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Targeting Gut Microbiota for the Prevention and Management of Diabetes Mellitus by Dietary Natural Products

Diabetes mellitus is one of the biggest public health concerns worldwide, which includes type 1 diabetes mellitus, type 2 diabetes mellitus, gestational diabetes mellitus, and other rare forms of diabetes mellitus. Accumulating evidence has revealed that intestinal microbiota is closely associated with the initiation and progression of diabetes mellitus. In addition, various dietary natural products and their bioactive components have exhibited anti-diabetic activity by modulating intestinal microbiota. This review addresses the relationship between gut microbiota and diabetes mellitus, and discusses the effects of natural products on diabetes mellitus and its complications by modulating gut microbiota, with special attention paid to the mechanisms of action. It is hoped that this review paper can be helpful for better understanding of the relationships among natural products, gut microbiota, and diabetes mellitus.


Introduction
Over the past 20 years, the prevalence of diabetes mellitus (DM) and its complications has rapidly increased across the world, which poses a serious threat to global health [1][2][3]. The international diabetes federation (IDF) estimated that one in eleven adults aged 20 to 79 (415 million) have DM globally in 2015, and predicts that the number can rise to 642 million by 2040 [4]. DM is a complex metabolic disorder characterized with the functional impairment or a lack of insulin-producing β-cells, alone or in combination with insulin resistance [5]. DM has several types, including type 1 diabetes mellitus (T1DM), type 2 diabetes mellitus (T2DM), gestational diabetes mellitus (GDM), and other rare forms of diabetes mellitus [6,7]. T1DM is an autoimmune disease that is associated with the aberrant immune responses to specific β-cell autoantigens, which causes the reduction of insulin [8][9][10][11]. T2DM is due to insulin resistance in the target tissue and a relative lack of insulin secreted by islet β-cells, and is mainly affected by the combination of genetic susceptibility and lifestyle factors [12][13][14][15]. GDM is a kind of diabetes during pregnancy due to the increased severity of insulin resistance and an impairment of the compensatory increase in insulin secretion [16][17][18]. Other rare forms of diabetes

The Association between Gut Microbiota and T1DM
The animal studies have shown that T1DM has been associated with the composition of the gut microbiota [88]. For example, the abundance of Proteobacteria showed a more marked change than that of the predominant phyla, such as Firmicutes and Bacteroidetes compared with the healthy control in the rat model of T1DM induced by streptozotocin [89]. In another study, the gut microbiota from non-obese diabetic (NOD) mice without myeloid differentiation primary response gene 88 (MyD88) was transferred to wild-type NOD mice, which reduced the intensity of insulitis and delayed the onset of autoimmune diabetes in recipients, by decreasing the amount of Lactobacillaceae and increasing the number of Lachnospiraceae and Clostridiaceae [59]. Moreover, the deficiency of TIR-domain-containing adapter-inducing interferon-β in NOD mice could prevent diabetes via the change in the gut microbiota, such as the proportion of Sutterella, Rikenella, and Turicibacter species [90]. In addition, the mouse β-defensin 14 (mBD14) in pancreatic endocrine cells in NOD mice could prevent autoimmune diabetes, while the microbiota dysbiosis could induce the deficiency of the pancreatic expression of mBD14, which results in the higher incidence of T1DM [91].
The relationship between gut microbiota and T1DM has been summarized in a review paper published in 2015 [54]. In addition, the epidemiological studies have reported that some intestinal microbiota like phylum Bacteroidetes, genus Clostridium, Bacteroides, and Veillonella as well as Candida albicans increased in T1DM patients, while some other gut microbiota like phylum Actinobacteria and Firmicutes, genus Lactobacillus, Bifidobacterium, and Faecalibacterium as well as the ratio of Firmicutes to Bacteroidetes decreased in T1DM patients [92][93][94][95][96].

The Association between Gut Microbiota and T2DM
The animal studies have found that there have been prominent differences in the composition of gut microbiota between T2DM animal models and control subjects. For example, an in vivo study found that the diabetic db/db mice had higher abundance of phyla Firmicutes, Proteobacteria, and Fibrobacteres [97]. Additionally, the streptozotocin-induced T2DM mice showed an increase in Brevibacterium, Corynebacterium, and Facklamia compared with wild type mice [98]. A decrease in the diversity of gut microbiota and a lack of butyrate-producing bacteria were observed in T2DM cats [99]. The level of serum fructosamine was inversely associated with the abundance of Prevotellaceae, and was positively associated with the abundance of Enterobacteriaceae. In addition, the transfer of Lachnospiraceae (strain AJ110941) from the feces of hyperglycemic obese mice to germ-free ob/ob mice contributed to the development of T2DM in ob/ob mice. The colonization of Lachnospiraceae could induce an increase in the level of fasting blood glucose (FBG) and a decrease in the level of plasma insulin as well as homeostasis model assessment-β (HOMA-β). The decrease in HOMA-β values indicated the dysfunction of pancreatic β-cells [100].
The epidemiological studies have also investigated the relationship between gut microbiota and T2DM patients [101][102][103][104]. For example, a case-control study has shown that the proportions of phylum Firmicutes and class Clostridia as well as the ratio of Firmicutes to Bacteroidetes decreased, whereas the abundance of class Betaproteobacteria increased in T2DM patients [101]. In addition, the concentration of Faecalibacterium prausnitzii and the abundance of genus Blautia was reduced in T2DM patients [102,103]. Moreover, a case-control study demonstrated that T2DM and obese patients had a lower quantity of Lactobacillus, especially its subgroups (L. acidophilus, L. plantarum, and L. reuteri) in comparison with controls [104].
Overall, several animal studies have revealed that the intestinal microbiota play an important role in T1DM and T2DM. In addition, many epidemiological investigations have demonstrated the closed association between gut microbiota and T2DM (Table 1). Furthermore, there are few prospective studies on the relationship between gut microbiota and DM, which should be further investigated in the future.

The Relationships among Natural Products, Gut Microbiota, and T2DM as well as T1DM Based on Animal Studies
Some natural products, such as vegetables, fruits, dietary fibers, and medicinal plants, have shown potential preventive effects against T2DM and T1DM as well as diabetic complications with mechanisms of action, at least partly, via the modulation of gut microbiota [80,[105][106][107][108].

Diabetes Mellitus
The extracts of vegetables and their bioactive components have been demonstrated to alleviate T2DM by modulating the gut microbiota [75,79,80]. Capsaicin could improve the glucose homeostasis and insulin tolerance in obese diabetic ob/ob mice by increasing the production of SCFAs and modulating gut microbiota. It could increase the ratio of Firmicutes to Bacteroidetes and the quantity of Roseburia, and reduce the quantities of Bacteroides and Parabacteroides at the genus level, which could decrease the levels of pro-inflammatory cytokines, such as TNF-α and IL-6 [79]. Additionally, pumpkin polysaccharide alleviated the T2DM in mice fed with a high-fat diet (HFD). Its antidiabetic effects were also related to the increase in SCFAs production and selective enhancement of some bacteria, such as Bacteroidetes, Prevotella, and Deltaproteobacteria [75]. Moreover, the carrot juice fermented with Lactobacillus rhamnosus GG was demonstrated to ameliorate T2DM in rats via the increase in SCFAs in the cecum and the change in the composition and abundance of gut microbiota, such as Oscillibacter and Akkermansia [80]. In addition, the anti-diabetic activity of Momordica charantia (bitter melon) was enhanced by the fermentation of Lactobacillus, which could induce an increase in the abundance of Bacteroides caecigallinarwn, Bacteroides thetaiotaomicron, Prevotella loescheii, Prevotella oralis, and Prevotella melaninogenica as well as the level of SCFAs [109].
It has also been reported that the extracts of several fruits and their bioactive components could improve T2DM via gut microbiota [105,110,111]. The phlorizin in many fruits could competitively inhibit the sodium-glucose symporters and regulate the level of blood glucose by decreasing the level of serum LPS and insulin resistance, and increasing the level of SCFAs, especially butyric acid. It could also increase the quantity of Akkermansia muciniphila and Prevotella, which modulates the gut microbial community structure [110]. In addition, polysaccharides from the mulberry fruit have shown protective effects against T2DM via gut microbiota in db/db mice, and the treatment with polysaccharides could enrich the functional bacteria, such as Bacteroidales, Lactobacillus, Allobaculum, Bacteroides, and Akkermansia [105]. The extracts from cinnamon bark were reported to improve glucose tolerance and insulin resistance by reducing the abundance of genus Peptococcus, and the extracts from grape pomace could reduce Desulfovibrio and Lactococcus, and increase Allobaculum and Roseburia in diabetic mice [111].
The inulin was also found to alleviate different stages of T2DM in diabetic mice by modulating gut microbiota. It increased the relative abundance of Cyanobacteria and Bacteroides, and reduced the relative abundance of Deferribacteres and Tenericutes [112]. In another study, the oral administration with inulin-type fructan could reduce the FBG level, increase the glucagon-like peptide-1 (GLP-1) level, and alleviate glucose intolerance as well as blood lipid in T2DM rats induced by HFD and streptozotocin [108]. The treatment with inulin also increased the level of GLP-1 and improved the gut microbiota dysbiosis by enriching probiotic bacteria Lactobacillus and SCFAs-producing bacteria Lachnospiraceae, Phascolarctobacterium, and Bacteroides. Furthermore, the supplementation of the long-chain inulin-type fructan fibers from chicory root to female NOD mice could delay the progression of T1DM via the modulation of gut microbiota. This fiber could increase the ratio of Firmicutes to Bacteroidetes and the abundance of Ruminococcaceae and Lactobacilli, which might contribute to the increase in the expression of tight junction proteins occludin and claudin-2 and an antidiabetogenic effect [106].
Some traditional Chinese medicine have been found to be effective in preventing and treating T2DM. The administration of ethanol extract of Atractylodis macrocephalae rhizoma improved the glucose metabolism by regulating the gut microbiota in diabetic db/db mice, and it could increase the abundance of Bacteroides thetaiotaomicron and Methanobrevibacter smithii [113]. In addition, the extract of Alpinia oxyphylla Miq. had protective effects against diabetes in T2DM db/db mice. It decreased the blood glucose levels by modulating the intestinal microbiota composition, which increased the abundance of Akkermansia and decreased Helicobacter [114]. The water extract of Potentilla discolor Bunge could alleviate diabetes, which might be related to the change in the gut microbiota. The treatment reduced the relative abundance of Proteobacteria in T2DM mice induced by HFD and streptozotocin [77]. It was related to an increase in levels of fecal acetic acid, butyric acid, and their specific receptors including the expression of G-protein-coupled receptor 41 (GPR41) and G-protein-coupled receptor 43 (GPR43), which increased the insulin sensitivity and decreased the fat accumulation. Additionally, the expressions of toll-like receptor-4 (TLR4), MyD88, nuclear factor-kappa B (NF-κB), and gut mucosal tight junction proteins (caudin-3 and occludin) were decreased, which could reduce the gut permeability and inflammation. In addition, the oral administration of total saponins and polysaccharides in Polygonatum kingianum could prevent T2DM by regulating the gut microbiota [115]. The treatment improved micro-ecology in the gut by reducing the abundance of Bacteroidetes and Proteobacteria, and increasing Firmicutes. Furthermore, the baicalein showed anti-diabetic effects on diabetic rats, which reduced the levels of blood glucose, LPS, and insulin resistance. The treatment of baicalein could regulate the gut microbiota by increasing the relative abundance of Bacteroides and Bacteroidales S24-7 [78]. It could also increase the production of SCFAs as well as the thickness of the gut mucus layer.
Overall, many studies have focused on the effects of different natural products, such as vegetables, fruits, dietary fibers, and medicinal plants, on T2DM by regulating gut microbiota and improving microecology in the gut. On the other hand, there are few studies about the effect of natural products on T1DM by modulating gut microbiota. In the future, more studies are needed to investigate the effects of different natural products and bioactive components on T1DM via gut microbiota-related mechanisms.

Diabetic Complications
Long-term diabetes increases the likelihood of its complications like diabetic erectile dysfunctions, nephropathy, cardiovascular diseases, retinopathy, and neuropathy [116,117]. The gut microbiota has also been found to have a close relationship with the development of the diabetic complications. In the rat models with T2DM erectile dysfunction, there was a reduction in the relative abundance of beneficial bacteria like Allobaculum, Bifidobacterium, Eubacterium, and Anaerotruncus. In addition, the relative abundance of opportunistic pathogens increased, such as Enterococcus, Corynebacterium, Aerococcus, and Facklamia [118]. Some natural products have been reported to have protective effects on diabetic nephropathy or kidney injury in diabetic mice by modulating the gut microbiota balance [107,119]. For example, the polyphenolic extract of Dendrobium loddigesii could improve the symptoms of diabetes and complications in diabetic db/db mice. These effects were likely related to the improvement of the intestinal flora balance, with an increase in the relative abundance of Prevotella and Akkermansia, and the reduction in the relative abundance of S24-7/Rikenella/Escherichia coli [119]. Additionally, the treatment with the water-ethanolic extract of green macroalgae Enteromorpha prolifera reduced the inflammation in the liver and kidney by significantly increasing the abundance of Lachnospiraceae and Alisties as well as regulating the insulin signaling pathway in T2DM mice induced by HFD and high sucrose diet (HSD) and streptozotocin [107].
To sum up, several studies have demonstrated that the gut microbiota has a tight association with diabetic complications. The increase or decrease in different types of gut microbiota could exert different effects on the progression of diabetic complications. Moreover, several natural products have showed potential efficacy on the prevention and management against diabetic complications, and these effects were mainly associated with the modulation of gut microbiota composition and abundance, regulation of the insulin signaling pathway, and inhibition of inflammation.
Lastly, the relationship among gut microbiota, T2DM, and its complications is shown in Figure 1, and the association among natural products, gut microbiota, T2DM, and its complications is given in Table 2 and Figure 2.   Decreasing the level of pro-inflammatory cytokines.
↑ The concentrations of fecal acetic acid butyric acid and their specific receptors including the expression of GPR41 and GPR43 ↓ The ratio of Firmicutes to Bacteroidetes and ↓ The abundance of Proteobacteria ↓ The expression of gut mucosal tight junction proteins (Claudin3, ZO-1, and Occludin), TLR4, MyD88, and NF-κB Diabetic rats induced by HFD, HSD, and STZ Decreasing the level of blood glucose. Improved inflammation and lipid metabolism.
↑ The relative abundances of Bacteroides and Bacteroidales S24-7 ↑ The production of SCFAs ↑ The thickness of the gut mucus layer [78] Dendrobium loddigesii polyphenols extract

Diabetic nephropathy
In vivo Diabetic nephropathy db/db mice Improving diabetic nephropathy. Decreasing blood glucose level. Increasing the level of insulin.

T2DM
In vivo

T2DM mice induced by HFD and HSD and STZ
Decreasing the level of FBG. Improving glucose tolerance.
Reducing inflammation. Preventing liver and kidney injury.

The Relationships among Natural Products, Gut Microbiota, and T2DM Based on Clinical Studies
Several clinical trials have evaluated the protective effects of natural products on T2DM via gut microbiota [120,121]. A double-blind, randomized, controlled clinical trial (RCT) involving 60 patients with T2DM found that the supplementation of 10 g/day inulin powder promoted the gut health by increasing the proportion of Akkermansia muciniphila [120]. Moreover, an RCT study found that the fiber-rich macrobiotic Ma-Pi 2 diet could regulate gut microbiome dysbiosis in obese T2DM patients by increasing the gut microbiota ecosystem diversity, recovering the SCFAs-producing bacteria community, such as Roseburia, Lachnospira, Faecalibacterium, Bacteroides, and Akkermansia, and inhibiting the increase of the pro-inflammatory group, such as Collinsella and Streptococcus [122]. Furthermore, an RCT study including 100 patients with T2DM reported that a Chinese herbal formula could ameliorate T2DM and improved the glucose and lipid homeostasis by increasing the abundance of Faecalibacterium spp. [116]. Another RCT study observed that the treatment with Gegen Qinlian decoction, which is a type of Chinese herbal formula, at the moderate and high doses, significantly reduced the mean alterations in adjusted FBG and glycated hemoglobin A1c (HbA1c) levels by enriching the beneficial bacteria, such as Faecalibacterium prausnitzii [121].
To sum up, several clinical trials demonstrate that natural products can ameliorate T2DM by regulating gut microbiota (Table 3). Some natural products, such as inulin and medicine plants, are able to enrich certain beneficial bacteria in T2DM patients, such as Akkermansia muciniphila, Faecalibacterium, and Lachnospira.

Conclusions
In conclusion, the relationship among diabetes mellitus (including T1DM and T2DM), gut microbiota, and natural products has been summarized and discussed. Animal and epidemiological studies found significant differences in intestinal microbiota composition and abundance between diabetic patients and healthy controls. The abundance of Bifidobacterium and Lactobacillus as well as the ratio of Firmicutes to Bacteroidetes decreased, while the abundance of Bacteroidetes and Proteobacteria increased in T1DM and T2DM patients. However, the consequences were not always consistent due to other factors like types of diets and progression of diseases. In addition, animal studies have demonstrated that gut microbiota is one of the most important factors for diabetes initiation and development. Several natural products possessing prebiotic effects like fruits, vegetables, and medicinal plants, have been found to ameliorate T2DM by modulating gut microbiota composition and abundance, reducing the gut permeability, increasing the production of SCFAs, decreasing the level of LPS, and inhibiting the inflammation. Moreover, clinical trials have further confirmed that several natural products are effective in preventing and treating T2DM, with regulation of gut microbiota as one of the potential mechanisms. In the future, it is necessary to explore the effects of more natural products and their bioactive components on DM via gut microbiota-related mechanisms. In addition, more well-designed clinical trials on natural products and their various bioactive compounds should be carried out to verify their effects on T2DM and its complications by regulating gut microbiota. Furthermore, functional foods based on natural products can be researched and developed by targeting gut microbiota for the prevention and management of T2DM. On the other hand, present studies mainly focus on modulating the action of natural products and their bioactive components on gut microbiota for preventing and managing T2DM. However, seldom studies have been carried out about the metabolism of natural products and their bioactive components by gut microbiota, even though the metabolites could directly play a role in the prevention and management of DM or, in return, could modulate gut microbiota. Thus, metabolisms of natural products and their bioactive components by gut microbiota should be widely studied in the future for the prevention and management of DM.