EIS markers in obesity
The accumulation of oligopeptides can be the result of enhanced activity of proteolytic enzymes [5, 18,19,20]. Obesity and relative diseases are associated with increasing proteolysis and oxidative stress [21,22,23]. Activation of free radical oxidation can increase endogenous intoxication and the increasing the MM levels in the blood . However, the mechanism of the enhanced oxidative stress in obesity is currently poorly understood, and evidence on links between uremia-related oxidative stress and MM levels is unclear; recently, it swas reported that β2-microglobulin, a traditional MM marker, may not by itself contribute to the pathogenesis of CKD by induction of leukocyte oxidative stress . It was evidenced that oxidative stress is associated with increased expression of genes encoding NADPH oxidase subunits and reduced expression of enzymatic antioxidants genes . Finally, another reason for the increasing of the MM level in obesity condition may be a violation of renal excretory capacity [26,27,28,29,30].
The Orlistat effects on MM levels can be explained by the side effects of this drug like steatorrhea, and inhibition of fat reabsorption in the gut antioxidants like vitamin A and E is disturbed [7, 9]. Development of oxidative stress, contributing to modification of proteins, can be involved in increasing their catabolism. Orlistat can decline glomerular filtration that also slows down the oligopeptide excretion [31, 32].
Unlike Orlistat, BNC based on Fenugreek significantly reduces the serum level of oligopeptides both in the control group and in the HCD rats. In the latter case, the value of the parameter did not significantly differ from the norm.
The reduction of EIS peptide markers under Fenugreek–BNC treatment is likely due to the antioxidative properties of polyphenols and bioflavonoids, saponin, vitamins, selenium, etc. present in Fenugreek. Fenugreek includes three main components as follows: diosgenin (the most important saponin of this plant), 4-hydroxyisoleucine (4-OH-Ile), and galactomannans exhibit powerful effects on the glucose and lipids metabolism, insulin signaling system, liver and kidneys’ functional states, and the development of inflammatory processes [10, 11, 13]. A number of studies indicate that the Fenugreek components have the ability to scavenge ROS, to inhibit the lipid peroxidation processes, and to promote the enzymatic antioxidants functions [14, 33]. The administration of Fenugreek improves renal function also by significantly decreasing calcification processes in the renal tissue, increasing the level of antioxidant protection, and reducing oxidative stress displays including lipid peroxidation inhibition [34, 35].
Kidney injury markers in obesity
A number of studies [28,29,30] have shown that the high-calorie diet alter kidney structure and renal morphometric parameters in rats and induce changes in the kidney weight, total kidney volume, volume of cortex, medulla, glomeruli, proximal and distal tubules, and nephron degeneration including glomerulosclerosis and tubular defects.
In normal physiological conditions in rats, urea is freely filtered at the glomerulus and then is reabsorbed in the inner medullary collecting duct (IMCD) of the kidneys, engaging in the processes of urine concentration [15, 36], but certain conditions, e.g., low-protein diet induce expression of genes of the proteins, secondary-active transport of urea is involved.
Detected differences among changes in serum levels of markers of kidney function, most likely, are due to the peculiarities of transport in the nephron, including energy-dependent reabsorption of urea and urate that involves a number of transport proteins and therefore may be subject to acute and chronic regulation. Some tendencies to increase creatinine levels under Fenugreek-based BNC treatment may occur due to the amount of muscle mass  and possible anabolic properties of Fenugreek. It may also explain the absence of reliable changes in creatinine levels when used in HCD rats (the oligopeptides levels at the same time are normalized since their content does not depend on the intensity of anabolic processes in muscle) . Creatinine elimination from the body occurs only in the kidneys by glomerular filtration processes and partly (10–40%) via active secretion in the proximal tubules of the nephron, which is mediated by specific transport proteins of basolateral (ОАТ2, ОАТ3, ОСТ2, ОСТ3) and apical (МАТЕ1 and МАТЕ2-К) membranes  (Fig. 7).
The general scheme of the study and action of Orlistat and Fenugreek-based BNC is presented on Fig. 8.
Acute regulation of urea transport is done by their phosphorylation. Chronic regulation involves altering protein abundance in response to changes in hydration status, low protein diets, adrenal steroids, sustained diuresis, or antidiuresis [39,40,41]. Secondary-active transport of urea in the rat kidneys is inducible and involves three sodium-dependent urea transporters which are involved into active urea reabsorption in the apical and basolateral membrane of initial part of IMCD, and also in active urea secretion in the apical membrane of terminal IMCD [42,43,44].
Unlike the human and higher primate organisms, where the levels of creatinine, urea, and uric acid in serum or plasma correlate with the state of ultrafiltration processes in the renal glomeruli, since they are not metabolized and excreted unchanged with the urine, creatinine and urea only can be considered as relevant markers in rats. Rats and most other mammals demonstrate high activity of uricase, the enzyme, which metabolize urate to allantoine and therefore its activity can significantly influence on urate content in the blood [15, 36].
About 90% of filtered urate is reabsorbed in the kidney; transport systems responsible for the reabsorption of urate in the nephron cells are found in both human and rats [36, 45,46,47]. The most important of them are URAT1 and GLUT9 [45,46,47]. Disruption of renal regulation of serum urate content is one of the pathological factors of hyperuricemia and gout. The analogous transport system in rats reveals 74% homology in amino acid sequence and has the same features and location, but a lower affinity for urate .
The observed reduction in serum levels of urea and urate under Orlistat treatment in obese rats is associated not only with the normalization of ultrafiltration in the glomeruli of nephron but also with the possible influence on the uricase activity.
Overall, despite the high efficiency of synthetic disaccharidase and lipase inhibitors, to reduce glucose and lipids levels in the blood, the antiobesogenic drugs, particularly Orlistat, have significant side effects that cause serious damage of the kidneys with accumulation of oligopeptides and consequently the development of the EIS.
Considering the above notes, the marker level changes under the BNC based on Fenugreek treatment along with normalization of the kidney glomeruli function may be partially associated with the same influence of biologically active Fenugreek components on complex mechanisms of reabsorption and secretion of urea and urate in the kidneys.
Reliable obesity animal models
Obesity models require multifactorial approach according to principle causality factors and reflect development in humans. Various rodent models of chronic kidney diseases have been developed to study pathogenesis and mechanisms to simulate neuroendocrine, nutrition, or genetically induced changes and effects of interventions; animal models of obesity have been developed, including spontaneous, genetic, and induced models. The high-calorie diet model  is simple and reliable approach. Recently, we have studied monosodium glutamate-induced obesity and concluded that the introduction of MSG to newborn rats caused the obesity in adulthood . However, these models do not exactly simulate human diseases, recurrent or progressive injuries in glomeruli, tubules, interstitium, and/or vasculature and furthermore, most of them are strain, gender, or age dependent . Rodent models remain the most popular species to approximate human disease. There has been a progressive increase in studies using mouse models of renal disease vs the use of rats [28, 50]. Several models addressed to collateral diseases consider cardiovascular parameters in the development renal injury [51,52,53]. The use of HFD in the C57BL/6 mouse is a suitable model to induce whole body and metabolic effects commonly seen in the human MetS and is associated with renal damage likely to lead to progressive renal disease .
T. foenum-graecum, a component of traditional diets, has a potential for prevention and treatment MetS
The beneficial properties of Fenugreek were reported, such as antidiabetic and galactagogue (lactation-inducer) effects; newer research has identified hypocholesterolemic, carminative, gastric stimulant, antioxidant, antilipidemia, antibacterial, hepatoprotective, antifungal, antiinflammatory, antilithigenic, antiulcer, anticarcinogenic, etc. [10, 11, 13, 14, 54,55,56,57,58].
Fenugreek does not produce any significant acute and cumulative toxicity at the doses administered . According to results by Kumar et al. , Fenugreek treatment offered significant protection against dyslipidemia and oxidative stress. Fenugreek demonstrated beneficial effects on blood glucose and insulin levels, antioxidant enzymes, lipid peroxidation, pyruvate kinase, lactate dehydrogenase, and protein kinase C in the heart, muscle, and brain of the alloxan-induced diabetic rats. Results of the study by Uemura et al.  showed that T. foenum-graecum can ameliorate hyperglycemia and diabetes. Authors determined the effects of Fenugreek on adipocyte size and inflammation in adipose tissues in diabetic obese KK-Ay mice, and identified the active substance in Fenugreek. Thus, Fenugreek miniaturized the adipocytes and increased the mRNA expression levels of differentiation-related genes in adipose tissues; Fenugreek also inhibited macrophage infiltration into adipose tissues and decreased the mRNA expression levels of inflammatory genes. Fenugreek, which contains diosgenin, may be useful for the management of diabetes-related hepatic dyslipidemias. Fenugreek might offer effective added advantages as prebiotic towards the enhancement of probiotic bacterial growth in the gastrointestinal environment  and is promising for development treatments on immune-related disorder .
In our study, the use of BNC based on Fenugreek reduces the oligopeptides level in control group rats, and normalizes in experimental group likely due to presence of active antioxidant ingredients in Fenugreek: diosgenin, galactomannans, vitamins–antioxidants, selenium, etc., along with the normalization of the lipids and carbohydrates metabolism, improve function of organ that provides release of endogenous toxins that belong to the oligopeptides group (proved by decreasing of urea and serum urate levels).
MetS, hyperuricemia, and nephropathy
Hyperuricemia, which is strongly associated with obesity and MetS and nephropathy can predict visceral obesity and insulin resistance, might be partially responsible for the pro-inflammatory endocrine imbalance in the adipose tissue, which is an underlying mechanism of the low-grade inflammation and insulin resistance in subjects with the MetS [60,61,62,63,64,65]. Gout is a common metabolic disorder, involving the liver, kidney, and joints; it is met more often in men, and is characterized by the deposition of reversible monosodium urate (MSU) crystals and occurs as a consequence of hyperuricemia which induces inflammatory arthritis and nephropathy. Serum uric acid levels are elevated in secondary insulin resistance syndromes (e.g., gout, transplantation, pre-eclampsia, and diuretic use [45,46,47, 65].
In the study by Köttgen et al. , genome-wide significant loci associated with serum urate concentrations were identified. Those alleles associated with increased serum urate concentrations can increase the risk of gout. The modulation of urate production and excretion by signaling processes that influence metabolic pathways, such as glycolysis and the pentose phosphate, seem to be central pathways including the genes from the newly identified associated loci. These findings may have implications for further research into urate concentration, lowering drugs to treat and prevent the common inflammatory arthritis and gout .
Uric acid predicts, and is an integral component and causal factor in the hypertension ; uric acid-induced endothelial dysfunction with impaired NO production might mediate development of insulin resistance and hypertension.
Evidence is growing, supporting involvement of uric acid in development of insulin resistance. Experimental hyperuricemia induces diabetes and hypertension in animals, while fructose intake may have a major role in the epidemic of MetS and obesity due to its ability to raise uric acid. Fructose ingestion increases serum uric acid and leads to hypertension, insulin resistance, obesity, and hypertriglyceridemia; these conditions are ameliorated by decreasing uric acid levels [68, 69].
Advanced imaging modalities including magnetic resonance imaging (MRI), ultrasound (US), computed tomography (CT), and dual energy CT have important applications in gout and can capture information about the composite, vascular nature of many tophaceous deposits [70,71,72].
More studies are required to establish the links with food additives (e.g., glutamate) and crystal deposition diseases, prior to the gout .