Background

The human gut harbors trillions of bacteria that live in a symbiotic relationship and produce important metabolic and immunologic effects in the host [1,2,3]. Those effects not only are achieved by complex interactions among commensal bacteria and different cell types from the human body but can also be mediated by products derived from microbiota metabolism [4]. Higher carbohydrate to protein bacterial fermentation contributes to the integrity of intestinal barrier, homeostasis of the gut ecosystem, and human health. It has been postulated that chronic kidney disease (CKD) induces negative changes in both the gut environment and the microbiome profile [5]. Intestinal barrier disruption [6, 7], inflammation [8], impairment of the immune system [9], and higher generation of colon-derived uremic toxins p-cresyl sulfate (PCS) and indoxyl sulfate (IS) [10] seem to be some of the consequences of an unbalanced gut ecosystem. Furthermore, the progressive accumulation of PCS and IS has been associated with cardiovascular disease [11], progression of CKD [12], and mortality [13, 14]. Thus, it is important to understand the complex interactions within and between the gut and other body systems in healthy individuals and in those with CKD.

The gut microbiota

The human microbiota consists of trillions of bacteria distributed in complex and specific communities in different parts of the body, such as the skin, respiratory tract, urogenital tract, oral cavity, and gastrointestinal tract. The human gut harbors an average of 100 trillion bacteria, and approximately 3 million bacterial genes are found only in the intestinal microbiome, which is 150-fold more genes than the human genome [1, 15]. Approximately 90% of bacterial species that make up the human intestinal flora belong to the phyla Bacteriodetes (Bacteroides, Prevotella, and Xylanibacter) and Firmicutes (Ruminococcus, Clostridium, Lactobacillus, Eubacterium, and Faecalibacterium). In lower proportions, species belonging to the phyla Actinobacteria, Proteobacteria, and Verrucomicrobia are also found. Methanogenic archaea, yeast, and viruses also appear in the composition of the intestinal microbiota [1, 16]. Recently, it has been suggested that a greater proportion of Proteobacteria could be the microbial signature of dysbiosis in gut microbiota [17].

The gut microbiota is vastly diverse. Each individual has a single microbiota that fluctuates over time [16]. Several factors can influence the development, composition, and activity of the intestinal microbiota, such as host genetics, early colonization, age, lifestyle and environment, diet, medications, and the health status of the host [18, 19].

This complex and specific gut community has been considered a “hidden organ” due to its important physiological effects on the body by interacting with different systems. The gut microbiota produces energy from fermentation of nondigested compounds, synthesizes vitamins (K and B groups) [20], activates bioactive food components (isoflavanoids, flavonoids, and plant lignans) [21], contributes to intestinal barrier integrity [22], upregulates mucin genes [23], stimulates secretion of antimicrobial peptides [24], produces bacteriocins that contribute to resistance or resilience to infection by pathogens [25], competes with pathogenic bacteria for binding to intestinal epithelial cells [26], and influences host metabolism [2] and the immunological system [3, 27]. More recently, it has been suggested that the gut microbiota modulates the functionality of the central nervous system. Indeed, a multidirectional dialog between the gut ecosystem (microbiota, mucosal immune system, and neuroendocrine system) and the central nervous system has been proposed [2, 28].

The bacterial fermentation of undigested carbohydrates contributes to several benefits, including the maintenance of appropriate intestinal pH, by generating lactic acid, acetic acid, and short-chain fatty acids (acetate, propionate, and butyrate). In addition to being an energy source for colonocytes, butyrate is also an essential factor regulating the differentiation of colonic T regulatory cells (Treg). These cells have an important role in the tolerance of the gut lumen towards microorganisms by modulating inflammatory responses [29,30,31]. In addition, the colonic microbiota can catabolize proteins and amino acids that have not been digested or absorbed in the upper portions of the gut. The end products of the proteolytic activity include short-chain fatty acids, branched-chain fatty acids, ammonia, amines, phenols, indoles, thiols, CO2, hydrogen, and hydrogen sulfide (H2S) [29]. Most of these metabolites are normally excreted in feces and urine. Among these products are biogenic amines, immunomodulatory compounds, and other signaling molecules, which seem to be involved in regulating the gut mucosal immune system through pro-inflammatory and anti-inflammatory mechanisms [4]. Although considered potentially toxic, these metabolites occur in the colon at such a concentration range that no toxic effects are expected either locally or systemically [32].

Among the bacterial species living in the human gut, there are saccharolytic bacterial species, which preferentially ferment carbohydrates and use proteins and amino acids to grow, and proteolytic bacterial species, which preferentially ferment proteins and amino acids to produce energy by deamination. The type and amount of substrate, and especially the ratio between undigested carbohydrates and proteins, are the key modulators of bacterial composition and metabolism. When the carbohydrate/protein ratio is reduced, the fermentation of proteins is favored over carbohydrates, resulting in an overgrowth of proteolytic species and a decrease in the growth of saccharolytic bacterial species, resulting in changes in the pattern of metabolites produced [33,34,35,36]. In addition to dietary components, other factors can influence gut microbiota composition and metabolism, such as intestinal pH, medication, time of intestinal transit, host genetics, innate and adaptive immunity, and presence of diseases. Therefore, the amount and type of metabolites produced by bacteria as well as the composition of gut microbiota are the result of complex interactions between the microbiota, the host, and the intestinal environment [29, 37].

The intestinal barrier

The gastrointestinal tract functions as a barrier between the external environment and the internal milieu of the body. As mentioned previously, the surface of the intestinal mucosa is constantly exposed to a complex and dynamic community of commensal microorganisms. To maintain a symbiotic relationship between host and microbiota, the gastrointestinal immune system is involved in several activities. The epithelial lining acts as a first line of defense for the mucosal immune system, both to protect against bacterial invasion and to establish tolerance of the gut lumen towards commensal microorganisms. Through cell adhesion and junction structures, the intestinal epithelium constitutes a physical barrier against bacteria from the lumen and a selectively permeable barrier allowing the passage of nutrients, ions, and water [38,39,40]. Numerous physiological and pathological stimuli dynamically regulate the permeability of the gut epithelium by changing structures involved in the mechanisms of cell adhesion and formation of cellular junctions [41]. In addition to enterocytes and colonocytes, other cell types constitute the intestinal epithelium and contribute to barrier function. Among them there are goblet cells (mucus secreting), Paneth cells (secreting antimicrobial peptides), M cells (antigen presenting) and enteroendocrine cells (producing and secreting peptide hormones). The activity and the communication of these cells with immune cells allow the gut to identify intestinal luminal contents and signals and to regulate and modulate several activities, both in the intestine and in other parts of the body, such as mucus secretion, intestinal motility, and aspects of host energy metabolism [42]. Furthermore, it has been suggested that the mucosal immune system plays an important role in the host-controlling microbiota composition [27].

The intestinal epithelial cells express numerous receptors, pro-inflammatory mediators, and other factors, such as B cell-activating factor (BAFF) and transforming growth factor (TGF)-β, allowing them to communicate with the immune system and contribute to the control of inflammatory responses and intestinal barrier integrity [4, 43, 44]. Through Toll-like receptors (TLRs), the intestinal epithelial cells identify bacterial cell components. These receptors have the ability to recognize specific pathogen-associated molecular patterns (PAMPs). For example, TLR-2 recognizes peptidoglycan, and TLR-4 recognizes lipopolysaccharide (LPS) [45, 46].

Dynamic control over TLR expression and activation in intestinal cells has an important role in tolerance towards commensal microorganisms and intestinal homeostasis. The activation of TLRs promotes several important activities, such as epithelial cell proliferation, immunoglobulin A (IgA) secretion in the intestinal lumen, production of antimicrobial peptides and pro-inflammatory cytokines, and reorganization of tight junction structures, which favors intercellular junctions, reducing intestinal permeability [47, 48]. However, if these receptors are over-activated, the symbiotic relationship between host and microbiota might be disrupted, resulting in damage to the intestinal homeostasis. Thus, under normal physiological conditions, the recognition of LPS via TLR-4 by intestinal epithelial cells is suppressed; continuous exposure to LPS induces desensitization of these cells against this molecule. This process is mediated by expression of inhibitory molecules of the signaling triggered by TLR-4 [49, 50]. The inflammatory cytokines also regulate the expression of TLRs in intestinal epithelial cells. Studies have shown that interferon-γ (IFN-γ) and tumor necrosis factor (TNF) stimulate TLR-4 gene transcription, while interleukin-4 (IL-4) and interleukin-13 (IL-13), secreted by type 2 helper cells (Th2), seem to reduce the expression of TLR-4 in intestinal epithelial cells, since these cytokines lead to a reduction in the responsiveness of intestinal epithelial cells to LPS [51,52,53,54]. Both dendritic cells and macrophages of the intestinal mucosa also appear to have reduced responsiveness to LPS when compared to similar cells from other body tissues [42, 55].

Since the maintenance of homeostatic interactions with a diverse resident microbiota is essential to maintaining a strict balance of pro- and anti-inflammatory responses, the activation of anti-inflammatory cells, such as regulatory T cells (Treg), is required to control the immune response in the gut and consequently avoid impairment of intestinal homeostasis. IL-10 secretion from Treg cells acts on different cell types, such as dendritic cells, effector T cells, macrophages, and intestinal epithelial cells, to attenuate pro-inflammatory responses [40, 56]. In addition, as previously mentioned, microbial molecules have a profound effect on host mucosal and systemic immune system development. These effects may occur, for example, by inducing the secretion of antimicrobial peptides and acting on the composition of T lymphocyte subsets, which in turn influences the balance between potentially pro- and anti-inflammatory cells in the intestinal mucosa [27].

In summary, the maintenance of a symbiotic relationship between host and microbiota occurs through broad and complex interrelationships among different cell types and the intestinal microbiota (Fig. 1).

Fig. 1
figure 1

Synbiotic relationship between host and microbiota. To maintain a symbiotic relationship between host and microbiota, the intestinal immune system is involved in several activities. The epithelial lining acts as the first line of defense of the mucosal immune system, both to protect against bacterial invasion and to establish tolerance of the gut lumen towards commensal microorganisms [39, 40]. Through Toll-like receptors (TLRs), epithelial cells recognize bacterial molecules and produce B cell-activating factor (BAFF), a proliferation-inducing ligand (APRIL), and transforming growth factor (TGF)-β; these cytokines promote immunoglobulin A (IgA) secretion in the intestinal lumen [149]. A strict balance between the function of CD4+ effector T cells and Treg cells is essential for homeostasis. Dendritic cell (DC) activation by bacterial molecules and/or epithelial cell-derived factors induces CD4+ T cell differentiation [150]. T helper 17 (TH17) cells secrete pro-inflammatory cytokines (IL-17A and IL-17F), IL-22 (which stimulates the secretion of antimicrobial peptides), and IL-21 (which inhibits Treg cell generation). T helper 1 (TH1) cells secrete interferon-γ (IFNγ), which activates subepithelial macrophages. Together, these cells sustain the intestinal barrier, in general, by protecting against bacterial invasion. On the other hand, in order to modulate the immune response and avoid acute inflammation, Treg cells are induced and these cells secrete IL-10 and/or TGF-β. These cytokines act on different cell types, such as dendritic cells, effector T cells, macrophages, and intestinal epithelial cells to attenuate pro-inflammatory responses. Simultaneously, Treg cells can stimulate IgA induction [40, 151]

Full size image

The gut in CKD

CKD is a condition characterized by a gradual loss of kidney function over time with consequent retention of a number of compounds collectively termed uremic toxins. Complications such as anemia, metabolic acidosis, bone and mineral metabolism disorders, malnutrition, acquired immunodeficiency, systemic inflammation, and cardiovascular disease are common in patients with CKD [57,58,59]. Recently, it has emerged that alterations observed in the composition of the gut microbiota and in intestinal permeability, as a consequence of CKD, may also contribute to the immunological disorders, uremic toxicity, and potentially to the progression of the disease. It is out of the scope of the current review to describe the mechanisms involved in kidney-gut related disturbances in depth. A comprehensive review of this topic can be found elsewhere [60].

Gut barrier function in CKD

There is indirect evidence linking CKD with damage to the barrier function of the intestinal mucosa (increased intestinal permeability), such as the presence of endotoxemia in the absence of infection [7, 61, 62], bacterial translocation [8], and detection of intestinal bacteria DNA in the blood [63].

Experimental studies have investigated the possible mechanisms and structures involved in gut barrier dysfunction in CKD. Vaziri et al. [6] conducted an in vivo study with uremic rats and observed a large reduction in the expression of tight junction proteins (claudin-1, occludin, and zonula occludens (ZO)-1) in the colonic mucosa, indicating marked damage to the intestinal barrier. In contrast, the amount of mRNA for these proteins was unchanged or even elevated, indicating that the reduction in tight junction proteins may have occurred in the mRNA translation and/or post-translation steps. Similar findings in the expression of those proteins were also described by the same group of researchers in an in vitro study of human colonocytes. These cells were incubated with uremic plasma and showed significant reductions in the expression of tight junction proteins. Thus, it is possible that the compounds present in uremic plasma might be involved in the process of intestinal damage caused by uremia [64].

The breakdown of intestinal epithelial tight junctions leads to entry of microbial components into the underlying tissue compartments, which triggers a local inflammatory process, which in turn contributes to perpetuating gut barrier damage [65, 66]. Indeed, in uremic rats, disruptions in tight junctions were associated with local inflammation in the gastrointestinal tissue [6, 67]. It has been proposed that the impaired nuclear factor-erythroid-2-related factor 2 (Nrf2) system is involved in uremia-associated intestinal inflammation and breakdown of intestinal epithelial tight junctions [68]. Nrf2 is a transcription factor that activates genes encoding antioxidant and phase II detoxifying enzymes and related proteins. This nuclear factor plays a central role in cellular defense against oxidative stress [69, 70]. The contribution of the impaired Nrf2 system to CKD-induced damage to the intestinal barrier was demonstrated when Lau et al. [68], using a potent Nrf2 activator, observed attenuation of colonic inflammation and restoration of epithelial tight junction proteins in the colon of CKD rats.

Edema and ischemia in the intestinal mucosa may also contribute to intestinal barrier damage. Therefore, hypervolemia, excessive use of diuretics, and hypotensive episodes during or after dialysis can aggravate CKD-induced intestinal barrier dysfunction [7, 71, 72].

Assessment of intestinal permeability disruption with non-invasive methods (serum metabolites) in clinical studies represents a challenge in CKD. A recently published systematic review that included 15 studies noted significantly increased permeability in the late stages of CKD despite the variety of methods used among the different studies. This result should, however, be interpreted with caution due to the possible influence of decreased renal clearance of the serum markers tested and to the lack of non-invasive validated method in CKD [73].

Gut microbiota in CKD

Studies have shown that CKD has been associated with altered colonic microbiota composition [5, 10, 74, 75]. Significant differences in the abundance of numerous microbial operational taxonomic units (OTUs) were observed when feces of CKD patients were compared with that of normal renal functional subjects. An overgrowth of bacteria species belonging to the phyla Proteobacteria, Firmicutes, and Actinobacteria was found in CKD patients. Among the 19 bacterial families that were dominant in individuals with CKD, 12 are urease producers, five are uricase producers, and three are producers of indole and p-cresol-forming enzymes. Among the four bacterial families that were reduced in these patients, two are producers of butyrate (short-chain fatty acid) forming enzymes [5, 10]. Similar results have been found recently. Compared with controls, CKD patients had a significant reduction in two butyrate-producing species (Roseburia spp. and Faecalibacterium prausnitzii) [76]. In general, these changes suggest potentially upregulated microbiota in the generation of specific uremic toxins and downregulated microbiota in the generation of beneficial products.

Changes in the composition of the microbiota do not necessarily implicate in alterations in the microbial metabolism. It is well known that several bacterial species can share similar functional gene profiles, so different microbiota profiles can perform the same functions [16]. In addition to the microbiota composition, the gut environment significantly influences the production of microbial metabolites [77]. As mentioned earlier, the gut environment plays an important role in both the composition and activities of the gut microbiota. Therefore, CKD-induced changes in the intestinal environment may potentially influence colonic microbial metabolism and play a pivotal role in alterations of the microbial community. The high concentration of urea in intracellular and extracellular compartments, due to reduced renal function, generates high influx of this compound into the gastrointestinal tract where microbial ureases catalyze the hydrolysis of urea and generate large amounts of ammonia. Ammonium hydroxide, a by-product of ammonia, increases the intestinal pH, which leads to mucosal irritation and interferes with the growth of commensal bacteria, favoring the establishment of intestinal dysbiosis [78, 79]. The negative impact of urea conversion by microbial urease over intestinal epithelial cells was demonstrated in an in vitro study when colonocytes were incubated with urea plus urease. After incubation, a large reduction in the expression of tight junction proteins was found [80]. In addition to urea, other compounds accumulated in the blood are also secreted in larger amounts into the intestinal lumen, such as uric acid and oxalate, which may also influence microbial composition and metabolism [81, 82].

Other CKD-related factors may also promote changes in the intestinal environment. These factors include low fiber intake due to restricted fruit and vegetable consumption to control serum potassium [83, 84], constipation [85,86,87], abnormal digestive function [88], impairment of digestion and absorption of protein [89, 90], oral iron supplementation [91], and use of phosphate binders to control serum phosphorus and antibiotics to treat infections [92, 93].

However, despite the presence of these CKD-related factors, the influence of CKD on colonic microbial metabolism remains poorly understood. As far as we know, there is only one study in humans on microbial metabolism in individuals with CKD. Poesen et al. [94] investigated fecal metabolite profiles of hemodialysis patients and healthy individuals and observed distinct colonic microbial metabolism between these two groups. They found upregulated generation of p-cresol, indoles, aldehydes, benzenes, furans, and branched-chain, medium-chain, and short-chain fatty acids and a reduced generation of ketones in patients on hemodialysis. The clinical impact of these alterations is unknown for the majority of these fecal metabolites. Studies with uremic rats also demonstrated significant differences in fecal metabolite profiles, indicating an important influence of loss of renal function [94, 95]. However, in humans, in contrast to rats, when the researchers compared the patients on hemodialysis with household contacts on the same diet, the influence of decreased renal function was inferior to the external factors, such as the diet [94].

In summary, the impact of CKD on the gut is complex and seems to involve a dynamic interplay between intestinal cells and microbiota, which are heavily influenced by the gut environment (Fig. 2). As a consequence, it has been proposed that CKD-induced intestinal changes may also influence the accumulation of specific uremic toxins and disturbances in the immune system described in the following sections.

Fig. 2
figure 2

Schematic representation of the potential impact of CKD on the gut ecosystem. The CKD-induced changes in the intestinal environment may potentially influence the colonic microbial metabolism and play a pivotal role in alterations in the microbial community. As shown in the figure, several CKD-related factors contribute to alterations in the intestinal environment. These factors may lead to increased gut pH and alterations in gut composition and microbial metabolism, favoring the establishment of intestinal dysbiosis [10, 79]. Together, these changes may impair the gut barrier, leading to entry of microbial components and uremic toxins into the underlying tissue compartments, which triggers a local inflammatory process. The increase of microbial components in the lamina propria consequently promotes secretion of pro-inflammatory cytokines by increasing the activation of dendritic cells (DCs), macrophages, and CD4+ effector T cells (Th1 and Th17), disrupting the strict balance between the function of CD4+ effector T cells and Treg cells. This inflammatory process contributes to perpetuating gut barrier damage [40, 120]. The continuous entry of these components (endotoxins and uremic toxins) into blood circulation may lead to a persistent systemic inflammation that links the gut with CKD-associated inflammation and immunodeficiency [9]

Full size image

Gut ecosystem in CKD and uremic toxins

The numerous metabolites that normally compose the mammalian blood metabolome originate from the colonic microbiota [96]. Among them, p-cresol and indole are generated from bacterial fermentation of tyrosine and tryptophan, respectively. p-Cresol is a phenolic compound that, by sulfate conjugation in the colonic mucosa and in the liver, generates p-cresyl sulfate, whereas indole generates indoxyl sulfate after sulfate conjugation in the liver [97].

The decrease in the excretion of p-cresyl sulfate and indoxyl sulfate, due to the loss or reduction of renal function, leads to blood retention of these compounds. Indeed, it has already been shown in clinical studies that the concentration of these uremic toxins is markedly elevated in all stages of CKD, with a gradual increase as renal function decreases [11, 98, 99]. Given the importance of colonic metabolism to the composition of the mammalian blood metabolome, it has been proposed that alterations in the colonic metabolism of CKD patients, such as upregulation of both p-cresol and indole generation, can amplify the accumulation of these uremic toxins [94].

The harmful effect of these uremic toxins on different cell types, such as kidney tubular cells [100,101,102], endothelial cells [103,104,105], immunological cells [106, 107], and bone cells [108,109,110], has been demonstrated in experimental studies. Moreover, the accumulation of these toxins in individuals with CKD has been associated with progression of CKD [12], inflammation [98, 111], cardiovascular disease [11], vascular calcification [13], and mortality from cardiovascular disease and from all causes [13, 14, 112, 113].

Gut ecosystem in CKD and immunity

CKD is simultaneously associated with systemic inflammation and acquired immunodeficiency [59, 114]. Persistent systemic inflammation contributes to atherogenesis, cardiovascular disease, anemia, cachexia, and bone loss, whereas acquired immunodeficiency leads to impaired response to vaccination and to microbial infections, increasing frequency and severity of microbial infections in these patients, which in turn contributes to systemic inflammation [115]. Together, these immunological disorders have a strong negative impact on the survival of CKD patients [59, 116, 117].

Functional and structural alterations in immune cell populations contribute to CKD-associated systemic inflammation and acquired immunodeficiency [118]. Several factors may contribute to these immunological disorders, including uremic toxicity, oxidative stress, metabolic acidosis, vitamin D deficiency, impaired protein catabolism, chronic and recurrent infection, decreased clearance of pro-inflammatory cytokines and the dialysis process itself [115, 119]. More recently, it has been proposed that CKD-induced changes in the gut microbiome and consequent dysfunction in the gut barrier may have potential effects on systemic immunity [9, 115].

Gut barrier dysfunction promotes leakage of bacterial components into the bloodstream, such as lipopolysaccharide (LPS) [120]. In fact, as previously mentioned, CKD patients are frequently exposed to significant endotoxemia in the absence of infection. This entry of bacterial components from the gut to the bloodstream could contribute to or explain, at least in part, the persistent systemic inflammation in patients with CKD by activation of the innate immune response [7, 9]. Given the current understanding of sepsis, which involves concomitant inflammation and immunodeficiency, it is supposed that persistent TLR stimulation by endotoxins derived from intestinal bacteria may lead to a subsequent refractory state in CKD, a phenomenon named “compensatory anti-inflammatory syndrome,” suppressing innate and adaptive immunity, which could explain the simultaneous association of CKD with systemic inflammation and acquired immunodeficiency [9].

A recent study with animals supports this hypothesis that uremic dysbiosis and intestinal barrier dysfunction are significantly implicated in CKD-related systemic inflammation. The eradication of facultative anaerobic microbiota with antibiotics in uremic rats prevented bacterial translocation and reduced serum endotoxin levels and markers of systemic inflammation [121]. Thus, the gut seems to be a source of inflammation in CKD. Since inflammation is a major mediator of CKD progression, alterations in the gut and gut microbiome are potentially involved in CKD progression. In addition, CKD progression will perpetuate and amplify alterations in the gut microbiome and intestinal barrier, forming a vicious circuit [120]. Indeed, butyrate-producing species (Roseburia spp. and Faecalibacterium prausnitzii) seem to be negatively related to renal function and microinflammation of CKD patients, suggesting a relation of these bacteria with CKD-associated inflammation and CKD progression [76]. As previously mentioned, butyrate is a short-chain fatty acid resulting from bacterial fermentation, and it is an important nutrient for colonocytes and has anti-inflammatory properties. The short-chain fatty acids seem to also have beneficial effects on the kidney. In a study of acute kidney injury, the three main short-chain fatty acids (acetate, propionate, and butyrate) improved renal dysfunction caused by injury and were associated with reduced local and systemic inflammation [122].

As previously mentioned, the gut environment has an in important role in the composition and metabolism of gut microbiota, and CKD induces changes in the intestinal environment. The contribution of the intestinal environment and potentially of microbial metabolism to the pathogenesis of CKD-induced damage in gut barrier function and consequent inflammation and CKD progression was shown in a recent study with animals. A significant attenuation in the alteration of tight junction proteins in colonic tissue, oxidative stress and inflammation, and CKD progression delay was found when uremic rats were treated with high fermentable fiber, which has a potential beneficial effect on enzyme activity and on microbiota composition [123]. These benefits are associated with reduced cecal pH and alterations in gut microbiota composition (reduced microbial diversity) and in metabolomic profiles [124].

It is important to highlight, however, that most of these studies are in animals. Future studies are needed to confirm and explore the relations between intestinal dysbiosis, barrier dysfunction, CKD-related systemic inflammation, and CKD progression in humans, which are very complex with several influencing factors.

Potential dietetic approaches targeting the gut environment and colon-derived uremic toxins in CKD

As previously described, the potential involvement of gut disorders in CKD has encouraged the investigation of alternatives to modulate colonic imbalance and its potential negative effects on the disease. Dietary management and supplementation with prebiotics, probiotics, and synbiotics have been highlighted in this scenario. These approaches have focused on the modulation of the carbohydrate/protein ratio delivered into the colon and/or on the composition and metabolism of microbiota. Despite the paucity of high-quality studies, promising data have been generated.

The prescription of a low-protein diet is an approach widely employed for NDD-CKD patients to control metabolic disturbances and uremic symptoms and to slow the progression of the disease [125]. Studies suggest that this dietary intervention may also exert some influence on the gut environment. Reduction of serum urea is generally achieved with dietary protein restriction, which in turn may limit the influx of urea into the gut and potentially contribute to preventing harmful changes in the gut ecosystem [126]. Moreover, the decrease in protein intake may reduce the delivery of amino acids into the colon and the consequent proteolytic fermentation that generates p-cresol and indole. The benefits of decreasing protein intake on colon-derived uremic toxins, especially on indoxyl sulfate (IS), were demonstrated in healthy subjects [36] and CKD patients [127]. Most importantly, the colonic balance between dietary protein and fiber appears to be more relevant in the reduction of serum p-cresyl sulfate (PCS) and IS than the protein intake alone [127,128,129]. Therefore, consuming diets with higher fiber in relation to protein might represent an important alternative to reduce the production and to attenuate the accumulation of PCS and IS in CKD, particularly in hemodialysis patients who require larger amounts of protein to maintain their nitrogen balance.

Fibers are non-digestible carbohydrates that, along with some nitrogen compounds, represent the sources of energy of the gut microbiota. As previously mentioned, although fibers are the preferable substrate for most bacteria, the colonic balance between fiber and protein determines whether the energy supply will derivate from saccharolytic or proteolytic fermentation. In the presence of enough fiber, nitrogen is used to the growth and development of microbiota, instead of being deviated to proteolytic fermentation, avoiding, at least in part, the generation of colon-derived uremic toxins [33, 97]. In addition, products of fiber fermentation stimulate intestinal transit time, enhancing nitrogen fecal excretion and contributing to the integrity and function of the intestinal barrier [123, 130, 131]. In CKD, a meta-analysis of controlled feeding trials showed that fermentable fibers, namely, prebiotics [132], were able to promote a reduction in serum urea and creatinine [133]. The potential of prebiotics to reduce colon-derived uremic toxins has been attributed to their bifidogenic effect and the consequent inhibition of the growth of uremic toxin-producing bacteria [134]. However, to date, no clinical trial has evaluated the association between changes in the microbiota and the decrease in colon-derived uremic toxins after prebiotic therapy in CKD. The few short-term prebiotic interventions performed so far have reported mixed effects on PCS and IS [131, 135,136,137,138]. The reasons for the heterogeneity in the results may be related to differences in type and amount of prebiotic used, factors that might be relevant for modulation of serum PCS and/or IS in CKD (Table 1).

Table 1 Dietary interventions targeting the gut microbiota and uremic toxins in CKD
Full size table

Probiotics have also been described as an alternative approach to promote the balance of the gut microbiota. They are defined as live microorganisms, which, when administered in adequate amounts, confer benefits to the host [139]. The benefits of probiotics are mediated by their interaction with the host’s microbiota, gut epithelial cells, and gut immune system, potentially providing improvement in gut barrier integrity and function, in the activity of the gut immune system and inflammatory response, and in the control of photobiont overgrowth [140]. However, the body of evidence supporting the use of probiotics in CKD is still insipient, and the effectiveness of the treatment over the reduction of colon-derived uremic toxins and inflammation is so far inconclusive (Table 1). Variability in bacterial strains, doses, and administration form hinder comparison of the results. In addition, since the genesis of “dysbiose” in CKD seems to involve the imbalance of the entire colonic ecosystem, the greater availability of probiotic bacteria per se (without adequate availability of colonic substrate), might not be sufficient to recover the gut milieu. Therefore, the association of probiotics with prebiotics, composing synbiotics, might promote better modulation of the gut ecosystem. Despite the differences in the combination and doses of probiotic and prebiotics and the short-term follow-up, studies with synbiotics seem to be better quality, in terms of methodological aspects compared to those with probiotics or prebiotics alone. Moreover, the overall result has been more consistent showing a reduction in PCS [141,142,143] or an increase in the relative abundance of fecal Bifidobacterium (Table 1) [141, 144]. However, the benefits of probiotics and synbiotics in the reduction of PCS and IS appear to be associated with the sustained use of the therapy, since an increase in those toxins was observed when the supplementation was discontinued [134, 141, 145]. In addition, potential harmful effects of probiotic supplementation, such as bacterial translocation in conditions of increased intestinal permeability and immunodeficiency [146], generally found in advanced stages of CKD have not been investigated so far.

The effectiveness of prebiotics, probiotics, and synbiotics in reducing colon-derived uremic toxins in CKD patients was tested in a meta-analysis. Combining those therapies, a decrease of approximately 6.4 mg/L was found in IS in hemodialysis patients. The study, unfortunately, did not evaluate the effectiveness on PCS or the differences among the treatments due to the limited number of applicable clinical trials at that time [134].

In summary, the dietary manipulation of the gut ecosystem with prebiotics, probiotics, and synbiotics seems to be a promising strategy in the management of uremic toxins in CKD. The studies conducted so far have large variability with regard to the type and dose of fiber/prebiotic, bacterial strain, or their combination as synbiotics, and most of them have relevant limitations such as short follow-up, small sample sizes, and/or the absence of dietary control. Therefore, the evidence for such strategies is still incipient to establish the most appropriate therapeutic approach for CKD. Until high-quality evidence is obtained in this field, a healthy dietary pattern that includes larger amounts of whole grains, fruits, and vegetables should be encouraged among CKD patients since there are additional benefits other than the decrease in uremic toxin production [147, 148].

Conclusion

It is becoming clear that the chronic loss of renal function can negatively impact the gut ecosystem and vice versa, and this bidirectional relationship contributes with uremic syndrome, inflammation, immunodeficiency, kidney damage, cardiovascular risk, and mortality among patients with CKD. In this view, the gut has been described as an important target to modify the burden of CKD, and interventions with prebiotics, probiotics, and synbiotics have been emerging as an attractive strategy in this field. However, since the evidence about the impact of those approaches is still incipient, this constitutes an important field of research in CKD.