SCFAs are one of the main metabolites of the microbially-mediated metabolism of fibre in the gut, leading to fermentation (He et al., 2020). It is a subset of fatty acids, containing six or fewer carbon molecules, and shows beneficial effects on the kidney. SCFAs play a role in bioregulation by inhibiting histone deacetylases (HDACs) and activating G protein receptors, thereby reducing the inflammatory response and lowering mean arterial pressure (Magliocca et al., 2022). SCFAs include acetate, propionate, and butyrate, which are produced by microbial communities through the fermentation of indigestible carbohydrates (Cong et al., 2022). Acetate and propionate are mainly produced by Bacteroidetes, whereas butyrate is mainly produced by Firmicutes (Levy et al., 2016).

The SCFA butyrate plays a positive role in the progression of diabetes and certain types of kidney disease (Machado et al., 2012; Guo et al., 2018; Xu et al., 2018), although its mechanism is not yet clear. Butyrate is suggested to alleviate DN by mediating the miR-7a-5p/P311/TGF-β1 pathway (Du et al., 2020). Transforming growth factor-β1 (TGF-β1) is the initial factor that triggers the fibrosis signalling cascade (Chang et al., 2016). P311 is an RNA-binding protein that stimulates TGF-β1 translation in several cell types (Yao et al., 2015). Butyrate supplementation attenuated kidney damage and apoptosis in a mouse model of genetic diabetes (db/db) as well as high glucose-induced fibrosis in SV40-MES-13 cells and inhibited TGF-β1 and P311 expression (Du et al., 2020). Oxidative stress is a key factor in the development of DN (Turkmen, 2017). The nuclear factor erythroid 2-related factor 2 (NRF2) plays a key role in cellular defence against oxidative stress (Kobayashi et al., 2016). Sodium butyrate (NaB), a known activator of NRF2, showed a DN-preventive effect (Zhou et al., 2022). A study that treated streptozotocin-induced diabetic C57BL/6Nrf2-knockout and the corresponding wild-type mice with NaB for 20 weeks found that diabetic mice not treated with NaB showed significant renal pathological changes such as oxidative damage, inflammation, apoptosis, fibrosis, and proteinuria. The results confirmed that NaB inhibited HDAC activity and increased the expression of NRF2 and its downstream targets, heme oxygenase 1 and NAD(P) H dehydrogenase quinone 1. Thus, NaB improved DN by activating NRF2 to inhibit HDAC activity (Dong et al., 2017). A recent study found anti-inflammatory effects of NaB on glomerular thylakoid cells in a mouse model of DN and high glucose-induced mouse glomerular mesangial cells (Gu et al., 2019). Gasdermin D (GSDMD) is a newly identified key executing protein for pyroptosis that is cleaved by inflammatory cysteine aspartase (Deng et al., 2021; Zuo et al., 2021). High glucose was found to increase propidium iodide (PI)-positive cells, promote the release of lactate dehydrogenase (LDH), interleukin-1 ß (IL-1β), and release of interleukin-18(IL-18); GSDMD, GSDMD N-terminal structural domain (GSDMD-n), and cleaved protein level caspase-1 levels were also elevated. NaB further enhanced the release of LDH by inhibiting caspase 1-GSDMD, which further enhanced LDH release and the number of PI-positive cells. In addition, NaB or Ac-YVAD-CMK, a caspase inhibitor, reversed the high glucose-induced nuclear factor-κB (NF-κB)/NF-κB inhibitor α (IκB-α) signalling pathway. Thus, NaB ameliorated high glucose-induced pyroptosis in glomerular endothelial cells via the caspase 1-GSDMD classical scorch death pathway involving the NF-κB/IκB-α signalling pathway. Exogenous NaB has been shown in different animal models to improve DN by reducing inflammation and oxidative stress as well as improving fibrosis, cell scorching, and DNA damage (Khan and Jena, 2014; Dong et al., 2017; Xu et al., 2018). Exogenous NaB also protected human glomerular thylakoid cells from high glucose-induced scorch death (Gu et al., 2019). These studies suggest that butyrate may serve as a potential therapeutic target for DN.

Whereas increasing evidence has indicated that butyrate may benefit the host through its anti-diabetic effects, some studies have suggested that acetate may exacerbate disease progression in DN. In a study using diabetic rats as a model, increased serum levels of acetate were found to be associated with increased expression of G protein-coupled receptor 43 (GPR43) in the kidney (Hu et al., 2020). Under high glucose conditions, acetate significantly increased the expression of GPR43 in human kidney 2 (HK-2) cells. Further analysis showed that GPR43 small interfering (si) RNA treatment significantly reduced lipid accumulation in HK-2 cells, even in the presence of acetate plus high glucose, which correlated with reduced protein levels of 3-hydroxy-3-methylglutaryl coenzyme A, low-density lipoprotein receptor, CD36, and chemokine ligand 16. This study confirmed that excess acetate production by the gut microbiota disrupted cholesterol homeostasis through the activation of GPR43, leading to tubulointerstitial damage in patients with DN Another animal study found that diabetic rats had abnormal gut flora, increased plasma acetate levels, increased urinary protein, thickened glomerular basement membranes, and loss of renal peduncle peduncles compared to controls (Lu et al., 2020). In addition, protein levels of angiotensin II, angiotensin-converting enzyme, and angiotensin II type 1 receptor were significantly increased in the kidneys of DM rats. The administration of broad-spectrum antibiotics to DM rats not only completely killed most intestinal flora but also significantly reduced plasma acetic acid levels, inhibited intrarenal activation of the renin-angiotensin (RAS) system (by inhibiting the aforementioned three indicators of RAS activation), and attenuated renal injury. This result suggests that dysregulation of the gut microbiota may be associated with intrarenal RAS activation in early DN. In contrast, plasma acetate levels positively correlated with intrarenal angiotensin II protein expression, inferring that acetate may also be involved in renal injury in early DN (Lu et al., 2020).

Acetic acid, propionic acid, and butyric acid are the main SCFA metabolites produced by gut microbes. However, several studies have focused on butyrate and acetate, and consequently, the potential effects of propionic acid on pathophysiological effects have been underestimated. There is a lack of robust research evidence on the effects of propionate on DN, although researchers have demonstrated that propionate can reduce fatty acid levels in the liver and plasma, reduce food intake, exert immunosuppressive effects, and possibly improve tissue insulin sensitivity (Al-Lahham et al., 2010; Pingitore et al., 2017; Yoshida et al., 2019). Propionate can reduce gluconeogenesis by activating AMP-activated protein kinase (AMPK) to downregulate glucose-6-phosphatase and phosphoenolpyruvate carboxykinase. In addition, siRNA-mediated knockdown of the propionate receptor GPR43 prevented propionate-induced AMPK activation and eliminated gluconeogenesis inhibition (Yoshida et al., 2019). A study group investigating the role of propionate in improving glucose homeostasis in vivo and its mechanism of action found that it enhanced glucose-stimulated insulin release and maintained ß-cell mass by inhibiting apoptosis (Pingitore et al., 2017). Propionate improves the high-glucose environment in patients with diabetes by modulating insulin sensitivity; thus, it is reasonable to infer that propionate has a positive impact on the progression of diabetic nephropathy.

TMAO is mainly derived from trimethylamine (TMA) oxidation by intestinal flora. Intestinal microorganisms metabolise and produce TMA from ingested nutrients such as lecithin and choline; these nutrients enter the liver via the portal circulation, where they are oxidised by flavin monooxygenase 3 or other flavin monooxygenases to produce TMAO (Bennett et al., 2013; Koeth et al., 2013). TMAO is a well-known pro-atherogenic sclerosing agent that promotes atherogenesis and is associated with cardiovascular risk (Bennett et al., 2013). Some studies have suggested that gut microbial dysbiosis contributes to diabetic microvascular damage, possibly because of the effects of changes in gut microbiota composition on atherosclerosis and endothelial dysfunction, but there is no conclusive evidence for this correlation (Roberts et al., 2018; Tanase et al., 2020). Evidence indicates that TMAO influences the progression of DN. In an experiment using streptozotocin-induced DN mice as a model, the mice were divided into mild proteinuria (MP) and severe proteinuria (SP) groups. The microbiota composition differed between the two groups, with a decrease in the abundance of thick-walled bacteria and a significant increase in heterobacteria in the SP group, and an increase in thick-walled bacteria in the MP group. This was associated with an increase in blood TMAO levels, which ultimately led to weight gain and increased glucose levels, accelerating the progression of DN. In addition, the lipopolysaccharide produced by the harmful bacteria led to a breakdown of the intestinal barrier, which allowed additional lipopolysaccharide and TMAO to enter the bloodstream and exacerbate renal damage (Li Y. et al., 2020a). In another animal study, TMAO exacerbated renal inflammation and promoted tubulointerstitial damage and renal fibrosis by activating NLRP3 inflammatory vesicles, thereby accelerating the development of DN (Fang et al., 2021b). A recent clinical trial also suggested that alternating gut microflora may be involved in the development of DN and that TMAO and chronic inflammation may be important factors in the development of DN (Yang et al., 2022).

Protein-bound uremic toxins such as indoxyl sulfate (IS), p-cresol sulfate (pCS), p-toluene glucosinolate (pCG) and phenyl sulfate are derived from the metabolism of aromatic amino acids, tyrosine, phenylalanine and tryptophan by intestinal microorganisms (Fang et al., 2021a). Escherichia coli, Proteus vulgaris, Paramecium coli, Bacillus liquefaciens, and Bacillus mimicus digest tryptophan to indole and metabolise it to IS in the liver (Keszthelyi et al., 2009). IS directly induces tubulointerstitial injury, oxidative stress and inflammation in post-gonadectomy mice (Ji et al., 2018; Tanaka et al., 2020) and human renal proximal tubular epithelial (HK-2) cells (Ellis et al., 2018), it has also been shown to be associated with the progression of DKD in patients with T1D and T2D, as well as in animal models of diabetes, and its elevation is mainly associated with changes in albuminuria and estimated glomerular filtration rate (Atoh et al., 2009; van der Kloet et al., 2012; Gooding et al., 2019).

Both pCS and pCG are derived from tyrosine, which is converted to p-cresol in the fermentation of Bacteroidetaceae, Bifidobacteria, Clostridia, Enterococcaceae, Eubacteriaceae, Clostridia, Lachnospiraceae, and Lactobacillariidae, and subsequently combined with sulphate or glucuronide to form pCS or pCG respectively (Russell et al., 2013). pCS can directly affect cell viability and induce cell death as well as induce reactive oxygen species and inflammatory cytokines in HK-2 cells (Watanabe et al., 2013; Park et al., 2019). pCG can cause phenotypic changes in proximal renal tubular cells (Mutsaers et al., 2015). These protein-bound uremic toxins are not only markers for the risk of DN development, but also risk factors that directly contribute to the development of DN. However, the molecular mechanisms of these urotoxins in DN are still unclear and needed further studies.

In a systematic review and meta-analysis, probiotics were shown to have beneficial effects on metabolic indicators in patients with DN, including renal function, glucose homeostasis, lipid metabolism, inflammation, and oxidative stress. Probiotics may be an effective and low-cost treatment for patients with DN (Dai et al., 2022). In a study assessing the effects of oral single-strain or multi-strain probiotic preparations on high-fat diet-induced obesity in mice, the separation between the ability of probiotics to suppress adipose tissue inflammation and limit weight gain was reported. The multi-strain mixture was able to improve fat deposition, insulin resistance, and dyslipidaemia through adipose tissue immune cell remodelling (mainly affecting macrophages). The mixture altered fatty acid uptake at the intestinal level and restored the expression levels of the SCFA receptor GPR43 (Alard et al., 2016). In another clinical trial, it was found that, compared with placebo, 12 weeks after the intake of probiotic supplements, the significantly reduced expression of hs CRP in serum, malondialdehyde (MDA), AGEs, urea, creatinine and IL-1 genes and the significantly increased concentration of glutathione (GSH) in plasma in patients with diabetes nephropathy were related, but did not affect other markers of inflammation and oxidative stress, as well as TNF- α And TGF- ß Gene expression (Mafi et al., 2018). Probiotic intake can help reduce inflammatory factors through SCFAs produced by the gut microbes and reduce the production of hydrogen peroxide radicals (March et al., 2017; Lopes et al., 2018). The beneficial effects of probiotics on oxidative damage may be mediated through the production of butyrate in follicles and the reduction of lipid peroxidation (Jiang et al., 2016). In an animal experiment, diabetic rats treated with Lactobacillus plantarum or Lactobacillus rhamnosus-fermented yoghurt showed the following effects: a significant reduction in blood glucose and α-amylase concentrations; a significant increase in high-density lipoprotein cholesterol concentration and improved lipid distribution; and inhibition of nuclear factor κB and increase in Bcl-2 concentrations (Negm El-Dein et al., 2022). This study conclusively established that the selected Lactobacillus spp. had hypoglycaemic potential and could be used as functional nutritional anti-diabetic supplements.

Of all the exogenous factors affecting the gut microbiome, a long-term diet appears to have, by far, the greatest impact (Xu and Knight, 2015), and it has attracted attention as a therapeutic route to re-establishing a symbiotic relationship. As hyperglycaemia is the main cause of the development of DN (VR et al., 2019), glycaemic control is the cornerstone of DN treatment. Several studies have suggested that dietary prebiotic or symbiotic supplements may modulate glucose metabolism through the modification of intestinal flora, thereby slowing the progression of diabetic complications (Nikbakht et al., 2018; Carvalho et al., 2019; Bock et al., 2021).

There are various types of prebiotics, including oligosaccharides, polysaccharides (e.g., Spirulina, Chlorella, etc.), some natural plant extracts (e.g., vegetables, herbs, wild plants, etc.), protein hydrolysates, polyols, etc. (Yadav et al., 2022). Oligofructose (FOS), a widely commercially available source of prebiotics, has also been shown to improve intestinal disorders and reduce inflammation in patients with diabetes (Pengrattanachot et al., 2022). In animal experiments using rats with type 1 diabetes induced by streptozotocin, FOS showed a protective effect on the kidneys and improvd inflammation and insulin sensitivity (Gobinath et al., 2010). Dietary fibre is a polysaccharide that modulates gut microbiota composition (Cronin et al., 2021). In another animal study, dietary fibre was found to protect mice from the clinical and histological manifestations of DN without altering blood glucose levels. Supplementation with the microbial-derived acetate and butyrate provided similar protection by binding to the metabolite-sensing receptors GPR43 and GPR109A. This study reveals a role for the gut microbiota in attempting to delay the progression of DN through dietary modulation, with SCFA being a key mediator of the protective effect produced by fibre depletion (Li Y. J. et al., 2020b).

Faecal microbiota transplantation (FMT) may affect gut microbiota composition (Wang et al., 2019). Its success depends largely on the diversity and composition of donor faecal microbes (Zheng et al., 2022). FMT is a new theory and promising technique for the treatment of intestinal microbiome disorders. However, its application is still greatly limited by the risk of disease transmission between donor and recipient, patient acceptance, poor outcomes, and the uncertain impact on the recipient’s immune system (Wang et al., 2019). FMT is a successful treatment for recurrent Clostridium difficile infection and is more commonly used in studies of inflammatory bowel disease (Imdad et al., 2018). However, it is now also increasingly being applied in extraintestinal diseases, including haematological disorders, neurological disorders, and metabolic diseases such as diabetes and autoimmune diseases, and has shown some positive results (Wang et al., 2019). In a mouse model of experimental DN, the potential role of gut microbiota was hypothesized to involve renal function regulation, which was validated by the role of the FMT gut microbiome and SCFA produced by it in the treatment of DN (Li Y. et al., 2020a). In addition, Faecalibacterium prausnitzi can be used as a diagnostic and therapeutic biomarker for FMT (Chen et al., 2020; Zhang et al., 2020). The gut structure can be restored by transplanting F. prausnitzii, which could be used as a potential treatment against inflammation and diabetes (Ganesan et al., 2018; Xu et al., 2020). Other studies have also demonstrated that FMT from healthy donors significantly reduced foot cell damage in diabetic rats, thereby delaying kidney damage (Lu et al., 2021).