Uremic Toxins: The Role of the Gut and the Kidneys

Abstract

Uremic toxins are a diverse group of substances that contribute significantly to the high cardiovascular disease burden in chronic kidney disease (CKD). When glomerular filtration begins to decrease, a disorder in the intestinal microflora known as dysbiosis occurs; this produces alterations in metabolic activities and decreased excretion of waste products. These substances have been identified and classified, accordingly to molecular weight and clearance. Biological and clinical effects have also been identified. These substances have different effects depending on the tissue or cell where they accumulate. The recommendations for a low-protein diet in pre-dialysis patients and the use of probiotics, prebiotics, and synbiotics added to the removal techniques in hemodialysis can help reduce the inflammatory effects and those associated with mortality.

1. Introduction

Uremic toxins are substances produced by protein metabolism. In persons with chronic kidney disease (CKD), these substances can accumulate and contribute to the diversity of symptoms produced by end-stage kidney disease.

In the gut, protein metabolism by bacteria produces ammonia, in the presence of urease enzymes. Diet has an important role, as animal protein generates more uremic toxins than vegetable proteins. Also, microbial fermentation of some amino acids and gut microbes can generate uremic toxins.

Patients with advanced chronic kidney disease in pre-dialysis and patients in renal replacement therapy need an adequate balance of protein in the diet, since dialysis can remove some uremic toxins, but not all and studies suggest that these toxins can contribute to cardiovascular risk and mortality.

2. Dysbiosis in chronic kidney disease

Uremic toxins are substances produced by protein metabolism. This substance accumulates in patients with chronic kidney disease (CKD) and produce deterioration of physiologic and biochemical functions that contribute to gastrointestinal symptoms, malnutrition, and progression of end-stage kidney disease [1].

Under normal circumstances, native bacteria are acquired at birth and during the first year of life, and some other transit bacteria are acquired during our daily food and fluid intake. Acids and bile from gastric and pancreatic enzymes secretion prevent most bacteria to grow in the first part of the gastrointestinal tract; but from past the duodenum until the distal colon, there is a population of some 100 billions bacteria that live in symbiosis with the human host [2], and that studies have shown that have roles in metabolic and nutritional function; protection from invasive infectious agents; proliferation and differentiation of intestinal epithelia; and modulation of immune system [3].

Approximately, 10 g of protein reaches the colon daily, where it is broken down by gut bacteria to metabolites such as ammonium, amines, thiols, phenols, and indoles. These products are normally eliminated by the feces, and a part are absorbed and eliminated by the kidney [4].

With progression of CKD, disorders of the intestinal microflora occur (dysbiosis), and there is an alteration in the quantity and quality of its composition and metabolic activities [5]. Rising urea levels and the spread of urease bacteria increase ammonium production in the intestinal lumen and induce changes in intestinal pH, altering the permeability of the intestinal mucosa, by affecting the enterocytes in the tight junction, producing an infiltration of mononuclear leukocytes in the lamina propria that is associated with an increased thickening of the colonic wall [6].

In addition to the decrease in the clearance of waste products, CKD patients use multiple medications, have a decrease in dietary fiber consumption, and need oral iron and frequent use of antibiotics that also produce alterations in intestinal transit [7]. Patients in hemodialysis have episodes of hypervolemia alternated with ultrafiltration and, sometimes, hypotension that can cause episodes of transient intestinal ischemia and also increase the permeability of the intestinal barrier and, with this, favor the passage of endotoxins [6].

There are more than 100 substances that have been identified as uremic toxins, and studies continue to suggest more each day. There are some characteristics that these substances must meet to be considered as uremic toxins: identification and measurement should be possible and levels should be elevated in CKD patients and when decreased, symptoms should improve [8, 9, 10].

There are differences in their physicochemical characteristics, as size, weight and clearance, and site of origin, that have allowed them to be classified into small water-soluble, small middle, medium-middle size, large-middle molecules, and protein-bound compounds. Some of these compounds are derived from endogenous metabolism and are water-soluble, but some are gut derived from dysbiosis and can be water-soluble or protein-bound (Table 1) [11, 12].

Small molecules; Water soluble; Protein bound^**; (< 0.5 kDa); (Gut derived); Low flux HD*; MCO HDx	Small-middle; Molecules; (0.5–15 kDa); High flux HD*	Medium-middle Molecule; Compounds; (>15–25 kDa); High flux HDF*	Large-middle molecules; (25–58 kDa); MCO HDx*	Large molecules; >58; HCO HD*
Carbamylated compounds	Β2-microglobuline	IL-1β, IL-6, IL-10 IL-18	AOPP	Modified albumin
ADMA	IL-8	myoglobin	FGF-23
urea		Kappa-FLC	Lambda-FLC
Uric acid		prolactin	TNFR1
TMAO		TNF-α	AGEs
SDMA		Complement factor D	CX3CL1
Indoxyl sulfate^**		FGF-2	CXCL12
P-cresyl sulfate^**			IL-2
Homocysteine^**			YKL-40

Table 1.

Classification of uremic toxins.

^*

All dialyzer types can remove small water soluble compounds, but for each molecular weight group, there is a dialysis modality with a higher capacity to remove each group of compounds: HD: hemodialysis; HDF: hemodiafiltration; MCO medium cutoff; HDx: expanded hemodialysis; HCO: high cutoff.

^** Molecules removed by MCO HDx.

Adapted from: Rosner et al. [12].

ADMA: asymmetric dimethylarginine; TMAO: trimethylamine-N-oxide, SDMA: symmetric dimethylarginine; FLC: free light chain; TNF: tumor necrosis factor; FGF: Fibroblast growth factor; AOPP: Advanced oxidative protein products; IL: Interleukin; AGEs: Advanced glycation end products; CX: chemokine; and YKL: chitinase like protein.

2.1 Small molecules (water-soluble and protein-bound)

The water-soluble small molecules have the characteristic that they can be easily removed with any type of dialysis (low-flux hemodialysis). Among these, we have creatine, creatinine, urea, and uric acid. The protein-bound small molecules, such as indoxyl sulfate, P-cresyl sulfate, and homocysteine, are difficult to remove by available dialysis techniques and are known to have toxic activity. Studies have confirmed that these molecules get better removal with a medium cutoff (MCO) membrane and expanded hemodialysis (HDx) [13, 14].

There are different indoles substances, produced by degradation of tryptophan by intestinal bacteria and subsequently sulfated in the liver. Indoxyl sulfate has been found to be the most abundant in uremic patients [15]. Studies have demonstrated a relation with renal fibrosis and progression of end-stage renal disease and an association with endothelial damage, as this substance can inhibit endothelial repair functions and free radical production [16].

P-cresol is a phenol produced by the metabolism of phenylalanine and tyrosine and then conjugated in the intestinal wall to p-cresyl sulfate and to p-cresyl glucuronide in the liver, being p-cresyl sulfate the main metabolite circulating in the cresol group [17].

2.2 Small middle molecules

These substances have a higher molecular weight. β2-microglobuline, PTH, and IL-8 can be removed by using high-flux hemodialysis (HD-high flux). The accumulation of β2-microglobuline in osteoarticular or viscera is known as dialysis-associated amyloidosis, and deposits contributes to destructive lesions of bones and joints and vascular damage [18].

2.3 Medium middle molecules

These molecules can be removed from plasma by using convective (hemofiltration or hemodiafiltration) or large-pore membrane dialysis techniques, such as peritoneal dialysis and high-flux hemodialysis. Some of these are myoglobin, prolactin, interleukins (IL-1B, IL-6, IL-10, and IL-18), and Kappa-free light chain, and they contribute to maintaining a state of chronic inflammation [19].

2.4 Large middle molecules

These group of molecules are composed by cytokines, growth factors, and signaling proteins; but since they are retained in uremia and clearance for these molecules is difficult with low-flux hemodialysis techniques, the accumulation of these substances is associated with endothelial dysfunction and cardiovascular disease [20].

2.5 Large molecules

In this group, we find that albumin, but in the presence of uremia, suffers an irreversible nonenzymatic post-translational modification, called carbamylation, that is associated with pro-atherogenic, endothelial dysfunction, monocyte adhesion, and cardiovascular mortality [21].

3. Biological and clinical effects of uremic toxins

The fermentation of non-digestible carbohydrates takes places mainly in the cecum and right colon. For bacteria, this is an important source of energy for proliferation and energy recovery from the diet that favors the absorption of ions (Ca, Ng, and Fe) in the cecum. The enteric flora also helps in the production of vitamins (K, B12, biotin, folic, and pantothenic acid) and the synthesis of amino acids from ammonia or urea [22]. Anaerobic metabolism of peptides and proteins that occur in distal segments of the colon generates ammonia, amines, phenols, thiols, and indoles [23].

There are some factors that favor bacterial translocation: bacterial proliferation in small intestine, increased permeability of the mucosal barrier, and deficiencies in immune response. CKD patients share those factors that can contribute to accumulation of uremic toxins [24].

Uremic toxins have different effects at tissue and cellular levels. At the endothelial level, there is an alteration in the balance of nitric oxide production, and there is an increase in the production of free oxygen radicals, inflammation, proliferation, and expression of tissue factor and ICAM-1 and MCP-1 [25]. In the smooth vascular fiber, there is production of free radicals and proliferation and an increase in the expression of osteoblastic proteins. Increased calcification, aortic stiffness, atherosclerosis, and increased leukocyte adhesion to the wall have been seen in the blood vessels [26].

In cardiac myocytes, the presence of these toxins causes alterations in cellular structure and function and has been associated with cardiac hypertrophy and fibrosis [27]. In bone, there are alterations in the differentiation and function of osteoclasts, and in osteoblasts, there is a decrease in the expression of the PTH receptor, with a decrease in cell viability and proliferation [28]; in adipocytes, deposits of uremic toxins have been associated with insulin resistance [29].

Uremic toxins have been associated with progression of end-stage kidney disease, cardiovascular complications, and alterations of mineral-bone metabolism, anemia, and insulin resistance. The deposits in the glomeruli, tubules, and interstitium produce monocyte infiltration, glomerulosclerosis, and tubulointerstitial fibrosis [30, 31]. There is interference in the production of erythropoietin, added to a decrease in the life of erythrocytes, which has been related to renal patient anemia [32, 33].

4. Treatment recommendations

It has been seen in general terms that the inhibition of the production of uremic toxins derived from the production of intestinal bacteria may have a potential use. There are certain bacteria that assist the small intestine; however, if its population is depleted or otherwise overstimulated and grows, then an increase in population may also cause a problem. There are some potential therapeutic effects on using decreasing the amount of protein in the diet and the use of some substances (Table 2) [34].

Low protein diet	Probiotics	Prebiotics	Synbiotics
Reduce production of nitrogenous waste products	Decrease production of toxic products and endotoxins	Modification of gut environment	Improving survival of intestinal bacteria
Decrease in production of toxic waste products and inflammatory substances

Table 2.

Suggested treatment strategies.

4.1 Low-protein diets

Low-protein diets could be helpful, since by decreasing the intake of amino acids, the production of uremic substances can be reduced. Diets with animal protein (meat) increase the production of nitrogenous waste products, increase the risk of constipation, and worsen uremia. Strategies to reduce the amount of animal protein and increase the intake of vegetable protein may have an impact on decreasing glomerular hyperfiltration [35].

It has been evaluated that low-protein diets can slow the progression of chronic kidney disease. However, patients do not reach the goal stipulated in the recommendations of low-protein diets of 0.6–0.8 g/kg/day. One strategy would be to supplement diets with essential amino acids or ketoanalogs [36]. It is important that diets for patients must be modulated, since a very strict diet could lead the patient to a state of malnutrition [37].

4.2 Probiotics

Probiotics are a group of living species of known bacteria (Lactobacillus acidophilus, bifidobacterium longum, and streptococci) that administered orally and have shown to decrease some levels of potentially toxic substances such as homocysteine, indoxyl sulfate, cytokines (TNF-α, IL-5, IL-6), and pro-inflammatory endotoxins in patients with chronic kidney disease [38, 39, 40].

4.3 Prebiotics

Prebiotics are a non-digestible food ingredient that promotes the growth of beneficial microorganisms in the intestines. The majority of them are a subset of carbohydrate groups and oligosaccharide carbohydrates (OSCs), like fructo-oligosaccharides and galacto-oligosaccharides. By providing energy sources for the gut microbiota, prebiotics can modulate the composition and function of these microorganisms and can modify the gut environment [41]. The fermentation products of prebiotics are mostly acidic, thus lowering the intestinal pH. This can contribute to a change in the composition and population of the intestinal microbiota, and in some cases, the fermentation of a prebiotic complex is a substrate for another microorganism [42]. Prebiotics are found in fruits, vegetables, and whole grains like apples, artichokes, asparagus, bananas, barley, wheat, oat, onions, and green vegetables but are also added to some foods. The use of prebiotics decreases the production of indoles and p-cresyl sulfate due to the production of short-chain fatty acids (SCFAs), which provides energy to the intestine, and allows the amino acids that reach the colon to be incorporated into the bacteria and therefore excreted, instead of being used to generate uremic solutes [43].

4.4 Synbiotics

Synbiotics are substances that contain a mixture of probiotics and prebiotics, with the intention of improving the activity and survival of bacteria in the intestine. Several studies in patients with stage 3 to 5 chronic kidney disease have evaluated some synbiotic-type products, managing to find a decrease in the values of uremic toxins in the blood. Reports of some laxatives agents [44] and oral activated charcoal adsorbent AST-120 [45] have suggested to reduce concentrations of some uremic toxins, but the effect wanes after stopping its use. Some studies in animals with chronic kidney disease have evaluated the use of sodium-glucose cotransporter (SGLT) 2 inhibitor Canagliflozin. This drug also has an inhibitory effect on SGLT 1, which has a gastrointestinal effect by promoting intestinal fermentation of carbohydrates and reducing plasma levels of p-cresyl and indoles [46, 47].

5. Conclusions

There is abundant evidence in the literature that the retention of uremic toxins contributes to generalized inflammatory damage in patients with chronic kidney disease [48]. The general recommendation of a balanced diet, with a low amount of protein and an adequate amount of fiber, depending on the renal stage, is important. Bacterial overgrowth produces retention of uremic toxins that cause intestinal dysbiosis. All the strategies that can be used to preserve residual renal function or reduce cellular inflammation are important to complete a multidisciplinary approach for the management of CKD that increasingly affects more and more patients worldwide.

Acknowledgments

Dr. Courville is a member of the Sistema Nacional de Investigadores SNI which is supported by Panama’s Secretaría Nacional de Cienia y Tecnología SENACYT.