Gut microbiota alteration in CKD: From toxicity mechanisms to supplementation

Chronic Kidney Disease (CKD) refers to progressive and irreversible kidney function loss; it is currently an important health problem due to its high social costs. Decreased Glomerular Filtration Rate (GFR) causes accumulation of Uremic Toxins (UT) that must be excreted by the kidney, increasing their serum concentrations, toxicity, and hence disease progression. Dysbiosis is the alteration in the composition and structure of the intestinal microbiota and is related to systemic inflammation. Patients with CKD present biochemical changes at the intestinal level that cause dysbiosis, altering the kidney-gut axis, which is implicated in the higher production of UT. Evidence suggests an association between UT and cardiovascular risk in CKD, and different mechanisms are involved in each of them. Modulation of the gut microbiota by specific nutrients is a new strategy for the nutritional approach to CKD. Novel strategies based on the use of probiotics and prebiotics aim to reduce the synthesis and accumulation of UTs to reduce disease progression; however, with current evidence, the effect and benefit of supplementation cannot be concluded, so more research in humans is needed to identify useful bacterial strains and doses to obtain beneficial effects in CKD patients.

Introduction

Chronic Kidney Disease (CKD) is the progressive and irreversible renal function failure; with a subclinical debut, in most cases the diagnosis is tardy, conferring a poor prognosis to patients [1-3]. Actually, is considered a public health problem due to the high social costs [1,4,5]. Literature reports that behavioral interventions are fundamental pillars to prevent the progression of the disease [6].

A kidney function is the excretion of Uremic Toxins (UT), by two mechanisms: one, by glomerular filtration, and the other by transporter-mediated tubular secretion; in CKD, decreased Glomerular Filtration Rate (GFR) leads to the accumulation of Uremic Retention Molecules (URMs) [7,8], resulting in the accumulation of UTs and increase in serum concentrations, thus contributing to the characteristic uremic syndrome [6,7,9].

In CKD there is an accumulation of UT produced by gut microbial metabolism of aromatic amino acids: tyrosine, phenylalanine, and tryptophan [6,7,10], evidence in vitro and In vivo shows that these UT are implicated in pathophysiological mechanisms of cardiovascular disease, the leading cause of death in renal failure patients [5,11-14]. This review presents an overview of gut microbiota alterations in CKD, the mechanisms underlying the main UT on disease progression and cardiovascular risk, and the nutritional strategies for these conditions.

Gut microbiota

The human gut microbiota is the community of more than 100 trillion microbial cells and more than 1000 species of bacteria that coexist in the host; in non-pathological conditions, the intestinal microbiota is mainly composed of Firmicutes, Bacteroidetes, Actinobacteria, Verrucomicrobia and, to a lesser extent, Proteobacteria; being Gram-negative Bacteroidetes and Gram-positive Firmicutes the most abundant [6,15].

The gut microbiota plays an essential role in the digestion of food and the metabolism of nutrients; also, it regulates the function of the immune system through the maintenance of epithelial homeostasis [6,15-17]; in addition, some microorganisms are involved in the synthesis of B vitamins, vitamin K and ascorbic acid, and the synthesis of Short-Chain Fatty Acids (SCFA) that participate in anti-inflammatory and antiproliferative mechanisms [2,6].

Diet is an environmental factor that modifies the composition of the intestinal microbiota, altering the metabolite profile of each individual [18]; low-fiber diets, for example, reduce SCFA synthesis, producing harmful metabolites such as lipopolysaccharides (LPS), thus contributing to intestinal dysbiosis [15,19,20].

Dysbiosis involves disturbances in the composition and structure of the gut microbiota resulting in endotoxemia, which is associated with elevated proinflammatory cytokines such as tumor necrosis factor α (TNF-α) and interleukin-6 (IL-6), and oxidative stress, triggering systemic inflammation present in CKD [6,20-23].

In CKD, microbiota metabolism is considered a non-traditional but modifiable risk factor influencing the progression of renal damage [24]; dysbiosis disrupts the kidney-gut axis, is linked to increased production of UT, combined with deterioration of renal function, maintains uremic status and results in accelerated decline in GFR [24].

Altered microbiota in CKD

Patients with CKD exhibit gut biochemical changes due to decreased fiber intake, frequent use of antibiotics, and metabolic acidosis [6,25,26]; in addition, elevated uremia increases their flow into the gastrointestinal tract, where urease-expressing bacteria metabolize it into CO2 and ammonia, damaging the epithelial barrier, triggering the immune system response and resulting in dysbiosis, as shown in Figure 1 [7,27-29].

Figure 1: Microbiota alterations in CKD patients.

Evidence supports quantitative and qualitative alterations of the microbiota in CKD [16,23,30,31]; renal patients’ microbiota has fewer families of Lactobaci laceae and Prevotellaceae, and of SCFA-producing bacteria, especially butyrate, that protect against inflammation [32], and 100-fold more species of Enterobacteriaceae and Enterococci [6,23]. Systematic reviews support the findings of changes in the microbiota during CKD, showing an abundance of proteolytic bacteria that produce toxic metabolites, such as the phylum Proteobacteria and Fusobacteria, the genus Escherichia/Shigella, and a lower abundance of Roseburia, Faecalibacterium, Pyramidobacter, Prevotellaceae and Prevotella [23,33].

A study identified that CKD patients have a lower abundance of Actinobacteria and a higher amount of Verrucomicrobia compared to healthy subjects, although the role of this change in CKD is unclear [23]. Other research concluded that end-stage disease is characterized by a decrease in the number of Bacteroides and an increase in the amount of Escherichia and Shigella [25], Also, other investigations report that microbiota contains a considerable number of bacterial families with urease, uricase, and tryptophanase activity, and increase of Escherichia/Shigella, Subdoligranulum, Fusobacterium in stage 5 of the disease [2,32].

Such alterations induce structural changes that affect intestinal epithelium, tight junctions and increase gut permeability, thus facilitating the leakage of bacterial metabolites into circulation and triggering immune responses, setting up an inflammatory microenvironment that accelerates the progression of CKD [6,7,27,34,35]. Dysbiosis induces proliferation and abnormal differentiation of B and T lymphocytes, resulting in the production of auto-antibodies and inflammatory molecules, also activates the renin-angiotensin system, along with increased UT production, contributing to the deterioration of renal function [33]. In CKD patients, dysbiosis is involved in establishing end-stage Protein-Energy Wasting (PEW) [35], and it is a determinant factor in altering neuroendocrine-immunological communication mechanisms in kidney disease [33].

Uremic Toxins (UT)

Renal function decline and uncompleted clearance of nitrogen compounds result in UT accumulation [36-38]. Several UTs are generated in the gut, as by-products of amino acid degradation by intestinal bacteria [37]; and they are associated with increased risk of CVD [2,26,30,34]. They include p-cresyl (pC), p-cresyl sulfate (pCS), p-cresyl glucuronide (PCG), indoxyl sulfate (IS), indole-3-acetic acid (IAA), and trimethylamine N-oxide (TMAO).

Evidence supports that serum IS concentration is predictive of CKD progression [39], and that pCS correlates negatively with GFR [40]. A study in hemodialysis patients identified that inflammatory markers, such as IL-6, are correlated to serum IS levels [2]; Yeh et al. in 2016 reported that IS and pCS activate inflammatory pathways that accelerate renal damage and increase cardiovascular risk [41]. Other studies have concluded that these UTs are determinants of increased expression of inflammatory markers, and increased oxidative stress, which enhances endothelial dysfunction and progression of CKD [32,34].

We now describe the different action mechanisms of each UT:

p-cresyl (pC) and p-cresyl sulfate (pCS): UT p-cresyl (pC) is a uremic toxin that binds to albumin for excretion by glomerular filtration [42], has a low molecular weight (MW: 108. 14 g / mol); it is synthesized from the catabolism of tyrosine and phenylalanine and is metabolized in the gut microbiota to its conjugates p-cresyl sulfate (pCS) and p-cresyl glucuronidate (pCG) [43]. Heir serum concentration is associated with GFR [44]. In the early stages of CKD, levels increase to 20.1 mg/L, and in the terminal stage, they reach 40.7 mg/L, with concentrations up to 17 times higher than in healthy subjects [42].

Their metabolites have been associated with immune system dysfunctions [45-48]. Evidence in vitro found that pCS suppresses STAT5 signaling and significantly reduces peripheral B cells [46], represses certain functions in innate immune system cells, decreases IL-12 synthesis, and increases IL-10 synthesis in peritoneal macrophages [47]. Another research reported that pCS interferes with antigen presentation by decreasing HLD-DR expression, reducing the adaptive immune response [48].

pCS is associated with cardiovascular damage and all its causes of mortality in CKD patients [43,49] due to the proinflammatory effect [12,50,51]. It is implicated in the formation of free radicals produced by leukocytes, endothelial cells, vascular smooth muscle cells, and renal tubular cells, and in the reduction of glutathione levels [52]. In vivo research reports certain mechanisms of damage of this UT, it causes mitochondrial damage by inducing AMPK-mediated mitochondrial hyperfusion [53]. Intervenes in endothelial dysfunction by correlating negatively with thrombospondin desintegrina metalloproteinase (ADAMTS) activity and positively with the markers of endothelial activation/damage ANGPT2 and MMP-7 [13]. An experiment in the cell model identified that it induced osteogenesis by triggering pERK / pJNK / pP38 MAPK signaling pathways and NF-κB translocation, which results in uremic vascular calcification [54].

Indoxyl sulfate (IS) and indole-3-acetic acid (IAA): In the colon, intestinal bacteria break down tryptophan into indole-3-acetic acid (IAA) and indole [55]; indole then enters portal circulation and the liver to be hydroxylated by cytochrome P450 2E1 (CYP2E1), then is sulfated by sulfotransferase 1A1 (SULT1A1) to produce indoxyl sulfate (IS) [37]. The evidence suggests that accumulation of these UTs in CKD is associated with a decline in renal function and a worse prognosis [55]. IS plasma levels in CKD patients are 54 higher than in healthy subjects [45]. IS favors dysbiosis by downregulating tight junction-expressing proteins, such as occludin and claudin-1; moreover, it favors oxidative stress-producing free radicals [2].

Several studies correlate these UT to increased cardiovascular risk [2,56]; certain evidence in CKD patients associate IS with adverse cardiovascular events independently of renal function [57], while others report an increased risk of mortality in CKD, but no increased risk of cardiovascular events [43]. Evidence in vitro identifies that IS promotes vascular smooth muscle cell (VSMC) proliferation and increases extracellular matrix (ECM) production and deposition, thus inducing calcifications [14], an additional investigation suggests that IS induces a macrophage-mediated inflammation which blocks cholesterol passage to high-density lipoproteins (HDL) [14]. It is also reported that it induces oxidative stress in endothelial cells [45]. An experimental study in rats showed that exposure to IS induces arterial thrombosis by decreasing aortic levels of sirtuin 1, a class III histone deacetylase involved in oxidative stress [58]. However, evidence in humans is limited [57].

They are also implicated in fibrosis mechanisms [12]; the aryl hydrocarbon receptor (AhR) recognizes IS or IAA and triggers signaling pathways activating nuclear factor kappa light chain enhancer of activated B cells (NFkB) and the expression of adhesion molecules; in addition, it induces the production of ROS activating p38 and p42 / p44 mitogen-activated protein kinases (MAPKs), promoting liberation of proinflammatory and profibrotic cytokines, such as transforming growth factor beta (TGF-β) and alpha-smooth muscle actin (α-sma) [2,14]. IS has also been associated with an increase in IL-6 [12].

Trimethylamine N-oxide (TMAO): Trimethylamine N-oxide (TMAO) is a low-molecular-weight metabolite (75 Da) derived from the metabolism of choline, carnitine, and betaine by intestinal bacteria; its synthesis involves two steps: release by gut microbiota from dietary Trimethylamine (TMA) precursors, actively absorbed into the bloodstream for oxidation to TMAO by the liver enzyme monooxygenase. Subsequently, TMAO and TMA can convert to dimethylamine (DMA) [59-62]. The elimination occurs via urine by glomerular filtration [61]. Consumption of foods such as meat, eggs, dairy products, and saltwater fish which are dietary sources of TMAO may influence its serum concentration as well as its metabolic precursors [60], however, the composition of gut microbiota is the main factor that regulates circulating TMAO concentration, along with BMF enzymatic activity and renal function [60,61].

The serum levels of TMAO increase as renal function decreases [61,62]. Various papers documented an increase in TMAO in CKD, with participation in renal fibrosis [37]. Hsu et al. in 2020 reported that stage 2-4 CKD children presented higher serum levels of DMA, TMA, and TMAO than in stage 1, whereas urinary levels of DMA and TMAO in advanced stages were lower [59]. Missailidis et al. reported that patients in stage 5 CKD had a 13-fold increase in TMAO levels compared to controls, and in stages 3-4 TMAO levels were inversely correlated with GFR [60]; Pelletier in 2019 confirmed increased TMAO in hemodialysis patients [61].

High levels of TMAO in CKD are associated with increased cardiovascular risk and it is an independent predictor of mortality in stages 3-5 [2,60], evidence shows that elevated serum TMAO levels in CKD patients are associated with a 70% higher risk of mortality [63]. Animal studies have reported that this UT enhances systemic inflammation through macrophage activation and foam cell formation; it is also involved in proatherogenic mechanisms, including disruption of cholesterol transport and platelet response, promoting thrombosis [60,62]. Zhang et al. reported in 2021 that mice fed choline and TMAO increased phosphorylation of SMAD family member 3 (Smad3) in the kidney, an important regulator of the TGF-β signaling pathway in fibrotic kidney disease [62].

Supplementation of prebiotics and probiotics in CKD

Manipulation of the gut microbiome in CKD patients by supplementation with probiotics, prebiotics, or synbiotics has the target to reduce the synthesis and accumulation of UT, and to increase SCFA synthesis, to reduce disease progression [64]. Studies have been conducted on the use of probiotics, mainly Lactobacillus and Bifidobacteria, as well as prebiotics in CKD, aimed at reversing intestinal dysbiosis [2], to increase SCFA concentration, as butyrate, which has protective effects maintaining intracellular intestinal pH, improving the composition of the gut microbiota [2]; and decreasing oxidative stress, systemic inflammation, and UT production [65].

Prebiotics are non-host-digestible food components that selectively induce the growth or activity of a limited number of intestinal microorganisms that contribute to well-being [6,66]. The main prebiotics are inulin, fructooligosaccharides, galactooligosaccharides, soy oligosaccharides, xylooligosaccharides, and pyrodextrins [6]. Although the impact on the intestinal microbiota of prebiotic treatment has not been investigated by omics sciences [66], they appear to be a strategy within nutritional treatment to regulate the fermentable carbohydrate-protein balance in the colon [26]; here is evidence of their effect on the symptomatology of patients with CKD.

A study conducted in patients with terminal CKD and supplemented with a prebiotic (starch with high corn-resistant amylose content) for 8 weeks concluded that the use of this prebiotic decreases intestinal inflammation and oxidative stress [34]. Likewise, Sirich et al. supplemented patients on hemodialysis therapy with this type of resistant starch for 6 weeks and found a significant reduction in IS and pC [67]; another investigation, also in patients on replacement therapy using the same type of supplement, reported decreased levels of total and LDL cholesterol, and inflammatory markers: TNF-α, IL-6, IL-8 and C-reactive protein (pCr) [68].

Other types of prebiotics studied for the reduction of UT, are oat and barley beta-glucans; a study performed with healthy subjects supplemented with this class of prebiotics identified that the serum level of pCS decreases, while the level of IS remains unchanged [21]. A 3-month, double-blind, randomized, controlled trial supplemented patients with end-stage CKD with 12 g of short-chain fructoligosaccharide (FOS)-type prebiotics, with no significant reduction in serum or urinary levels of pCS, IS, or IAA, markers of intestinal permeability such as zonulin: or markers of inflammation IL-6 and pCr [26].

Probiotics are live microorganisms that are beneficial to host health in adequate amounts [19,65]. Some of their functions include increasing the fermentation of dietary fiber, reducing intestinal pH, decreasing uremia by degrading urea and uric acid, and modulating the composition of the intestinal microbiota [65]. Evidence suggests that the gut microbiota of CKD patients receiving probiotic supplementation utilizes metabolic waste as a substrate, leading to a decrease in toxic metabolites [65]. There are contradictory results on the effect of probiotics on the composition of the microbiota: in some studies, an increase in the proportion of genera with no effect on the composition and diversity of the gut microbiota has been observed, while in others changes in its composition have been observed [19].

Evidence suggests that probiotics positively affect the immune system, generating increased expression of anti-inflammatory cytokines, such as IL-10, and decreased expression of proinflammatory cytokines, such as IL-6 and TNFα [65]. Studies with probiotic supplementation in CKD patients reported encouraging results against uremia and inflammation, although long-term studies are needed [27]. SYNERGY (Symbiotics alleviating renal failure by improving gut microbiology) is one of the first trials in this field, no changes in GFR or proinflammatory markers were found, but changes in the nutritional status of patients were observed by increasing serum albumin levels and slowing the progression of proteinuria, as well as significant increases of Bifidobacterium in the composition of the microbiota [69].

A randomized clinical trial administering a probiotic supplement containing 16 × 109 CFU/day of L. casei shirota to patients with stage 3-4 CKD for two months, reported a decrease in serum urea levels; similar results were observed in a pilot study using different bacterial strains (L. acidophilus, B. longum and S. thermophilus) were used at doses of 9 × 109 CFU/day for three months, finding a reduction in urea nitrogen levels; meanwhile, a placebo-controlled clinical trial with oral administration of B. longum capsules to HD patients reported a reduction in serum phosphorus concentration [65]. Meanwhile, an animal model study reported that treatment with 1 x 10 CFU/kg/day of Lactobacillus increases urinary protein excretion [70].

The identification of bacterial strains or intermediate metabolites as therapeutic targets to modulate altered gut microbiota could be a target in the treatment of CKD [6,19]. Modulation of the gut microbiota by prebiotics and probiotics provides an attractive approach in CKD with interesting findings in animal models for the reduction of UT and intestinal permeability [24,70,71].

Conclusion

Uremic toxins accumulated as a result of impaired renal function modulate pathways of oxidative stress, systemic inflammation, and dysbiosis in the context of CKD, so the study of their pathophysiological mechanisms is important to better understand their role in CKD progression.

Restoration of the gut microbiota in CKD patients by prebiotic and probiotic supplementation is an alternative treatment but requires further research and evaluation to identify the type of prebiotic or useful bacterial strains, dose, and duration of treatment for beneficial effects in CKD patients.

Author contributions

Conceptualization, C.J.D.l.C.-A and S.R.-D.l.S; investigation, C.J.D.l.C.; writing—original draft preparation, C.J.D.l.C.-A and S.R.-D.l.S.; writing—review and editing, C.J.D.l.C.-A and S.R.-D.l.S.; supervision, J.F.T.-R. and S.R.-D.l.S. All authors have read and agreed to the published version of the manuscript.

The authors acknowledge the effort of researchers whose work was used in this review. The image was created using BioRender.com

1. Lv JC, Zhang LX. Prevalence and Disease Burden of Chronic Kidney Disease. Adv Exp Med Biol. 2019;1165:3-15. doi: 10.1007/978-981-13-8871-2_1. PMID: 31399958.
2. Plata C, Cruz C, Cervantes LG, Ramírez V. The gut microbiota and its relationship with chronic kidney disease. Int Urol Nephrol. 2019 Dec;51(12):2209-2226. doi: 10.1007/s11255-019-02291-2. Epub 2019 Oct 1. PMID: 31576489.
3. Wang YN, Ma SX, Chen YY, Chen L, Liu BL, Liu QQ, Zhao YY. Chronic kidney disease: Biomarker diagnosis to therapeutic targets. Clin Chim Acta. 2019 Dec; 499:54-63. doi: 10.1016/j.cca.2019.08.030. Epub 2019 Aug 30. PMID: 31476302.
4. Bao YW, Yuan Y, Chen JH, Lin WQ. Kidney disease models: tools to identify mechanisms and potential therapeutic targets. Zool Res. 2018 Mar 18;39(2):72-86. doi: 10.24272/j.issn.2095-8137.2017.055. PMID: 29515089; PMCID: PMC5885387.
5. Hatem-Vaquero M, de Frutos S, Luengo A, González Abajo A, Griera M, Rodríguez-Puyol M, Rodríguez-Puyol D, Calleros L. Contribution of uraemic toxins to the vascular fibrosis associated with chronic kidney disease. Nefrologia (Engl Ed). 2018 Nov-Dec;38(6):639-646. English, Spanish. doi: 10.1016/j.nefro.2018.07.008. Epub 2018 Oct 15. PMID: 30337107.
6. Al Khodor S, Shatat IF. Gut microbiome and kidney disease: a bidirectional relationship. Pediatr Nephrol. 2017 Jun;32(6):921-931. doi: 10.1007/s00467-016-3392-7. Epub 2016 Apr 29. PMID: 27129691; PMCID: PMC5399049.
7. Gryp T, De Paepe K, Vanholder R, Kerckhof FM, Van Biesen W, Van de Wiele T, Verbeke F, Speeckaert M, Joossens M, Couttenye MM, Vaneechoutte M, Glorieux G. Gut microbiota generation of protein-bound uremic toxins and related metabolites is not altered at different stages of chronic kidney disease. Kidney Int. 2020 Jun;97(6):1230-1242. doi: 10.1016/j.kint.2020.01.028. Epub 2020 Feb 17. PMID: 32317112.
8. Takada T, Yamamoto T, Matsuo H, Tan JK, Ooyama K, Sakiyama M, Miyata H, Yamanashi Y, Toyoda Y, Higashino T, Nakayama A, Nakashima A, Shinomiya N, Ichida K, Ooyama H, Fujimori S, Suzuki H. Identification of ABCG2 as an Exporter of Uremic Toxin Indoxyl Sulfate in Mice and as a Crucial Factor Influencing CKD Progression. Sci Rep. 2018 Jul 24;8(1):11147. doi: 10.1038/s41598-018-29208-w. PMID: 30042379; PMCID: PMC6057959.
9. Böhringer F, Jankowski V, Gajjala PR, Zidek W, Jankowski J. Release of uremic retention solutes from protein binding by hypertonic predilution hemodiafiltration. ASAIO J. 2015 Jan-Feb;61(1):55-60. doi: 10.1097/MAT.0000000000000166. PMID: 25419832.
10. Rysz J, Franczyk B, Ławiński J, Olszewski R, Ciałkowska-Rysz A, Gluba-Brzózka A. The Impact of CKD on Uremic Toxins and Gut Microbiota. Toxins (Basel). 2021 Mar 31;13(4):252. doi: 10.3390/toxins13040252. PMID: 33807343; PMCID: PMC8067083.
11. van Gelder MK, Middel IR, Vernooij RWM, Bots ML, Verhaar MC, Masereeuw R, Grooteman MP, Nubé MJ, van den Dorpel MA, Blankestijn PJ, Rookmaaker MB, Gerritsen KGF. Protein-Bound Uremic Toxins in Hemodialysis Patients Relate to Residual Kidney Function, Are Not Influenced by Convective Transport, and Do Not Relate to Outcome. Toxins (Basel). 2020 Apr 7;12(4):234. doi: 10.3390/toxins12040234. PMID: 32272776; PMCID: PMC7232478.
12. Madero M, Cano KB, Campos I, Tao X, Maheshwari V, Brown J, Cornejo B, Handelman G, Thijssen S, Kotanko P. Removal of Protein-Bound Uremic Toxins during Hemodialysis Using a Binding Competitor. Clin J Am Soc Nephrol. 2019 Mar 7;14(3):394-402. doi: 10.2215/CJN.05240418. Epub 2019 Feb 12. PMID: 30755453; PMCID: PMC6419294.
13. Glorieux G, Vanholder R, Van Biesen W, Pletinck A, Schepers E, Neirynck N, Speeckaert M, De Bacquer D, Verbeke F. Free p-cresyl sulfate shows the highest association with cardiovascular outcome in chronic kidney disease. Nephrol Dial Transplant. 2021 May 27;36(6):998-1005. doi: 10.1093/ndt/gfab004. PMID: 33508125.
14. Yamamoto S. Molecular mechanisms underlying uremic toxin-related systemic disorders in chronic kidney disease: focused on β2-microglobulin-related amyloidosis and indoxyl sulfate-induced atherosclerosis-Oshima Award Address 2016. Clin Exp Nephrol. 2019 Feb;23(2):151-157. doi: 10.1007/s10157-018-1588-9. Epub 2018 Jun 5. PMID: 29869756; PMCID: PMC6510801.
15. Haro C, Rangel-Zúñiga OA, Alcalá-Díaz JF, Gómez-Delgado F, Pérez-Martínez P, Delgado-Lista J, Quintana-Navarro GM, Landa BB, Navas-Cortés JA, Tena-Sempere M, Clemente JC, López-Miranda J, Pérez-Jiménez F, Camargo A. Intestinal Microbiota Is Influenced by Gender and Body Mass Index. PLoS One. 2016 May 26;11(5):e0154090. doi: 10.1371/journal.pone.0154090. PMID: 27228093; PMCID: PMC4881937.
16. Mishima E, Fukuda S, Mukawa C, Yuri A, Kanemitsu Y, Matsumoto Y, Akiyama Y, Fukuda NN, Tsukamoto H, Asaji K, Shima H, Kikuchi K, Suzuki C, Suzuki T, Tomioka Y, Soga T, Ito S, Abe T. Evaluation of the impact of gut microbiota on uremic solute accumulation by a CE-TOFMS-based metabolomics approach. Kidney Int. 2017 Sep;92(3):634-645. doi: 10.1016/j.kint.2017.02.011. Epub 2017 Apr 8. PMID: 28396122.
17. García-Montero C, Fraile-Martínez O, Gómez-Lahoz AM, Pekarek L, Castellanos AJ, Noguerales-Fraguas F, Coca S, Guijarro LG, García-Honduvilla N, Asúnsolo A, Sanchez-Trujillo L, Lahera G, Bujan J, Monserrat J, Álvarez-Mon M, Álvarez-Mon MA, Ortega MA. Nutritional Components in Western Diet Versus Mediterranean Diet at the Gut Microbiota-Immune System Interplay. Implications for Health and Disease. Nutrients. 2021 Feb 22;13(2):699. doi: 10.3390/nu13020699. PMID: 33671569; PMCID: PMC7927055.
18. Ramos-Romero S, Léniz A, Martínez-Maqueda D, Amézqueta S, Fernández-Quintela A, Hereu M, Torres JL, Portillo MP, Pérez-Jiménez J. Inter-Individual Variability in Insulin Response after Grape Pomace Supplementation in Subjects at High Cardiometabolic Risk: Role of Microbiota and miRNA. Mol Nutr Food Res. 2021 Jan;65(2):e2000113. doi: 10.1002/mnfr.202000113. Epub 2020 Dec 16. PMID: 33202108.
19. Gomes AC, Hoffmann C, Mota JF. Gut microbiota is associated with adiposity markers and probiotics may impact specific genera. Eur J Nutr. 2020 Jun;59(4):1751-1762. doi: 10.1007/s00394-019-02034-0. Epub 2019 Jun 27. PMID: 31250099.
20. Ojo O, Ojo OO, Zand N, Wang X. The Effect of Dietary Fibre on Gut Microbiota, Lipid Profile, and Inflammatory Markers in Patients with Type 2 Diabetes: A Systematic Review and Meta-Analysis of Randomised Controlled Trials. Nutrients. 2021 May 26;13(6):1805. doi: 10.3390/nu13061805. PMID: 34073366; PMCID: PMC8228854.
21. Cosola C, De Angelis M, Rocchetti MT, Montemurno E, Maranzano V, Dalfino G, Manno C, Zito A, Gesualdo M, Ciccone MM, Gobbetti M, Gesualdo L. Beta-Glucans Supplementation Associates with Reduction in P-Cresyl Sulfate Levels and Improved Endothelial Vascular Reactivity in Healthy Individuals. PLoS One. 2017 Jan 20;12(1):e0169635. doi: 10.1371/journal.pone.0169635. PMID: 28107445; PMCID: PMC5249102.
22. Santos-Marcos JA, Haro C, Vega-Rojas A, Alcala-Diaz JF, Molina-Abril H, Leon-Acuña A, Lopez-Moreno J, Landa BB, Tena-Sempere M, Perez-Martinez P, Lopez-Miranda J, Perez-Jimenez F, Camargo A. Sex Differences in the Gut Microbiota as Potential Determinants of Gender Predisposition to Disease. Mol Nutr Food Res. 2019 Apr;63(7):e1800870. doi: 10.1002/mnfr.201800870. Epub 2019 Feb 13. PMID: 30636111.
23. Li F, Wang M, Wang J, Li R, Zhang Y. Alterations to the Gut Microbiota and Their Correlation With Inflammatory Factors in Chronic Kidney Disease. Front Cell Infect Microbiol. 2019 Jun 12;9:206. doi: 10.3389/fcimb.2019.00206. PMID: 31245306; PMCID: PMC6581668.
24. Cosola C, Rocchetti MT, Sabatino A, Fiaccadori E, Di Iorio BR, Gesualdo L. Microbiota issue in CKD: how promising are gut-targeted approaches? J Nephrol. 2019 Feb;32(1):27-37. doi: 10.1007/s40620-018-0516-0. Epub 2018 Aug 1. PMID: 30069677.
25. Jiang S, Wang B, Sha T, Li X. Changes in the Intestinal Microbiota in Patients with Stage 5 Chronic Kidney Disease on a Low-Protein Diet and the Effects of Human to Rat Fecal Microbiota Transplantation. Med Sci Monit. 2020 Jun 27;26:e921557. doi: 10.12659/MSM.921557. PMID: 32592577; PMCID: PMC7336834.
26. Ramos CI, Armani RG, Canziani MEF, Dalboni MA, Dolenga CJR, Nakao LS, Campbell KL, Cuppari L. Effect of prebiotic (fructooligosaccharide) on uremic toxins of chronic kidney disease patients: a randomized controlled trial. Nephrol Dial Transplant. 2019 Nov 1;34(11):1876-1884. doi: 10.1093/ndt/gfy171. PMID: 29939302.
27. Meijers B, Evenepoel P, Anders HJ. Intestinal microbiome and fitness in kidney disease. Nat Rev Nephrol. 2019 Sep;15(9):531-545. doi: 10.1038/s41581-019-0172-1. Epub 2019 Jun 26. PMID: 31243394.
28. Chaves LD, McSkimming DI, Bryniarski MA, Honan AM, Abyad S, Thomas SA, Wells S, Buck M, Sun Y, Genco RJ, Quigg RJ, Yacoub R. Chronic kidney disease, uremic milieu, and its effects on gut bacterial microbiota dysbiosis. Am J Physiol Renal Physiol. 2018 Sep 1;315(3):F487-F502. doi: 10.1152/ajprenal.00092.2018. Epub 2018 Apr 25. PMID: 29693447; PMCID: PMC6172581.
29. Kim JE, Kim HE, Park JI, Cho H, Kwak MJ, Kim BY, Yang SH, Lee JP, Kim DK, Joo KW, Kim YS, Kim BS, Lee H. The Association between Gut Microbiota and Uremia of Chronic Kidney Disease. Microorganisms. 2020 Jun 16;8(6):907. doi: 10.3390/microorganisms8060907. PMID: 32560104; PMCID: PMC7355700.
30. Turkmen K, Erdur FM. The relationship between colonization of Oxalobacter formigenes serum oxalic acid and endothelial dysfunction in hemodialysis patients: from impaired colon to impaired endothelium. Med Hypotheses. 2015 Mar;84(3):273-5. doi: 10.1016/j.mehy.2015.01.010. Epub 2015 Jan 19. PMID: 25630805.
31. Wang X, Yang S, Li S, Zhao L, Hao Y, Qin J, Zhang L, Zhang C, Bian W, Zuo L, Gao X, Zhu B, Lei XG, Gu Z, Cui W, Xu X, Li Z, Zhu B, Li Y, Chen S, Guo H, Zhang H, Sun J, Zhang M, Hui Y, Zhang X, Liu X, Sun B, Wang L, Qiu Q, Zhang Y, Li X, Liu W, Xue R, Wu H, Shao D, Li J, Zhou Y, Li S, Yang R, Pedersen OB, Yu Z, Ehrlich SD, Ren F. Aberrant gut microbiota alters host metabolome and impacts renal failure in humans and rodents. Gut. 2020 Dec;69(12):2131-2142. doi: 10.1136/gutjnl-2019-319766. Epub 2020 Apr 2. PMID: 32241904; PMCID: PMC7677483.
32. Jiang S, Xie S, Lv D, Wang P, He H, Zhang T, Zhou Y, Lin Q, Zhou H, Jiang J, Nie J, Hou F, Chen Y. Alteration of the gut microbiota in Chinese population with chronic kidney disease. Sci Rep. 2017 Jun 6;7(1):2870. doi: 10.1038/s41598-017-02989-2. PMID: 28588309; PMCID: PMC5460291.
33. Zhao J, Ning X, Liu B, Dong R, Bai M, Sun S. Specific alterations in gut microbiota in patients with chronic kidney disease: an updated systematic review. Ren Fail. 2021 Dec;43(1):102-112. doi: 10.1080/0886022X.2020.1864404. PMID: 33406960; PMCID: PMC7808321.
34. Headley SA, Chapman DJ, Germain MJ, Evans EE, Hutchinson J, Madsen KL, Ikizler TA, Miele EM, Kirton K, O'Neill E, Cornelius A, Martin B, Nindl B, Vaziri ND. The effects of 16-weeks of prebiotic supplementation and aerobic exercise training on inflammatory markers, oxidative stress, uremic toxins, and the microbiota in pre-dialysis kidney patients: a randomized controlled trial-protocol paper. BMC Nephrol. 2020 Nov 26;21(1):517. doi: 10.1186/s12882-020-02177-x. PMID: 33243160; PMCID: PMC7689649.
35. Lin TY, Wu PH, Lin YT, Hung SC. Characterization of Gut Microbiota Composition in Hemodialysis Patients With Normal Weight Obesity. J Clin Endocrinol Metab. 2020 Jun 1;105(6):dgaa166. doi: 10.1210/clinem/dgaa166. PMID: 32296838.
36. Chao CT, Lin SH. Uremic Toxins and Frailty in Patients with Chronic Kidney Disease: A Molecular Insight. Int J Mol Sci. 2021 Jun 10;22(12):6270. doi: 10.3390/ijms22126270. PMID: 34200937; PMCID: PMC8230495.
37. Lim YJ, Sidor NA, Tonial NC, Che A, Urquhart BL. Uremic Toxins in the Progression of Chronic Kidney Disease and Cardiovascular Disease: Mechanisms and Therapeutic Targets. Toxins (Basel). 2021 Feb 13;13(2):142. doi: 10.3390/toxins13020142. PMID: 33668632; PMCID: PMC7917723.
38. Duranton F, Cohen G, De Smet R, Rodriguez M, Jankowski J, Vanholder R, Argiles A; European Uremic Toxin Work Group. Normal and pathologic concentrations of uremic toxins. J Am Soc Nephrol. 2012 Jul;23(7):1258-70. doi: 10.1681/ASN.2011121175. Epub 2012 May 24. Erratum in: J Am Soc Nephrol. 2013 Dec;24(12):2127-9. PMID: 22626821; PMCID: PMC3380651.
39. Wu IW, Hsu KH, Lee CC, Sun CY, Hsu HJ, Tsai CJ, Tzen CY, Wang YC, Lin CY, Wu MS. p-Cresyl sulphate and indoxyl sulphate predict progression of chronic kidney disease. Nephrol Dial Transplant. 2011 Mar;26(3):938-47. doi: 10.1093/ndt/gfq580. Epub 2010 Sep 29. PMID: 20884620; PMCID: PMC3042976.
40. Meijers BK, Claes K, Bammens B, de Loor H, Viaene L, Verbeke K, Kuypers D, Vanrenterghem Y, Evenepoel P. p-Cresol and cardiovascular risk in mild-to-moderate kidney disease. Clin J Am Soc Nephrol. 2010 Jul;5(7):1182-9. doi: 10.2215/CJN.07971109. Epub 2010 Apr 29. PMID: 20430946; PMCID: PMC2893077.
41. Yeh YC, Huang MF, Liang SS, Hwang SJ, Tsai JC, Liu TL, Wu PH, Yang YH, Kuo KC, Kuo MC, Chen CS. Indoxyl sulfate, not p-cresyl sulfate, is associated with cognitive impairment in early-stage chronic kidney disease. Neurotoxicology. 2016 Mar; 53:148-152. doi: 10.1016/j.neuro.2016.01.006. Epub 2016 Jan 18. PMID: 26797588.
42. Opdebeeck B, D'Haese PC, Verhulst A. Molecular and Cellular Mechanisms that Induce Arterial Calcification by Indoxyl Sulfate and P-Cresyl Sulfate. Toxins (Basel). 2020 Jan 19;12(1):58. doi: 10.3390/toxins12010058. PMID: 31963891; PMCID: PMC7020422.
43. Lin CJ, Wu V, Wu PC, Wu CJ. Meta-Analysis of the Associations of p-Cresyl Sulfate (PCS) and Indoxyl Sulfate (IS) with Cardiovascular Events and All-Cause Mortality in Patients with Chronic Renal Failure. PLoS One. 2015 Jul 14;10(7):e0132589. doi: 10.1371/journal.pone.0132589. PMID: 26173073; PMCID: PMC4501756.
44. Poesen R, Viaene L, Verbeke K, Claes K, Bammens B, Sprangers B, Naesens M, Vanrenterghem Y, Kuypers D, Evenepoel P, Meijers B. Renal clearance and intestinal generation of p-cresyl sulfate and indoxyl sulfate in CKD. Clin J Am Soc Nephrol. 2013 Sep;8(9):1508-14. doi: 10.2215/CJN.00300113. Epub 2013 Jun 27. PMID: 23813557; PMCID: PMC3805062.
45. Lin CJ, Chen HH, Pan CF, Chuang CK, Wang TJ, Sun FJ, Wu CJ. p-Cresylsulfate and indoxyl sulfate level at different stages of chronic kidney disease. J Clin Lab Anal. 2011;25(3):191-7. doi: 10.1002/jcla.20456. PMID: 21567467; PMCID: PMC6647585.
46. Shiba T, Makino I, Sasaki T, Fukuhara Y, Kawakami K, Kato I, Kobayashi T. p-Cresyl sulfate decreases peripheral B cells in mice with adenine-induced renal dysfunction. Toxicol Appl Pharmacol. 2018 Mar 1; 342:50-59. doi: 10.1016/j.taap.2018.01.025. Epub 2018 Jan 31. PMID: 29407365.
47. Shiba T, Makino I, Kawakami K, Kato I, Kobayashi T, Kaneko K. p-Cresyl sulfate suppresses lipopolysaccharide-induced anti-bacterial immune responses in murine macrophages in vitro. Toxicol Lett. 2016 Mar 14; 245:24-30. doi: 10.1016/j.toxlet.2016.01.009. Epub 2016 Jan 16. PMID: 26784855.
48. Azevedo ML, Bonan NB, Dias G, Brehm F, Steiner TM, Souza WM, Stinghen AE, Barreto FC, Elifio-Esposito S, Pecoits-Filho R, Moreno-Amaral AN. p-Cresyl sulfate affects the oxidative burst, phagocytosis process, and antigen presentation of monocyte-derived macrophages. Toxicol Lett. 2016 Nov 30; 263:1-5. doi: 10.1016/j.toxlet.2016.10.006. Epub 2016 Oct 17. PMID: 27760375.
49. Paroni R, Casati S, Dei Cas M, Bignotto M, Rubino FM, Ciuffreda P. Unambiguous Characterization of p-Cresyl Sulfate, a Protein-Bound Uremic Toxin, as Biomarker of Heart and Kidney Disease. Molecules. 2019 Oct 15;24(20):3704. doi: 10.3390/molecules24203704. PMID: 31618977; PMCID: PMC6832250.
50. Maciel RA, Rempel LC, Bosquetti B, Finco AB, Pecoits-Filho R, Souza WM, Stinghen AE. p-cresol but not p-cresyl sulfate stimulate MCP-1 production via NF-κB p65 in human vascular smooth muscle cells. J Bras Nefrol. 2016 Jun;38(2):153-60. English, Portuguese. doi: 10.5935/0101-2800.20160024. PMID: 27438970.
51. Veldeman L, Vanmassenhove J, Van Biesen W, Massy ZA, Liabeuf S, Glorieux G, Vanholder R. Evolution of protein-bound uremic toxins indoxyl sulphate and p-cresyl sulphate in acute kidney injury. Int Urol Nephrol. 2019 Feb;51(2):293-302. doi: 10.1007/s11255-018-2056-x. Epub 2019 Jan 2. PMID: 30604232.
52. Edamatsu T, Fujieda A, Itoh Y. Phenyl sulfate, indoxyl sulfate and p-cresyl sulfate decrease glutathione level to render cells vulnerable to oxidative stress in renal tubular cells. PLoS One. 2018 Feb 23;13(2):e0193342. doi: 10.1371/journal.pone.0193342. PMID: 29474405; PMCID: PMC5825083.
53. Huang TH, Yip HK, Sun CK, Chen YL, Yang CC, Lee FY. P-cresyl sulfate causes mitochondrial hyperfusion in H9C2 cardiomyoblasts. J Cell Mol Med. 2020 Aug;24(15):8379-8390. doi: 10.1111/jcmm.15303. Epub 2020 Jul 8. PMID: 32639656; PMCID: PMC7412408.
54. Chang JF, Hsieh CY, Liou JC, Liu SH, Hung CF, Lu KC, Lin CC, Wu CC, Ka SM, Wen LL, Wu MS, Zheng CM, Ko WC. Scavenging Intracellular ROS Attenuates p-Cresyl Sulfate-Triggered Osteogenesis through MAPK Signaling Pathway and NF-κB Activation in Human Arterial Smooth Muscle Cells. Toxins (Basel). 2020 Jul 24;12(8):472. doi: 10.3390/toxins12080472. PMID: 32722241; PMCID: PMC7472002.
55. Liabeuf S, Laville SM, Glorieux G, Cheddani L, Brazier F, Titeca Beauport D, Valholder R, Choukroun G, Massy ZA. Difference in Profiles of the Gut-Derived Tryptophan Metabolite Indole Acetic Acid between Transplanted and Non-Transplanted Patients with Chronic Kidney Disease. Int J Mol Sci. 2020 Mar 16;21(6):2031. doi: 10.3390/ijms21062031. PMID: 32188143; PMCID: PMC7139556.
56. Lin YL, Liu CH, Lai YH, Wang CH, Kuo CH, Liou HH, Hsu BG. Association of Serum Indoxyl Sulfate Levels with Skeletal Muscle Mass and Strength in Chronic Hemodialysis Patients: A 2-year Longitudinal Analysis. Calcif Tissue Int. 2020 Sep;107(3):257-265. doi: 10.1007/s00223-020-00719-x. Epub 2020 Jul 20. PMID: 32691117.
57. Fan PC, Chang JC, Lin CN, Lee CC, Chen YT, Chu PH, Kou G, Lu YA, Yang CW, Chen YC. Serum indoxyl sulfate predicts adverse cardiovascular events in patients with chronic kidney disease. J Formos Med Assoc. 2019 Jul;118(7):1099-1106. doi: 10.1016/j.jfma.2019.03.005. Epub 2019 Mar 28. PMID: 30928187.
58. Karbowska M, Kaminski TW, Znorko B, Domaniewski T, Misztal T, Rusak T, Pryczynicz A, Guzinska-Ustymowicz K, Pawlak K, Pawlak D. Indoxyl Sulfate Promotes Arterial Thrombosis in Rat Model via Increased Levels of Complex TF/VII, PAI-1, Platelet Activation as Well as Decreased Contents of SIRT1 and SIRT3. Front Physiol. 2018 Nov 28; 9:1623. doi: 10.3389/fphys.2018.01623. PMID: 30546314; PMCID: PMC6279869.
59. Hsu CN, Chang-Chien GP, Lin S, Hou CY, Lu PC, Tain YL. Association of Trimethylamine, Trimethylamine N-oxide, and Dimethylamine with Cardiovascular Risk in Children with Chronic Kidney Disease. J Clin Med. 2020 Jan 25;9(2):336. doi: 10.3390/jcm9020336. PMID: 31991725; PMCID: PMC7074377.
60. Missailidis C, Hällqvist J, Qureshi AR, Barany P, Heimbürger O, Lindholm B, Stenvinkel P, Bergman P. Serum Trimethylamine-N-Oxide Is Strongly Related to Renal Function and Predicts Outcome in Chronic Kidney Disease. PLoS One. 2016 Jan 11;11(1):e0141738. doi: 10.1371/journal.pone.0141738. PMID: 26751065; PMCID: PMC4709190.
61. Pelletier CC, Croyal M, Ene L, Aguesse A, Billon-Crossouard S, Krempf M, Lemoine S, Guebre-Egziabher F, Juillard L, Soulage CO. Elevation of Trimethylamine-N-Oxide in Chronic Kidney Disease: Contribution of Decreased Glomerular Filtration Rate. Toxins (Basel). 2019 Nov 1;11(11):635. doi: 10.3390/toxins11110635. PMID: 31683880; PMCID: PMC6891811.
62. Zhang W, Miikeda A, Zuckerman J, Jia X, Charugundla S, Zhou Z, Kaczor-Urbanowicz KE, Magyar C, Guo F, Wang Z, Pellegrini M, Hazen SL, Nicholas SB, Lusis AJ, Shih DM. Inhibition of microbiota-dependent TMAO production attenuates chronic kidney disease in mice. Sci Rep. 2021 Jan 12;11(1):518. doi: 10.1038/s41598-020-80063-0. PMID: 33436815; PMCID: PMC7804188.
63. Tang WH, Wang Z, Kennedy DJ, Wu Y, Buffa JA, Agatisa-Boyle B, Li XS, Levison BS, Hazen SL. Gut microbiota-dependent trimethylamine N-oxide (TMAO) pathway contributes to both development of renal insufficiency and mortality risk in chronic kidney disease. Circ Res. 2015 Jan 30;116(3):448-55. doi: 10.1161/CIRCRESAHA.116.305360. Epub 2014 Nov 5. PMID: 25599331; PMCID: PMC4312512.
64. Vacca M, Celano G, Lenucci MS, Fontana S, Forgia FM, Minervini F, Scarano A, Santino A, Dalfino G, Gesualdo L, De Angelis M. in vitro Selection of Probiotics, Prebiotics, and Antioxidants to Develop an Innovative Synbiotic (NatuREN G) and Testing Its Effect in Reducing Uremic Toxins in Fecal Batches from CKD Patients. Microorganisms. 2021 Jun 17;9(6):1316. doi: 10.3390/microorganisms9061316. PMID: 34204263; PMCID: PMC8235484.
65. Lopes RCSO, Balbino KP, Jorge MP, Ribeiro AQ, Martino HSD, Alfenas RCG. Modulation of intestinal microbiota, control of nitrogen products and inflammation by pre/probiotics in chronic kidney disease: a systematic review. Nutr Hosp. 2018 Apr 27;35(3):722-730. English. doi: 10.20960/nh.1642. PMID: 29974784.
66. Vandeputte D, Falony G, Vieira-Silva S, Wang J, Sailer M, Theis S, Verbeke K, Raes J. Prebiotic inulin-type fructans induce specific changes in the human gut microbiota. Gut. 2017 Nov;66(11):1968-1974. doi: 10.1136/gutjnl-2016-313271. Epub 2017 Feb 17. PMID: 28213610; PMCID: PMC5739857.
67. Sirich TL, Plummer NS, Gardner CD, Hostetter TH, Meyer TW. Effect of increasing dietary fiber on plasma levels of colon-derived solutes in hemodialysis patients. Clin J Am Soc Nephrol. 2014 Sep 5;9(9):1603-10. doi: 10.2215/CJN.00490114. Epub 2014 Aug 21. PMID: 25147155; PMCID: PMC4152802.
68. Xie LM, Ge YY, Huang X, Zhang YQ, Li JX. Effects of fermentable dietary fiber supplementation on oxidative and inflammatory status in hemodialysis patients. Int J Clin Exp Med. 2015 Jan 15;8(1):1363-9. PMID: 25785138; PMCID: PMC4358593.
69. Rossi M, Johnson DW, Morrison M, Pascoe E, Coombes JS, Forbes JM, McWhinney BC, Ungerer JP, Dimeski G, Campbell KL. SYNbiotics Easing Renal failure by improving Gut microbiologY (SYNERGY): a protocol of placebo-controlled randomised cross-over trial. BMC Nephrol. 2014 Jul 4; 15:106. doi: 10.1186/1471-2369-15-106. PMID: 24996842; PMCID: PMC4094543.
70. Yoshifuji A, Wakino S, Irie J, Tajima T, Hasegawa K, Kanda T, Tokuyama H, Hayashi K, Itoh H. Gut Lactobacillus protects against the progression of renal damage by modulating the gut environment in rats. Nephrol Dial Transplant. 2016 Mar;31(3):401-12. doi: 10.1093/ndt/gfv353. Epub 2015 Oct 20. PMID: 26487672.
71. Khoshbin K, Camilleri M. Effects of dietary components on intestinal permeability in health and disease. Am J Physiol Gastrointest Liver Physiol. 2020 Nov 1;319(5):G589-G608. doi: 10.1152/ajpgi.00245.2020. Epub 2020 Sep 9. PMID: 32902315; PMCID: PMC8087346.