Increased levels of BCAA in the blood have been linked to the emergence of insulin resistance, according to studies. BCAAs may encourage better metabolic phenotypes, such as increased insulin sensitivity and glucose uptake (51). Insulin resistance and BCAAs have a mutually reinforcing interaction. The impaired function of BCAT and BCKDH, important pathways in the metabolism of insulin resistance, is thought to be a primary genetic abnormality. The accumulation of related metabolites and branched-chain ketone acids after elevated insulin levels may lead to further insulin resistance (52). High levels of BCAA polymorphisms are linked to a high risk of type 2 diabetes, according to a Mendelian randomization study that used genes related to BCAA metabolism to evaluate the causality of an influence on insulin resistance (53). Another possible mechanism is related to the activation of mTOR. BCAA overload may activate the mTOR pathway and inhibit the PI3K-Akt intracellular pathway, causing insulin resistance (29, 54). mTOR is also a key factor in inhibiting autophagy in renal tubular epithelial cells, and Leu may regulate cellular autophagy through the mTOR pathway to mitigate renal injury (55). Additionally, BCAA can sustain cellular energy metabolism, control mitochondrial function, and lower ROS generation to decrease oxidative stress through BCAT, which in turn lessens DKD.

According to Garibotto et al., renal failure can lower the glomerular filtration rate, which in turn can cause acidosis and impact the catabolism of BCAAs. As DKD continues, it can also cause a decline in BCAA levels because of kidney damage (56). Therefore, changes in branched-chain amino acid levels can be consistent with the progression of DKD and also demonstrated that BCAAs can be used as one of the early diagnostic markers of DKD. Wang et al. showed that plasma levels of BCAAs can be used as one of the early diagnostic markers of diabetes by metabolomics liquid chromatography–tandem mass spectrometry (48). Also, BCAAs are closely related to insulin resistance, which can lead to the development of DKD, and therefore elevated levels of BCAAs can be detected in patients with DKD (40). In a prospective study of type 2 diabetic patients, Zhu et al. hypothesized that high levels of Val and isoleucine may be one of the risk factors for the development of DKD (47). Wang et al. study concluded by metabolomic analysis that serum branched-chain amino acid levels decreased with disease progression, suggesting that elevated BCAA could predict the development of diabetes (19). By using metabolomic analysis on 637 individuals with type 1 diabetes, Tofte et al. established a strong positive cross-sectional connection between the BCAAs isoleucine, leucine, Val, and eGFR. They came to the conclusion that BCAAs are intimately associated with diabetes (49).

Tryptophan (Trp), one of the aromatic amino acids, is crucial for the early detection of DKD. Niewczas et al., found that greater levels of C-glycosyl tryptophan, O-sulfotyrosine, and N-acetyl threonine were linked to quicker glomerular filtration rate (eGFR) slopes in 158 diabetic patients with proteinuria and chronic renal disease (41). According to Solini et al.’s screening of metabolomics in serum and urine samples from 286 T2DM patients, the combination of serum metabolites (C-glycosyl tryptophan, pseudouridine, and N-acetyl threonine) predicted lower eGFR and increased the predictability of clinical parameters (42). Zhang et al. performed a metabolomic analysis of 95 patients with urinary albumin nephropathy (PKD). They concluded that metabolites such as threonine, Trp, methionine, homovanillic acid sulfate, and Cit could be potential biomarkers for early diagnosis (43).

By conducting a meta-analysis of metabolites, Roointan et al. discovered that Gln in the blood is one of the most significant non-invasive early diagnostic biomarkers and that several important metabolites, including glutamate, arginine, glycine, and Gln, are highly centralized (57). According to urine metabolomics research performed by Feng et al., patients with chronic kidney disease had a large rise in the metabolite -phenylacetylglutamine (44). Wang et al. demonstrated a significant decrease in serum Gln levels in the urinary albumin group. As a marker of kidney injury, it can indicate that changes in Gln levels can reflect kidney injury in DKD patients and may be used as one of the early diagnostic markers of DKD (44). Zhang et al. showed that serum γ-glutamine transferase (γ-GGT) was significantly elevated in patients in the massive albuminuria group. While γ-GGT could show abnormalities earlier in the absence of trace urine protein. It suggests the significance of Gln in the early diagnosis (45).

Following metabolite functional enrichment analysis in DKD, Roointan et al. found that the urea cycle was one of the most variable sets of metabolites (57). The urea cycle also called the ornithine (Orn) cycle, plays a key role in proteolytic metabolism, while insulin can also reduce the biosynthetic capacity of urea (58). Citrulline (Cit) is typically absorbed by the kidneys and converted to urea via arginine. Orn and Cit are all components of the urea cycle. Urea is the major product of nitrogen metabolism and is formed from free ammonia and aspartate (40). Arginine can be transformed into Orn or Cit in the urea cycle, which aids arginase in the production of urea or NO molecules (57). Chuang et al. showed that the concentration of amino acids involved in the urea cycle was significantly increased in patients with end-stage renal disease (50). Jiang et al. came to the conclusion that Cit was independently linked with the development of diabetes mellitus in conjunction with microproteinuria (40). Therefore, Cit can be used as one of the early diagnostic markers of DKD. By analyzing 95 individuals with PKD’s serum and urine using metabolomic techniques, Zhang et al. came to the conclusion that Cit in blood and urine had the highest sensitivity in early diagnosis (43).

Elevated blood glucose leads to a decrease in mitochondrial membrane potentials (MMPs), GSH, and NO levels, prompting the downregulation of eNOX to activate SOD and finally induce DKD through structural damage. Taurine can regulate oxidative stress induced by hyperglycemia to its normal level through antioxidant activity (15). By preventing calcium excess and enhancing mitochondrial respiratory performance, taurine was demonstrated by Zhang et al., to minimize the formation of ROS in the mitochondria (26).

In addition, it has been demonstrated that oxidative stress can activate NF-kb, which can increase the expression of pro-inflammatory factors like IL-6, IL-8, and TGF-1. Taurine inhibits the expression of pro-inflammatory factors by reducing ROS and NF-kb activity, which is then involved in reducing their inflammatory response brought on by oxidative stress (15). TauCl has bactericidal and anti-inflammatory properties. It can inhibit pro-inflammatory cytokine production, which in turn is involved in alleviating its inflammatory response induced by oxidative stress (25). Meanwhile, DKD alters apoptotic markers, and hyperglycemia induces the mitochondria to release cytochrome C (Cytc) and produce apoptotic complexes, which in turn promotes apoptosis. Taurine can regulate the apoptotic parameters of apoptotic markers through the protective and anti-apoptotic effects of mitochondrial membranes. Thus reducing the structural damage of renal cortex caused by DKD, and Xie et al. showed that taurine can alleviate endothelial cell apoptosis by regulating miR-126 gene expression (Figure 1) (15, 59).

Secondly, taurine supplementation can prevent insulin resistance due to hyperglycemia through its antioxidant effect, anti-inflammatory response, and reduction of apoptosis. It can also treat DKD by reducing the production of proteinuria and inhibiting glomerular hypertrophy and tubulointerstitial fibrosis (60). Taurine, cysteine, and methionine are amino acids that contain sulfur and can be used as adjuvant therapy to prevent diabetes and its associated problems. According to research by Manna et al., eNOX produced by the nitric oxide system can induce oxidative stress, which can be alleviated by taurine through antioxidant effects (61). While arginine is a precursor of NO and the nitric oxide pathway inhibition of the nitric oxide pathway can lead to systemic hypertension, reduced glomerular filtration rate and effective renal flow. Arginine supplementation can alleviate glomerulosclerosis and thus delay renal injury (62).

First, the antioxidant and anti-inflammatory properties of Gln may be responsible for its nephroprotective effects in DKD. Nasri et al. have shown that oral Gln can reduce renal inflammation as well as oxidative stress biomarker levels in type 2 - diabetic rats, while improving pathological changes such as glomerulosclerosis and leukocyte infiltration (63). Meanwhile, in DKD, the production of large amounts of SOD induces iron death in the renal cell membrane through lipid peroxidation. GSH can play an antioxidant role through the GPX4 enzyme to reduce oxidative stress caused by iron death (11).

In addition, the heat shock response through HBP interacts with Gln metabolism (34). Transforming growth factor-β (TGF-β)-upregulation in rat renal mesangial cells has been demonstrated to be correlated with the hexosamine pathway, which is a key factor in the pathogenesis of DKD. It suggests that high glucose levels induce the production of fibronectin, especially in the glomeruli of DKD patients, this effect is eliminated by Gln inhibition through GFAT (64). GFAT is an important rate-limiting enzyme for Gln entry into the hexosamine pathway, and by inhibiting fructose-6-phosphate cells, it can eliminate the effector component of the extracellular matrix of the renal mesangial (37, 64). Glucose metabolism to fructose-6-phosphate enters into the hexosamine pathway, which is under the control of GFAT to produce the ultimate product UDP-GlcNAc. UDP-GlcNAc is a high-energy complex that regulates energy homeostasis and promotes insulin resistance in glucose transport, insulin secretion, and gene expression (36, 65). In renal cells, UDP-GlcNAc mediates renal cell dysfunction through the OGT-Polycomb-Knot-Sns pathway (66). Meanwhile, Leite et al. showed that UDP-GlcNAc can also inhibit glycogen synthesis by blocking the glycogen synthase GSK-3β. On the other hand, it can release HSF1 receptors, thus enhancing the heat shock gene expression and stimulating the inflammatory reaction as a way to induce diabetic nephropathy (Figure 2) (34).

Gln, as the most important intermediate in gluconeogenesis and amino acid metabolism, can improve insulin resistance and reduce renal injury through the hexosamine pathway. Additionally, it can promote gluconeogenesis, control glucose metabolism, and prevent protein hydrolysis in the liver and skeletal muscle, promoting protein synthesis in those tissues (67). Perriello et al. showed that the normal absorption of Gln in the population can regulate gluconeogenesis in vivo (68). Hu et al. demonstrated that Gln by administration attenuated sepsis-induced renal injury in mice (69). As scavengers of reactive oxygen species (ROS), aspartate and glutamate help lessen the oxidative stress and inflammatory response brought on by DKD (70).

A dipeptide called myostatin is produced in living cells from β-alanine and L-histidine. It can protect podocytes and thylakoid cells through antioxidant and anti-glycosylation properties, thereby alleviating DKD and slowing down aging (71). Angelica et al. showed that myostatin affects glomerular intracellular pressure through vasomodulation of afferent small arteries, thereby alleviating renal injury in DKD (72). Peters et al. demonstrated that myostatin treatment reduced glomerular ultrastructural damage, albuminuria production, and prevented alterations in renal myostatin metabolism in diabetic mice (73). Zhu et al. showed that myostatin could alleviate foot cell injury by targeting cystatin-1 to inhibit its scorching effect as a way to alleviate DKD (74).

Furthermore, different kinds of amino acids might be useful in treating DKD. Luo et al. showed that Orn metabolites can alleviate renal injury by activating activated mTOR signaling and cytoskeletal remodeling in podocytes, and glycine can reduce proximal tubular injury produced by hypoxia by increasing glomerular filtration rate (75). Yatzidis et al. demonstrated that supplementation with glycine, Asp., Glu, His, and arginine increased glomerular filtration rate in normal subjects or in some patients with renal failure. By the way, cysteine and acetylcysteine regulated insulin secretion and plasma glucose levels. Methionine increased mitochondrial DNA density in skeletal muscle and improved insulin sensitivity (61, 67). Tyrosine, Trp, and phenylalanine are low aromatic amino acids that Barba et al. demonstrated reduced renal damage and prevented renal fibrosis in mice (76). Acetylcysteine has been shown by Li et al. to maintain mitochondrial redox homeostasis by activating the SIRT3-SOD2/Gpx4 pathway, which reduces iron death in DKD (77).