical studies provide evidence of their potential use as markers of NAFLD progression.
Bile acids (BA) are steroidal molecules synthesized after cholesterol oxidation by enzymes presented in hepatocytes which are important in the regulation of glucose and lipid metabolism. They participate in the digestion and solubilization of lipids and regulate hepatic glucose and inflammation. Also, they are able to control their own synthesis through the activation of farnesoid X receptor (FXR) [
Gut microbiota is a critical modulator of BA pool size and composition and the process of dysbiosis could substantially alter systemic concentrations of conjugated and/or secondary bile acids, as well as increasing their synthesis. An increased level of BA causes an activation of cell death pathway mediated by inflammatory and oxidative stress cascades in liver tissue [
Previous investigations have demonstrated a BA increase in biological fluids of patients with NASH compared to subjects with healthy livers and an evident association with intestinal dysbiosis [
In children with NAFLD, changes in circulating BA profile have been reported too [
Experimental studies have demonstrated that dysbiosis of the gut microbiota can modulate the activity of FXR in the intestine, affecting as consequence lipid metabolism in the liver [
Considering the numerous experimental and clinical published studies associating gut dysbiosis, bile acids, and NAFLD, it is expected that these molecules could be proposed as potential noninvasive markers of the disease, specifically the secondary bile acids deoxycholic acid (DCA) and lithocholic acid (LCA), which cannot be produced without bacterial fermentation [
The liver is exposed to potentially harmful substances derived from the gut, considered as pathogen-associated molecular patterns (PAMPs), including translocated bacteria, lipopolysaccharide (LPS), DNA, RNA, and endotoxins, which are potent inducers of tissue inflammation [
There is evidence that dysbiosis causes permeability changes that increase portal levels of gut-derived TLR ligands (LPS or endotoxin), which further activate TLR4 on hepatic Kupffer and stellate cells [
In addition, TLR signaling, as a result of gut dysbiosis, can also lead to the production of inflammasomes, in peripheral and parenchymal cells, which activate a variety of processes, including cleavage of procaspase-1 to form active caspase-1, resulting in cell death dependent on caspase-1 and caspase-3 [
Previous studies have demonstrated that endotoxemia markers, as a result of gut dysbiosis, were associated with the pathogenesis and severity of NAFLD [
Multiple experimental studies have demonstrated that a high-fat diet can increase the proportion of LPS derived from gut bacteria and administration of endotoxin has been shown to induce insulin resistance and weight gain [
There is an evident relation between gut dysbiosis, bacterial-derived components, inflammatory response, and NAFLD; therefore these bacterial mediators, especially circulating TLRs, might be used as potential noninvasive markers of disease progression.
Dysbiosis due to changes in microbiome composition profile, specifically in
Several studies have detected increased levels in nondietary ethanol, derived from bacteria, in obese patients [
Gut dysbiosis with the increase in ethanol-producing bacteria (
In summary, the proinflammatory and prooxidative damage as a result of endogenous ethanol in the liver, which might contribute to the pathogenesis of NAFLD, has been demonstrated; and the previous reports may support its use as a noninvasive marker of disease progression.
Choline is an essential nutrient obtained through both dietary intake and endogenous synthesis, being an important constituent of membrane phospholipids. The human gut microbiome actively metabolizes dietary components, including choline, and dysbiosis may alter its cellular disponibility and predispose the body to a deficiency of choline. Alterations in choline and phosphatidylcholine metabolism may have an impact on several physiological pathways, which could induce NAFLD. Choline deficiency prevents synthesis and excretion of very-low density lipoprotein (VLDL), leading to hepatic triglyceride accumulation and liver steatosis [
In addition, choline can be metabolized to its derivate trimethylamine (TMA) by the intestinal microbiota. TMA reaches the liver via portal circulation and is subsequently oxidized by hepatic flavin-containing monooxygenases in the liver, forming trimethylamine-N-oxide (TMAO), which is then released into blood circulation [
A few studies have examined the association of choline with the fatty liver disease in animals and humans. A small number of human studies have shown that the consumption of a low-choline diet promotes fatty liver and liver damage [
On the other hand, the metabolite TMAO has been associated with the occurrence of NAFLD, and TMAO raised levels correlate with the severity of steatosis, and it has been proposed as an independent risk marker for the disease [
In summary, the evidence demonstrated that choline and TMAO are associated with progression of NAFLD, indicating the potential use of these gut-derived mediators of dysbiosis as markers of disease progression.
Intestinal dysbiosis can trigger intestinal inflammation and increase permeability of the gut epithelial barrier, exposing the hepatobiliary system to gut-derived mediators of dysbiosis, such as bacterial components or metabolites, which may induce NAFLD progression. Gut-derived mediators of dysbiosis contribute to steatosis activate the immune system, induce inflammatory and oxidative pathways, enhance inflammation, and promote fibrogenesis (Figure
A schematic figure of the role of gut dysbiosis in the development and progression of nonalcoholic fatty liver disease (NAFLD) on the basis of the gut-liver axis. Environmental factors as obesity, high fat diet, or infection (among others) may induce intestinal dysbiosis and also increased intestinal permeability (malfunction of tight junctions). Substances such as short-chain fatty acids, bile acids, bacterial components, choline, and endogenous ethanol reach the liver and activation of toll-like receptors (TLRs) occurs. This activation induces insulin resistance, hepatic inflammation, lipogenesis, and oxidative stress, inducing NAFLD. BA, bile acids; LPS, lipopolysaccharides; SCFA, short chain fatty acid; TLR, toll-like receptor; TMAO, trimethylamine oxide.
Despite the evident association between gut dysbiosis and obesity and NAFLD, derived from experimental studies, very few studies have been conducted in patients with NAFLD in order to explore the role of gut-microbiota derived mediators of dysbiosis in the occurrence and progression of the disease. Most of the previous evidence has been focused on gut microbiota as a therapeutic target to prevent or to treat NAFLD, interfering in gut dysbiosis with probiotic, prebiotic, and symbiotic supplements. Nevertheless, few studies have been focused in gut-derived mediators of dysbiosis as noninvasive markers of disease progression. The study of specific gut-derived mediators of dysbiosis—bacterial components and metabolites—may provide an opportunity to develop a specific diagnostic biomarker for NAFLD. In this sense, we propose the metabolomic study of these and other metabolites involved, in order to achieve a metabolomic profile that could be used as biomarkers for evaluating the status of NAFLD.
The authors declare no conflicts of interest.
Gemma Aragonès and Sergio González-García contributed equally to this work.
This study was supported by the Fondo de Investigación Sanitaria and Fondo Europeo de Desarrollo Regional (FEDER, Grant no. PI16/00498, to Teresa Auguet), by funds from Agència de Gestió d’Ajuts Universitaris de Recerca (AGAUR 2009 SGR 959 to Cristóbal Richart) and the Grup de Recerca en Medicina Aplicada URV (2016PFR-URV-B2-72 to Cristóbal Richart), and by the Fundación Biociencia.