Feb 12, 202216 min read

Gut barrier disruption and chronic disease

Highlights

Maintenance of gut barrier integrity is indispensable for health as the #gut barrier protects the host against gut microbes, food antigens, and toxins. Many factors such as enteric #infection, #antibiotics, low-fiber diets, #circadian rhythm disruption, and psychological #stress can affect gut barrier integrity and lead to systemic, low-grade #inflammation due to translocation of bacteria and their components. While the body can resist transient gut barrier disruption, it may be overwhelmed by mild insult due to genetic predisposition, chronic stress, and aging, which may contribute to the development of autoimmune, metabolic, and mental health disorders. Consideration of the various intrinsic and extrinsic factors that affect gut barrier integrity and microbiota composition is needed to maintain or restore human health.

Abstract

The intestinal barrier protects the host against gut microbes, food antigens, and toxins present in the gastrointestinal tract. However, gut barrier integrity can be affected by intrinsic and extrinsic factors, including genetic predisposition, the Western diet, antibiotics, alcohol, circadian rhythm disruption, psychological stress, and aging. Chronic disruption of the gut barrier can lead to translocation of microbial components into the body, producing systemic, low-grade inflammation. While the association between gut barrier integrity and inflammation in intestinal diseases is well established, we review here recent studies indicating that the gut barrier and microbiota dysbiosis may contribute to the development of metabolic, autoimmune, and aging-related disorders. Emerging interventions to improve gut barrier integrity and microbiota composition are also described.

Structure and function of the gut barrier

Maintenance of epithelial and endothelial barriers in the gut, skin, blood vessels, respiratory tract, and the brain is critical for human health [1]. The intestine forms the largest and one of the most important internal barriers in the body as it protects the host from noxious substances and microbes present in the gut lumen. The gut barrier consists of the mucus layer, commensal bacteria, epithelial cells, and immune cells residing in the lamina propria (see Glossary) (Figure 1A). In the intestinal epithelium, goblet cells secrete mucus glycoproteins that prevent direct contact between gut microbes and colonocytes [2], while the mucus in the small intestine is loose and allows passage of bacteria [3]. In the small intestine, Paneth cells secrete antimicrobial proteins that can specifically lyse bacterial cells [4]. In the lamina propria, B cells secrete IgA that can bind to bacteria and their toxins to prevent their translocation into the body [5].

Commensal microbes of the gut microbiota help to maintain gut homeostasis in various ways (Figure 1A). For instance, they oppose colonization by pathogens [6] and promote differentiation of regulatory T (Treg) cells, which induce tolerance to lumen antigens [7]. When sequestered into the lumen, microbe-associated molecular patterns (MAMPs) such as flagellin, lipopolysaccharide (LPS), and peptidoglycan strengthen the gut barrier by binding to Toll-like receptors (TLRs) on the apical surface of intestinal cells to induce production of antimicrobial proteins [5]. Commensal bacteria can induce the production of mucus from goblet cells by activating interleukin (IL)-22 secretion by innate lymphoid cells [8]. Commensals also convert dietary fiber into short-chain fatty acids (SCFAs), which protect the gut barrier in various ways, including by providing energy for colonocytes and stimulating the production of mucus, antimicrobial proteins, and Treg cells [5]. Depletion of commensals and their replacement by pathogens, a condition termed dysbiosis, may therefore affect the gut barrier and produce detrimental effects on the host (Figure 1B–F).

Absorption of nutrients and water by the intestine can occur via the transcellular and paracellular pathways (Figure 2). Intestinal cells are linked by a series of proteins forming junctional complexes consisting of tight junctions, adherens junctions, and desmosomes, allowing absorption of water and small solutes (<8 Å) via the ‘pore’ pathway [9,10]. Transient and reversible rearrangement of the actin cytoskeleton and tight junctions allows passage of larger molecules (<100 Å) via the ‘leak’ pathway [9,10] (Figure 2). For instance, activation of glucose-Na+ co-transport following food intake increases the leak pathway [11], which enhances absorption of food nutrients but also small food antigens and MAMPs (Figure 2). Proinflammatory cytokines such as tumor-necrosis factor-alpha (TNF-α) [12] can also activate the leak pathway and cause diarrhea, which may help to expulse proinflammatory stimuli into the gut lumen. Intestinal erosion, ulceration, and epithelial cell death may allow larger particles, including MAMPs and bacteria, to unrestrictedly cross the intestinal epithelium and induce inflammation [9] (Figure 2).

Effect of gut barrier dysfunction on intestinal diseases

Gut barrier dysfunction is involved in intestinal diseases such as enteric infections, intestinal bowel disease (IBD), and celiac disease [13]. For example, pathogenic bacteria and viruses such as Salmonella and rotaviruses can breach the intestinal epithelium and alter tight junctions, causing diarrhea via water and electrolyte loss into the gut lumen [14,15] (Figure 1B). Gastrointestinal infections can lead to bacterial translocation to the gut mucosa (Figure 1B), producing inflammation that further increases gut barrier dysfunction and may result in a vicious cycle [16].

IBD, which consists of Crohn’s disease and ulcerative colitis, is characterized by excessive immune reaction towards the gut microbiota and mucosa of the small and large intestine, respectively. More than 200 single nucleotide polymorphisms (SNPs) in various genes coding for NOD-like receptors (NLRs), antimicrobial proteins, and cytokines have been implicated in IBD [17]; however, only a fraction of ulcerative colitis subjects have a family history of IBD [18], indicating the importance of environmental triggers. Mice that lack the major mucus glycoprotein, mucin-2, show gut barrier dysfunction and spontaneously develop colitis and colorectal cancer [19,20], illustrating the role of mucus in maintaining homeostasis [21]. Moreover, mice fed a high-glucose diet develop more severe dextran sodium sulfate (DSS)-induced colitis than controls due to increased mucolytic bacteria in the gut and reduced mucus barrier, leading to bacterial translocation to the lamina propria [22] (Figure 1B). In humans, weakening of the colonic mucus barrier is an early event in the development of ulcerative colitis [23] and bacterial DNA is increased in the blood of IBD subjects compared with healthy controls [24]. The development of IBD is therefore associated with genetic and environmental factors that lead to intestinal erosion and inflammation in susceptible individuals.

Celiac disease is a well-known condition involving gut barrier disruption. In this disease, gluten from wheat and other grains has been identified as the environmental trigger of autoimmune reactions in genetically susceptible individuals (Table 1). Gliadin proteins found in gluten induce the release of the protein zonulin from the gut epithelium [25] (Figure 1B). Zonulin is a mammalian ortholog of the zonula occludens toxin (Zot) from the cholera pathogen Vibrio cholerae. Similar to the cholera toxin, zonulin induces tight junction disassembly and increases gut permeability to peptides larger than three amino acids [25]. Celiac patients harbor SNPs in various genes, including human leukocyte antigens (HLAs) (e.g., HLA-DQ2 and HLA-DQ8) that render gliadin peptides capable of activating T cells and inducing autoimmune reactions [26]. In addition, increased gut permeability and antibodies against LPS and flagellin have been observed in people with non-celiac gluten sensitivity [27], which may involve SNPs in non-HLA genes [26]. Given the difficulty in identifying gluten sensitivity and the observation that gliadin can induce the release of zonulin even in healthy individuals [26], it is likely that gluten sensitivity is more prevalent than presently recognized. A combination of genetic and environmental factors thus affects gut barrier integrity and is involved in the development of intestinal diseases.

Effects of the diet on gut barrier integrity

Given that absorption of nutrients, including glucose, amino acids, and fatty acids, occurs in the small intestine, the bulk of the gut microbiota, which resides in the colon, must rely on indigestible dietary fiber as their main source of energy and nutrients. However, people who consume a Western diet usually eat less than 15 g of fiber per day, in sharp contrast with the recommended daily intake (25 g/day for women and 38 g/day for men) [28] or the diet of our ancestors (100 g/day) [29]. Following intake of a fiber-depleted, Western diet, many bacterial taxa that normally feed on fiber gradually disappear from the gut microbiota [30], while other commensals such as Bacteroides thetaiotaomicron shift their metabolism to degrade mucus glycans that normally protect the intestine [31]. A low-fiber diet reduces SCFA producers in the gut microbiota, which can affect gut barrier integrity by reducing production of mucus, antimicrobial proteins, and Treg cells [5], as well as affecting tight junction assembly [32] (Figure 1C). Accordingly, the Western diet reduces mucus thickness and increases gut permeability and proinflammatory markers in mice [33] (Table 1).

The Western diet also induces systemic inflammation associated with changes in bile acid synthesis and signaling [34]. Bile acids are emulsifiers that affect mucus properties [35] and increase gut permeability in intestinal cell monolayers [36] and mice [37]. Emulsifiers are also added to processed foods to improve lipid–water mixing and food texture. Chronic intake of relatively low amounts of emulsifiers (1% w/v) for 12 weeks is sufficient to induce gut permeability, low-grade inflammation, and metabolic syndrome in wild-type mice and produce robust colitis in inflammation-prone IL-10-deficient mice [38]. Emulsifiers may affect gut barrier integrity by inducing gut dysbiosis and affecting mucus thickness [38]. Other food additives that can disrupt the gut barrier and induce dysbiosis in animal models include fructose [39], salt [40], and artificial sweeteners [41] (Table 1).

The seminal work of Cani et al. was instrumental in elucidating a link between diet, gut barrier dysfunction, and metabolic diseases. This group showed that feeding a high-fat diet (HFD) to mice for 4 weeks increases serum LPS levels, and subcutaneous injection of LPS is sufficient to increase fasted glycemia, insulinemia, and body weight similar to feeding a HFD [42]. The HFD reduces tight junction expression and induces MAMP translocation into the lamina propria, which promotes inflammation via activation of TLRs on the basolateral surface of enterocytes or on immune cells [43] (Figure 1C). In humans, analysis of a 3-day food survey shows that people who consume a high-energy diet, but not a high-carbohydrate diet, have higher levels of plasma LPS [44], a condition called endotoxemia. Diabetic subjects show signs of translocated bacteria into the blood and treatment with probiotics can prevent this process [45]. Moreover, systemic influx of microbial products correlates with levels of hemoglobin A1c (HbA1c), a marker of poor glycemic control in humans [46]. Endotoxemia is also observed in subjects with non-alcoholic fatty liver disease [47], and individuals with high endotoxemia levels have a threefold higher risk of atherosclerosis [48].

These observations indicate that gut barrier dysfunction, dysbiosis, and inflammation induced by poor diet affect organs connected to the gut (e.g., pancreas, liver, and blood vessels), thus contributing to the development of various metabolic conditions, including type 2 diabetes, obesity, non-alcoholic fatty liver disease, and cardiovascular disease.

The autoimmune connection

Increased gut permeability, dysbiosis, and altered mucosal immunity have also been observed in autoimmune diseases (Figure 1D). Mice carrying a transgenic T cell receptor specific for a beta cell autoantigen spontaneously develop gut permeability and mucus layer dysfunction, leading to increased Th17 cells, reduced Treg cells, and development of type 1 diabetes [49]. Increased intestinal permeability also precedes type 1 diabetes onset in humans [50]. Systemic lupus erythematosus can also be triggered by TLR7-induced gut dysfunction in various mouse models [51]. For instance, continuous topical treatment with imiquimod, a TLR7 agonist, induces bacterial translocation and lupus-like symptoms in wild-type mice [52]. Bacterial translocation of Enterococcus gallinarum to lymph nodes and the liver occurs in lupus-prone mice carrying a TLR7 gene duplication and in human lupus subjects [53]. Moreover, the human gut commensal Prevotella histicola protects against neurodegeneration by reducing gut permeability and increasing levels of Treg cells and tolerogenic dendritic cells in a murine model of multiple sclerosis [54]. Bacterial DNA of intestinal origin is found in the circulation of subjects with psoriasis [55], suggesting the involvement of gut barrier dysfunction in this autoimmune condition as well. A time-course experiment showed that increased gut permeability preceded intestinal inflammation and the onset of arthritis in mice [56]. However, another study showed that co-transfer of splenocytes and gut microbiota from arthritic mice is required to induce gut permeability and arthritic symptoms in recipient mice [57], indicating that a combination of immune and microbiota-related factors may be needed to disrupt gut barrier integrity and induce autoimmune diseases.

The Western diet may contribute to the development of autoimmune diseases in part via its effects on the gut microbiota and SCFAs. Lupus subjects are more likely to consume a low-fiber diet [58], which may affect gut barrier integrity by reducing SCFAs, mucus, IgA, and Treg cell levels [3,5,59] (Figure 1D). Translocation of Lactobacillus reuteri was observed in lupus-prone mice and a subset of lupus subjects and a fiber-rich diet containing resistant starch reduced lupus mortality in mice by increasing SCFA levels [60]. The SCFAs butyrate and acetate improved gut barrier integrity, increased Treg cell levels, and reduced autoreactive T cells in a mouse model of type 1 diabetes [61], supporting the use of fiber-enriched diets against autoimmune diseases.

Lifestyle and the gut barrier

Antibiotics affect microbiota composition and gut barrier integrity and their overuse has been implicated in the development of various health issues (Box 1). Likewise, long-term intake of non-steroidal anti-inflammatory drugs (NSAIDs) can induce gastroduodenal ulcers, inflammation, and bleeding by increasing gastric acid secretion, reducing mucus production, and inducing gut barrier leakage and endotoxemia [62] (Table 1). Another group of widely used drugs, proton pump inhibitors, enhance alcohol-induced fatty liver disease in mice and humans by reducing gastric acid, which increases growth of commensal bacteria in the small intestine and leads to translocation of microbial products and liver inflammation [63]. One of the most commonly used toxins that disrupt gut barrier integrity and the intestinal microbiota is alcohol. As such, chronic alcohol abuse is associated with increased gut barrier leaks, gut dysbiosis, and translocation of bacterial components into internal organs, which contribute to alcohol-induced health complications even before liver disease development [64].

Psychological stress also increases gut permeability in rodents and humans (Table 1). Water-avoidance stress increases gut permeability and affects the cohesive properties of mucin in mice [65]. Maternal separation induces the release of corticotropin-releasing factor by the hypothalamus and acetylcholine by cholinergic enteric neurons in rat pups, which results in increased gut permeability to large proteins [66]. In humans, a public speech test increased gut permeability in a subset of healthy subjects who had concomitant increased salivary cortisol levels [67]. Chronic stress can exacerbate symptoms in IBD patients by increasing gut permeability and inducing low-grade inflammation [68,69]. Conversely, vagus nerve stimulation reduces gut permeability and intestinal injury in mouse models of burn injury [70], indicating that stress management strategies may help to mitigate the effects of psychological stress on the gut barrier.

Sleep deprivation also induces gut permeability and increases proinflammatory cytokines in mice [71]. Night shift workers are more prone to develop various disorders, including gastric and duodenal ulcers [72,73], obesity, type 2 diabetes, and cardiovascular disease [74], which are associated with gut barrier dysfunction. In view of the pleiotropic effects of the circadian rhythm in regulating physiological functions, chronic disruption of the circadian rhythm by sleep deprivation or unrestricted, ad libitum feeding patterns has emerged as a critical factor in the development of metabolic disease [75,76]. Accordingly, circadian rhythm disruption through genetic or environmental means increases colonic permeability, endotoxemia, and hepatosteatosis in alcohol-treated mice [77] (Table 1).

Proinflammatory cytokines produced as a result of increased gut permeability and chronic inflammation may also affect the blood–brain barrier, leading to fatigue and possibly contributing to behavioral disorders and depression [78,79] (Figure 1E). Increased anti-LPS antibodies have been observed in subjects with major depressive disorder [80,81]. Endotoxemia and proinflammatory cytokines induce sickness behavior, which includes fatigue, anxiety, and depressive symptoms [79]. Gut barrier dysfunction and dysbiosis have been linked to other mental health issues such as chronic fatigue syndrome [82], schizophrenia [83], suicide [84], and autism [85], although the mechanisms remain to be examined in more detail. While mental health disorders are multifactorial in nature, gut barrier defects may represent a contributing factor in some individuals.

Exercise can also affect gut barrier integrity (Table 1). Strenuous exercise for more than 2 hours at 60% VO2 max appears to be the threshold above which increased gut permeability and endotoxemia are observed in healthy subjects and trained athletes, irrespective of fitness status [86]. These effects may be attributed to stress hormones and increased heat, which induces a redistribution of blood flow to the periphery to dissipate heat, thereby reducing blood flow and oxygen levels in the gastrointestinal tract [87]. However, swimming at moderate intensity improved gut barrier integrity in rodents [88], implying the occurrence of biphasic hormesis responses (Box 2).

Do poor diet and lifestyle habits that increase gut permeability, such as a low-fiber diet and psychological stress, necessarily lead to autoimmune, metabolic, and cognitive disorders? Obviously not (Box 2). However, it is likely that chronic ruptures of the gut barrier, as may occur in individuals with a modern lifestyle, may overwhelm the body’s defenses and contribute to inflammation and disease development, especially in aging individuals.

When the fence grows old

Major changes occur in the gut during aging, including increased intestinal cell apoptosis, reduced mucus thickness and antimicrobial peptides, intestinal stem cell exhaustion, and dysbiosis [89., 90., 91., 92.] (Figure 1F). The term ‘inflammaging’ has been used to describe the association between aging and chronic, low-grade inflammation [93,94]. For instance, serum levels of zonulin and HMGB1 (a nuclear protein whose extracellular form is known to induce inflammation) increase in healthy, old individuals compared with young subjects [95]. Germ-free mice fail to show an aging-dependent increase in inflammatory mediators and an aging-dependent increase in gut permeability and proinflammatory cytokines can be transferred to germ-free mice that are co-housed with aged, but not young, mice [96]. These data suggest that gut dysbiosis observed during aging promotes gut permeability and inflammation. While proinflammatory cytokines initially act to contain microbial infection and promote tissue regeneration following injury or infection [97], these molecules may accumulate in aging individuals, producing a vicious cycle of gut barrier loss and chronic inflammation [98].

Research even suggests that increased gut permeability may be a critical event leading to death from ‘natural cause’. In Drosophila, in which gut permeability can be observed as accumulation of a blue dye throughout the body following feeding (thus creating blue flies called ‘Smurfs’), gut permeability has been found to be more accurate than chronological age for predicting the time of death [99]. Aging-induced gut permeability is also observed in nematodes and zebrafish and it increases quasi linearly with age, occurring in all individuals prior to death, regardless of chronological age [100]. Notably, increased gut permeability is observed in critically ill patients prior to multiple organ failure [101]. In addition to the gut, increased epithelial and endothelial permeability in the kidneys, lungs, and the liver is observed in critically ill patients [102], indicating that loss of integrity in various barriers may contribute to the demise of the human body.

Interventions to maintain gut barrier integrity

Many interventions can improve gut barrier integrity in human and animal studies (Table 2). Arguably, one of the main strategies to improve health is the reintroduction of dietary fiber in the Western diet [103]. For example, adding 10 g of fiber per day for 6 months reduced serum zonulin, insulin resistance, and serum liver enzymes and improved fatty liver score in non-alcoholic fatty liver disease subjects [104]. Given that human subjects show notoriously poor compliance with dietary modifications, dietary fiber supplements may be a viable option for these patients.

Many phytochemicals such as berberine, curcumin, quercetin, and resveratrol improve gut barrier integrity and reduce inflammation in animal models (Table 2). While these phytochemicals show poor bioavailability, we suggested earlier that they may produce beneficial effects by serving as prebiotics and biological stress inducers [105]. Phytochemicals may thus reduce inflammation by fortifying the gut barrier and preventing the translocation of microbial components, as shown in recent studies for curcumin [106], resveratrol [107], and urolithin A [108] in mice and cultured cells. Conversely, phytochemicals from hops, chili pepper, and marigold may instead increase gut permeability [109] and a diet deficient in specific phytochemicals and lectins may be highly beneficial in individuals who show signs of inflammation [110].

In addition to maintaining gut barrier integrity by inducing the production of mucus, antimicrobial proteins, IgA, and Treg cells (Figure 1), commensals can reinforce the gut epithelium by promoting tightening of intestinal cellular junctions [111]. First-generation probiotics such as Lactobacillus and Bifidobacterium stimulate resistance against gastrointestinal diseases, enteric infection, antibiotic-induced diarrhea, depression, and anxiety [112., 113., 114.]; however, negative results were also obtained, for instance, against anxiety [115] or gastroenteritis [116,117], possibly due to variations in diet, lifestyle, and gut microbiota composition.

Second-generation probiotics such as Akkermansia muciniphila, Roseburia spp., and Parabacteroides spp. are currently being studied to improve gut homeostasis. Our group observed that Parabacteroides goldsteinii reduced gut permeability, serum endotoxemia, body weight gain, insulin resistance, and inflammation in HFD-fed mice [118]. In humans, supplementation with heat-killed A. muciniphila for 3 months reduced serum endotoxemia, slightly decreased body weight and fat mass, and improved insulin sensitivity in overweight and obese volunteers [119]. A membrane protein (i.e., Amuc 1100) from A. muciniphila may be responsible for these benefits as it reduced signs of obesity and diabetes and improved gut barrier integrity in mice, possibly by inducing TLR2 signaling [120].

Corticosteroids, anakinra (IL-1 receptor antagonist), and anti-TNF-α antibodies can reduce inflammation and improve gut barrier integrity in IBD subjects [121,122]. Glutamine reduced intestinal permeability and improved inflammatory bowel syndrome severity scores in a randomized placebo-controlled clinical trial of patients following enteric infection [123]. In a mouse model of arthritis, treatment with butyrate, cannabinoid type 1 receptor agonists, or larazotide (a zonulin antagonist) reduced gut permeability and arthritis symptoms [56]. Notably, larazotide is currently in Phase III clinical trials for the treatment of celiac disease. In addition, oral supplementation with alkaline phosphatase, which can dephosphorylate and inactivate LPS, can reduce endotoxemia, gut barrier dysfunction and liver damage induced by alcohol in mice [124].

Caloric restriction, the fasting-mimicking diet, and time-restricted feeding can also produce benefits for the gut (Table 2). A 4-week caloric restriction diet (800 kcal/day) reduces gut permeability, body weight, insulin resistance, and inflammation markers in obese women [125]. The fasting-mimicking diet, which involves consuming a low-protein, low-calorie diet (800 kcal/day) for 4 or 5 consecutive days per month, reduces markers of aging, diabetes, cancer, and cardiovascular disease in humans [126]. The fasting-mimicking diet increased intestinal stem cells and reversed dysbiosis, inflammation, and pathological scores in the mouse model of DSS-induced colitis [127]. Time-restricted feeding in which food is consumed within 4–8 hours (during daytime for humans or nightime for rodents) also reduces gut barrier disruption and ischemia-reperfusion injury in mice [128].

An interesting strategy to improve gut homeostasis and reduce inflammation consists of combining nonpersonalized interventions centered around limiting the inappropriate use of antibiotics coupled with personalized interventions involving consumption of plant-based, high-fiber foods, engineered fiber-enriched foods, prebiotics supplementation, and reintroduction of volatile and/or associated negatively with industrialized societies of humans (VANISH) taxa in the gut microbiota [103].

Concluding remarks and future perspectives

Multiple factors involving poor diet and lifestyle habits affect gut barrier function and the composition of the gut microbiota. These factors play a role in the development of intestinal diseases as well as metabolic, autoimmune, and mental disorders involving inflammation. Many promising therapies have been tested in animal models and require confirmation in clinical trials (see Outstanding questions). However, treatments such as probiotics or pharmaceutical drugs aimed at restoring gut barrier integrity are likely to fail when taken in isolation. Instead, we believe that lifestyle changes involving consideration of diet composition, pharmaceutical drug and alcohol use, exercise, sunlight exposure and vitamin D levels, circadian rhythm regulation, and stress management are more likely to show positive results. Recent advances related to gut barrier integrity and gut microbiota composition therefore offer the opportunity to reassess the importance of the diet and lifestyle in the prevention and treatment of chronic diseases associated with gut barrier disruption, inflammation, and aging.