Updated Progress on Polysaccharides with Anti-Diabetic Effects through the Regulation of Gut Microbiota: Sources, Mechanisms, and Structure–Activity Relationships

Diabetes mellitus (DM) is a common chronic metabolic disease worldwide. The disturbance of the gut microbiota has a complex influence on the development of DM. Polysaccharides are one type of the most important natural components with anti-diabetic effects. Gut microbiota can participate in the fermentation of polysaccharides, and through this, polysaccharides regulate the gut microbiota and improve DM. This review begins by a summary of the sources, anti-diabetic effects and the gut microbiota regulation functions of natural polysaccharides. Then, the mechanisms of polysaccharides in regulating the gut microbiota to exert anti-diabetic effects and the structure–activity relationship are summarized. It is found that polysaccharides from plants, fungi, and marine organisms show great hypoglycemic activities and the gut microbiota regulation functions. The mechanisms mainly include repairing the gut burrier, reshaping gut microbiota composition, changing the metabolites, regulating anti-inflammatory activity and immune function, and regulating the signal pathways. Structural characteristics of polysaccharides, such as monosaccharide composition, molecular weight, and type of glycosidic linkage, show great influence on the anti-diabetic activity of polysaccharides. This review provides a reference for the exploration and development of the anti-diabetic effects of polysaccharides.


Introduction
Diabetes mellitus (DM) is a metabolic disease characterized by hyperglycemia, which is a typical and complex chronic metabolic disease worldwide.Generally, it is classified into type 1 DM (T1DM), type 2 DM (T2DM), gestational DM and specific types of diabetes due to other causes like monogenic diabetes syndromes [1].T1DM is an autoimmune illness caused by the destruction of β cells, which typically results in a lack of insulin, accounting for 5-10% of the prevalence of DM [2].About 90% of DM cases are T2DM, which is caused by a gradual decrease in β-cell insulin secretion [3].
The incidence of DM is rising as a result of the changing of lifestyles and living circumstances.DM is considered a global disease and more and more people suffer from it.According to a prediction of the International Diabetes Federation (IDF), there may be up to 700 million people living with DM by 2045.Persistent hyperglycemia can cause chronic damage to and dysfunction of numerous organs and tissues, such as eyes, kidneys, heart, blood vessels, and nerves [4,5].For example, abnormal blood glucose metabolism is usually accompanied by blood lipid metabolism disorder [6].Additionally, patients with DM have a notable increase in microvascular risk, which can lead to the development of hypertension [7].What is more, DM may predispose toward Parkinson-like pathology [8,9].These complications seriously endanger human health.Nowadays, the prevention and treatment of DM mainly focus on improvement in lifestyle and control of blood sugar by drugs.Although there are many drugs on the market that can be used to treat DM, like biguanides, acarbose, sulfonylureas and thiazolidinediones, these drugs have some limitations that cannot be ignored, such as side effects or high cost [10,11].The development of lower-toxicity and more effective drugs is necessary.
Polysaccharides are carbohydrate macromolecules composed of at least more than 10 monosaccharides linked to each other by glycosidic bonds, and can be mainly found in plants, fungi, and marine organisms in nature.Numerous functions, including hypoglycemic, anti-inflammatory, immunomodulatory, neuromodulatory, and anticancer activities, have been demonstrated by studies on polysaccharides [12][13][14][15][16].In addition, there are many advantages of polysaccharides, including good biodegradability and biocompatibility and few side effects [17,18].Polysaccharides are utilized extensively in the food and pharmaceutical industries as emulsifiers, adjuvants for vaccines, and materials to treat oral diseases [19][20][21].Therefore, polysaccharides can be regarded as a promising strategy for the prevention and treatment of DM.
The human gut is a crucial organ that is directly connected to the outside environment.The human the gut microbiota comprises trillions of microorganisms, including archaea, eukaryotes, viruses and bacteria at the domain level [22].At the phylum level, it mainly consists of Firmicutes, Bacteroidetes, Actinobacteria, Proteobacteria, Verrucomicrobia and Fusobacteria [23][24][25].They participate in nutrient metabolism, immune regulation and chemical modifications of dietary components, and the health of the host is closely related to the diversity and stability of the gut microbiota [26][27][28].In normal conditions, the gut microbiota and its metabolites are maintained in a reasonably steady ecological balance with the host, but many factors can affect the composition and function of the gut microbiota such as dietary habits and hereditary and environmental factors [29].The destruction of the gut microbiota may lead to many diseases, such as osteoporosis, obesity, T2DM, non-alcoholic fatty liver, and hypertension [30].Research has found that a reduction in gut microbiota biodiversity and gut-associated metabolites contributes to heart failure [31].The dysbiosis state of the gut microbiota and disruption in the energy metabolism contribute to the pathogenic mechanisms of neurodegeneration [32].The modulation of gut microbiota biodiversity and gut-associated metabolites is associated with anti-diabetic activities [33][34][35][36].
The host cannot digest and absorb the majority of polysaccharides.In the human body, polysaccharides can interact with the gut microbiota and ferment into absorbable substances, mainly short-chain fatty acids (SCFAs) in the colon [37,38].SCFAs consist of free fatty acids with two to five carbons, such as acetic acid, propanoic acid, and butyric acid, participating in signal transmission, nutrition provision and many other healthy protection functions in human body [39].During these processes, polysaccharides are involved in the balance of the gut microbiota and host as a kind of prebiotic [40].In turn, the gut microbiota can influence the host digestive efficiency by altering the bioavailability of polysaccharides [41].
In our earlier research, we discovered that mulberry leaf polysaccharides (MLPs) had a substantial negative link with the risk of obesity and might considerably lower the Firmicutes/Bacteroidetes (F/B) ratio, which may result in diminished energy absorption in mice [42].Additionally, we discovered that Lentinula edodes soluble dietary fibers (LESDF) promoted Bacteroides production, indicating that LESDF was one of the dietary elements with an impact on the composition and relative abundance of the gut microbial community in vitro [43].Nowadays, polysaccharides are highly valued by researchers in the field of medicine and health food and more and more polysaccharides have been discovered to prevent and treat DM.In addition, the gut microbiota is a hot topic in the management of DM.Although there are some reviews about the anti-diabetic effects and gut microbiota regulation function of polysaccharides, there is no comprehensive review about how polysaccharides exert antidiabetic effects through the regulation of the gut microbiota and the related mechanisms.This article gives a summary of the relationship between polysaccharides and the gut microbiota in the prevention and treatment of DM, which may provide a reference for developing more effective anti-diabetic strategies.

Sources of Polysaccharides and Their Anti-Diabetic Effects
In the 21st century, scientific researchers paid more attention to natural products.Polysaccharides, natural compounds with low toxicity and multiple functions, can be regarded as lead compounds to be screened for developing new drugs and health foods [44].Researchers discovered that the gut microbiota were linked to the onset of DM and played an important role in the maintenance of good health [23].Although the human digestive system is capable of breaking down and absorbing certain polysaccharides like starch, there are polysaccharides that cannot be digested by the host because of high molecular weight and low bioavailability.They can enter the colon and interact with the gut microbiota, exerting dietary or medicinal effects [45,46].For example, researchers found that polysaccharides from Dendrobium officinale were fermented and degraded into SCFAs in the large intestine and modulated bioactivities associated with the gut microbiota in rats [47].
The sources of polysaccharides and the structure of some examples are shown in Figure 1.The anti-diabetic effects of polysaccharides from different sources and their regulation effects on the gut microbiota are summarized in the following.with an impact on the composition and relative abundance of the gut microbial community in vitro [43].Nowadays, polysaccharides are highly valued by researchers in the field of medicine and health food and more and more polysaccharides have been discovered to prevent and treat DM.In addition, the gut microbiota is a hot topic in the management of DM.Although there are some reviews about the anti-diabetic effects and gut microbiota regulation function of polysaccharides, there is no comprehensive review about how polysaccharides exert anti-diabetic effects through the regulation of the gut microbiota and the related mechanisms.This article gives a summary of the relationship between polysaccharides and the gut microbiota in the prevention and treatment of DM, which may provide a reference for developing more effective anti-diabetic strategies.

Sources of Polysaccharides and Their Anti-Diabetic Effects
In the 21st century, scientific researchers paid more attention to natural products.Polysaccharides, natural compounds with low toxicity and multiple functions, can be regarded as lead compounds to be screened for developing new drugs and health foods [44].Researchers discovered that the gut microbiota were linked to the onset of DM and played an important role in the maintenance of good health [23].Although the human digestive system is capable of breaking down and absorbing certain polysaccharides like starch, there are polysaccharides that cannot be digested by the host because of high molecular weight and low bioavailability.They can enter the colon and interact with the gut microbiota, exerting dietary or medicinal effects [45,46].For example, researchers found that polysaccharides from Dendrobium officinale were fermented and degraded into SCFAs in the large intestine and modulated bioactivities associated with the gut microbiota in rats [47]. The

Polysaccharides Extracted from Plants
Plants are one of the main sources of polysaccharides.Polysaccharides can be extracted from different kinds of plants like herbs, woody plants, and shrubs [56].For example, Camellia sinensis, which is commonly called a tea tree, is a kind of shrub and the source of tea polysaccharides with famous anticancer and anti-diabetic effects in vitro or

Polysaccharides Extracted from Plants
Plants are one of the main sources of polysaccharides.Polysaccharides can be extracted from different kinds of plants like herbs, woody plants, and shrubs [56].For example, Camellia sinensis, which is commonly called a tea tree, is a kind of shrub and the source of tea polysaccharides with famous anticancer and anti-diabetic effects in vitro or in vivo [57,58].Various plant parts, including the leaves, stems, roots, and flowers, can also be used to extract polysaccharides.Polysaccharides extracted from the leaves and fruits of baobab (Adansonia digitata L.) have different molecular weights, solution viscosity-enhancing properties and other characteristics [59].In our previous study, we found that polysaccharides from plants showed good anti-diabetic effects.Polysaccharides from Camellia sinensis showed α-glucosidase inhibitory capacity and hypoglycemic effect on L6 cells by promoting glucose uptake [60].MLP treatment considerably improved metabolic diseases by lowering the Firmicutes/Bacteroidetes (F/B) ratio and increasing the amounts of Bifidobacterium, Lactobacillus, and Akkermansia, which were linked to the generation of butyric acid and/or propionic acid in mice [42].
Many other polysaccharides from various plants also showed anti-diabetic effects and the gut microbiota regulation function.The sources, structures, anti-diabetic effects and the gut microbiota regulation function of polysaccharides from plants are shown in Table 1.
Polysaccharides from plants show different gut microbiota regulation function.On the one hand, polysaccharides increase the diversity of probiotics in gut.Probiotics in the gut show positive effects on DM, such as decreasing lipid accumulation, ameliorating oxidative stress and inflammation, and increasing SCFAs in the gut [61].Lactobacillus and Bifidobacterium are two main probiotics at the genus level and can even be used commercially [62,63].Lactobacillus participates in glycolipid metabolism and produces SCFAs like lactic acid and acetic acid [64].One study found that Lactobacillus could also reduce hepatic insulin resistance and liver deterioration in diabetic mice [65].Bifidobacterium has great saccharolytic capability, plays an anti-inflammatory role, manages obesity, protects mucosal barrier integrity, and participates in the production of SCFAs in the gut [66,67].
Polysaccharides from kiwifruit (Actinidia chinensis), ginseng, and soy hull increase the abundance of Lactobacillus in mice or rats [68][69][70].Polysaccharides from the tuberous root of Ophiopogon japonicus significantly enrich Bifidobacterium in mice [71,72].On the other hand, polysaccharides regulate the gut microbiota by decreasing pathogenic bacteria.For example, the in vitro fermentation of human fecal microbiota showed that polysaccharides from Polygonatum kingianum decrease the abundance of Proteobacteria, which was positively correlated with dysbiosis of the gut microbiota and the occurrence of DM.It exhibited similar effects in diabetic rats [73,74].

Polysaccharides Extracted from Fungi
Fungi have been widely eaten and used by humans all over the world for thousands of years.Polysaccharides from fungi can be extracted from different parts like fruiting body, mycelium, or fermentation broth, and β-glucan accounts for the main structure of fungi polysaccharides [75,76].Fungi polysaccharides possess many health-enhancing activities, such as immune modulation, fatigue relief, anti-aging and hypolipidemic activities in vivo or in vitro [77][78][79][80][81].For example, researches revealed that oral β-1,3/1,6 glucan per day prevented the occurrence or reduced the severity of upper respiratory tract infection in older individuals [82].High-purity yeast β-glucan induced innate immune regulation in gut mucosa, protecting against T1DM in mice [83].Grifola frondose polysaccharides improved the disorder in lipid metabolism in high-fat diet (HFD)-and STZ-induced mice by increasing hepatic Acox1 expression and reducing the hepatic free fatty acid levels [84].
Polysaccharides from fungi can play an anti-diabetic role through modulation of the gut microbiota.The sources, structures, anti-diabetic effects and gut microbiota regulation function of polysaccharides from fungi are shown in Table 2.
Polysaccharides from fungi have anti-diabetic and the gut microbiota regulation functions in different respects.For example, Grifola frondosa polysaccharides improved the diversity and composition of the gut microbiota in HFD-induced diabetic mice [85].Polysaccharides from Agaricus bisporus promoted the growth of beneficial bacteria, including Prevotella, Megamonas, and Bacteroides during the in vitro fermentation process [86].Hirsutella sinensis mycelium (HSM) polysaccharides showed protective effects on gut integrity by selectively promoting the growth of a commensal bacterium, Parabacteroides goldsteinii, whose level was reduced in HFD-induced mice.HSM polysaccharides reduced inflammation associated with metabolic disorders, relieved insulin resistance, and improved lipid metabolism to treat obesity and T2DM [87].In a recent study, we discovered that soluble dietary fiber from Lentinula edodes byproducts combined with Lactobacillus plantarum LP90 repaired intestinal epithelial injury in dextran sulfate sodium-induced colitis mice.Additionally, the synbiotic therapy enhanced the production of butyric acid and upregulated the expression of tight junction proteins [101].

Polysaccharides Extracted from Marine Organisms
Marine organisms are a huge source of material for drug development.Many marine organisms are used as food ingredients and supplements due to their great taste and nutrition.Researchers are paying more attention to marine active ingredients because of the unique environmental characteristics like the high salt, high pressure and the lack of light.For example, Ishige okamurae extract showed multiple activities such as anti-obesity, anti-diabetic, anti-inflammatory and antioxidant activities in vitro or in mice [102][103][104][105].As one of the major components of the active ingredients from marine organisms, polysaccharides play various activities.Two polysaccharides (laminaran SdL, fucoidan SdF) extracted from brown alga Sargassum duplicatum had different structures and fucoidan SdF showed anticancer effect in vitro on colon cancer cells [106].Undaria pinnatifida polysaccharides exhibited α-glucosidase inhibitory activity in vitro and alleviated HFD/STZ-induced hyperglycemia by increasing glucose tolerance, relieving insulin resistance and the histopathological injuries of pancreas and liver [107].They also showed anti-inflammatory activity by inhibiting IFN-γ expression in mice [108].In addition to the examples above, the sources, structures, anti-diabetic effects and gut microbiota regulation function of polysaccharides from marine organisms are shown in Table 3.
An in vitro digestion experiment showed that polysaccharides from Gracilaria lemaneiformis were mostly degraded in the fermentation process, promoting the production of SCFAs and inhibiting the growth of the Firmicutes community [109].Another in vitro experiment illustrated that fermentation of fucosylated chondroitin sulfate from Stichopus chloronotus contributed to Bacteroidetes and Fusobacteria and benefited host health by lowering the F/B ratio in turn [110].
Experiments in vivo showed similar results.For instance, the seaweed polysaccharides laminaran and ulvan were slowly fermented by Bifidobacterium as well as stimulated the growth of Bifidobacterium and promoted the production of acetate, propionate and lactate [111].The mixture of algal polysaccharides ulvan and astaxanthin increased the level of Bacteroidia, Bacilli, Clostridia, and Verrucomicrobia [112].Sulfated sea cucumber Stichopus japonicus significantly enriched the relative abundance of Parabacteroides and Akkermansia, as well as reduced the level of Proteobacteria [113,114].Sargassum fusiforme polysaccharides decreased the relative abundance of Bacteroidetes and increased that of Oscillospira, Mucispirillum, and Clostridiales [115].Laminaria japonica polysaccharides upregulated Turicibacter, which produced SCFAs, especially lactic acid [116].

Mechanism of the Anti-Diabetic Effects of Polysaccharides through Regulating Gut Microbiota
The gut microbiota is closely related to human health maintenance.Research has shown that there is an important relationship between the gut microbiota and DM [122].The gut microbiota forms a network regulating the gut barrier, the production and utilization of metabolites, and the immune function of the gut [123].It also influences the use of glucose and the conversion of glycogen, and its composition is related to the progression of insulin resistance in T2DM as well as the development of complications of DM [124,125].Below, the anti-diabetic mechanisms of polysaccharides through regulating the gut microbiota are summarized and are shown in Figure 2.

Repairing the Gut Barrier
The mucus layer and the intestinal apical junctional protein complex make up the complex gut barrier, which protects host health from many acute and chronic illnesses [126].Inflammatory bowel disease (IBD), colon cancer, and many other illnesses were related to damage to the gut barrier [127].In diabetic mice, the destruction of the gut barrier was shown in a reduction in the number of goblet cells, shorter and irregular villi, a reduction in mucosal space, an influx of inflammatory cells, and weakened epithelial cells [121,128,129].When the gut barrier was damaged, its function was decreased or even disturbed.Many studies have shown that polysaccharides showed great protective effects on gut barrier integrity.For example, Pleurotus eryngii polysaccharide interacted with intestinal mucus layer after in vitro fermentation by the gut microbiota [130].Ganoderma atrum polysaccharides maintained the intestinal barrier and its permeability in rats [131].The mixture of hawthorn flavonoids and two kinds of polysaccharides from Auricularia auricula and Tremella improved injuries to the intestinal barrier and epithelial cells in rats [132].Dandelion polysaccharides repaired the intestinal barrier and improved the structure of the gut microbiota in mice [133].
Polysaccharides can also relieve the injury of the gut barrier through improving tight junction proteins.Cultured Cordyceps sinensis polysaccharides, Dictyopteris divaricate polysaccharides, and Phellinus linteus polysaccharides upregulated tight junction proteins such as occludin, claudin-1, and ZO-1 in intestinal barrier injury in mice, maintaining the gut structure and barrier permeability in mice or rats [47,121,134].Mucin-2 protein √ means improvement.

Repairing the Gut Barrier
The mucus layer and the intestinal apical junctional protein complex make up the complex gut barrier, which protects host health from many acute and chronic illnesses [126].Inflammatory bowel disease (IBD), colon cancer, and many other illnesses were related to damage to the gut barrier [127].In diabetic mice, the destruction of the gut barrier was shown in a reduction in the number of goblet cells, shorter and irregular villi, a reduction in mucosal space, an influx of inflammatory cells, and weakened epithelial cells [121,128,129].When the gut barrier was damaged, its function was decreased or even disturbed.Many studies have shown that polysaccharides showed great protective effects on gut barrier integrity.For example, Pleurotus eryngii polysaccharide interacted with intestinal mucus layer after in vitro fermentation by the gut microbiota [130].Ganoderma atrum polysaccharides maintained the intestinal barrier and its permeability in rats [131].The mixture of hawthorn flavonoids and two kinds of polysaccharides from Auricularia auricula and Tremella improved injuries to the intestinal barrier and epithelial cells in rats [132].Dandelion polysaccharides repaired the intestinal barrier and improved the structure of the gut microbiota in mice [133].
Polysaccharides can also relieve the injury of the gut barrier through improving tight junction proteins.Cultured Cordyceps sinensis polysaccharides, Dictyopteris divaricate polysaccharides, and Phellinus linteus polysaccharides upregulated tight junction proteins such as occludin, claudin-1, and ZO-1 in intestinal barrier injury in mice, maintaining the gut structure and barrier permeability in mice or rats [47,121,134].Mucin-2 protein (MUC2) is a kind of heavily glycosylated mucin proteins secreted by intestinal cells protecting the human gut from potentially harmful bacteria and substances [135].Polysaccharides from the fruit of Lycium barbarum increased the mRNA expression of MUC2 in mouse colon [136].Quinoa seed polysaccharides increased the production of intestinal mucus in rats, protecting host from infection [137].

Changing Gut Microbiota Composition and Metabolites
Research found that the production of bacteria-derived metabolite damaging to human health was strongly associated with gut dysbiosis reversal [89].Polysaccharides, regarded as a kind of prebiotics, were found to promote the growth and/or activity of beneficial bacteria and to suppress pathogenic bacteria in rat gut [47].Polysaccharides also influenced the metabolism of SCFAs and other metabolites indirectly by changing the composition and diversity of the gut microbiota in rats [138].The metabolites of the gut microbiota influenced the gut and system function by circulation or acting as ligands for cell receptors [139].For example, increased expression of GPCR 41/43 in the intestinal L cells caused by higher SCFAs may cause the production of GLP-1 [140].
At the phylum level, the Gram-positive Firmicutes and Gram-negative Bacteroidetes were two dominant bacteria in the gut microbiota, comprising more than 90% of total 16S rRNA-targeted sequences from bacteria [141].Firmicutes and Bacteroidetes had many carbohydrate metabolism pathways, which promoted the expression of CAZymes to degrade most polysaccharides to produce SCFAs, participating in the regulation of glycol metabolism [142,143].One study found that the F/B ratio was related to metabolic disorders, and compared with a normal group, the F/B ratio was considerably increased in diabetic model mice [68].Regulation of the F/B ratio is one important way for polysaccharides to modulate the composition of the gut microbiota and improve the gut metabolites.
Dendrobium officinale leaf polysaccharides change gut microbiota composition and derived microbial compounds.They decrease the F/B ratio and upregulate butyrate production to repair the intestinal microenvironment in mice [128].The treatment of polysaccharide from Cyclocarya paliurus leaves attenuated the decrease in the F/B ratio induced by diabetes, while in nondiabetic rats, polysaccharide administration did mot have the same effect [144].Astragalus membranaceus polysaccharide treatment decreased the F/B ratio and promoted the production of acetic acid, butyric acid, and propanoic acid in feces from db/db mice.Konjac glucomannans (KGMs) increased Bacteroidetes and decreased Firmicutes abundance in diabetic mice.KGMs increased the abundance of Muribaculaceae, part of Bacteroidetes, and produced SCFAs to improve pancreatic β-cell function and reduce the release of proinflammatory cytokines [145].
It was also found that Akkermansia is closely related to human health and some DM related indices and is a potential probiotic for the treatment of diabetes [146].Akkermansia is a kind of representative bacterium in the mucus layer of the intestine that participates in the degradation process of mucin, thereby protecting the intestinal mucosal barrier and reducing protein deposition [147].Metformin therapy enhances the relative abundance of Akkermansia in mice [148].Polysaccharides from pumpkin, gougunao tea, and Gastrodia elata increased Akkermansia in T2DM mice [149][150][151].
Ruminococcus, which is abundant and common in the mammalian gut environment, can degrade polysaccharides to produce SCFAs and is a predominant acetogen producing acetic acid and propionate [152].It was reported that Ruminococcaceae UCG-005 is a key genus for protecting against diabetes [144].One study found that Ruminococcus was negatively connected with FBG and positively related to body weight [74].Ruminococcaceae UCG-014 and UCG-005 genera were negatively correlated with indices including liver weight, AST and ALT levels, hepatic steatosis and inflammation degree, which revealed their hepatoprotective effect [153].Grifola frondosa, Ganoderma atrum and Cordyceps militaris polysaccharide supplementation enriched the relative abundance of Ruminococcus in HFD rats [154].
Furthermore, a number of studies have shown that reducing the amount of Alistipes in the gut may decrease the generation of antimicrobial peptides, which in turn promotes the colonization of harmful microbes on the gut barrier and results in liver illnesses.For example, Alistipes play a crucial part in the prevention of dextran sulfate sodium-induced colitis in mice [155].Polysaccharides from Grifola frondose and Lentinula edodes significantly increased the abundance of Alistipes in mice [156,157].Auricularia auricula polysaccharides markedly increased the level of Bifidobacterium animalis, Morchella esculenta, and Inonotus obliquus polysaccharides enhanced the level of Lactobacillus in mice [77,158,159].

Regulating Anti-Inflammatory Activity and Immune Function
Inflammation and disorder of the immune system in the host, e.g., IBD, systemic lupus erythematosus, asthma, arthritis, and many other diseases, are impacted by changes in the normal gut microbiota [161,162].A low-grade inflammatory and autoimmune condition that is closely linked to T2DM may be caused directly by changes in the gut microbiota's composition.According to the results of Pearson analysis, the relative abundance of some of the gut microbiota was closely correlated with indices of immunological organs like the spleen and thymus and immunoglobulins like IgG and IgM, and CD4 + T cells [163].Investigation of the connection between the gut microbiota and biochemical profiles showed Bifidobacterium was involved in inflammation and Fusobacterium modulated host immune responses [93,164].Similarly, relevant studies showed that members of the Lachnospira family in the human gut express two "superantigens" that stimulate the IgA response and are crucial for intestinal homeostasis [165].Another study found that an increase in Proteobacteria resulted in immune dysregulation in the host [166].Agaricus blazei Murill polysaccharides decreased Proteobacteria and increased Lachnospiraceae and Lactobacillaceae, suppressing the inflammation response in mice [167].
The intervention of polysaccharides can improve the inflammatory and immune response.For example, water extract of Berberis kansuensis reduced inflammatory factors such as TNF-α, IL-1β and IL-6 in diabetic mice [168].Lycium berry polysaccharides downregulated proinflammatory cytokines including IL-1β and IL-18 and M1 macrophage markers and increased anti-inflammatory cytokines like IL-4 and IL-10 and M2 macrophage markers to facilitate the shifting of the epithelial immunity from the pro-to the anti-inflammatory microenvironment in mice [169].Nigella sativa seed polysaccharides decreased the levels of cytokines, including IL-6, IL-1β and TNF-α [94].Dictyopteris divaricate polysaccharides downregulated the expression of IL-1β, IL-2, IL-6, TNF-α and IFN-γ in T1DM mice [121].

Regulating the Signal Pathway
Many signal pathways have been found to be related to glucose metabolism as well as DM.For example, the mTOR signal pathway regulates energy intake and is related to metabolic disorders like T2DM [170].The PI3K/Akt pathway is a key signaling pathway and regulates insulin signal and glucose metabolism [171,172].Inactivation of the PI3K/Akt signaling pathway in liver reduces the synthesis of liver glycogen, resulting in insulin resistance, a typical symptom of T2DM [173].The phosphorylation of ISR1, PI3K and Akt in succession activates GLUT4/GLUT2, leading to glucose uptake of cells [174].Many polysaccharides relieve the symptoms of DM by regulating the PI3K/Akt signal pathway.For examples, azuki bean (Vigna angularis) polysaccharides control glucose metabolism and oxidative stress by considerably upregulating the expression of INSR, IRS-1, PI3K, Akt, and GLUT2 in this signal pathway [175].Brasenia schreberi polysaccharides increase the expression of PI3K and Akt in T2DM mice [90].Oligosaccharides from seaweed Sargassum confusum significantly upregulate the expression of IRS1 and PI3K genes and downregulate the expression of JNKs,h contributing to DM in high-fat/high-sucrose-fed hamsters [176].
Besides the signal pathways in the liver and muscle for glucose uptake, the signal pathways in the gut and other organs are also affected.Coix seed polysaccharides increase the expression of insulin-like growth factor 1 (IGF1) and insulin-like growth factor 1 receptor (IGF1R), which participates in the secretion of insulin [92].Dietary inulin and Lycium barbarum polysaccharides improve the expression of Toll-like receptor 2 (TLR2) on the surface of γδ T cells, which can be recognized by the metabolites of the gut microbiota and increase the integrity of the gut microbiota in rats [177].At the same time, a Scutellaria-Coptis herb couple downregulated the expression of Toll-like receptor 4 (TLR4) and MyD88 protein in the colon, decreasing the secretion of proinflammatory factors in mice [178].It has been reported that polysaccharides can interact with enteroendocrine L cells directly and enhance the production of glucagon-like peptide-1 (GLP-1) through the cAMP signaling pathway and Ca 2+ /calmodulin/calmodulin-dependent protein kinase 2 signaling pathway [136].Cyclocarya paliuru polysaccharides can increase the mRNA expression of G-protein-coupled receptors (GPRs) like GPR41, GPR43 and GPR109a, which are receptors of SCFAs in colon tissue in T2DM mice [118].

Action on Related Tissue and Organs
The gut microbiota is closely related to other tissue and organs, and the mostly studied include muscle, liver, kidney and pancreas.The relationship of the gut microbiota and to these organs is shown in Figure 3.The metabolites of the gut microbiota enter the blood and interact with receptors on muscle, liver, pancreas, and other organs, influencing their functions [179].The microbiota-liver axis, microbiota-brain axis, microbiota-kidney, microbiota-lung, and other microbiota-organ axes have been studies [180][181][182][183][184]. For example, one study found that hepatic markers were associated with Ruminococcaceae family [153].The regulation effects and communication with other tissue and organs of the gut microbiota has aroused researchers' interest.Polysaccharides can play protective roles by regulating intestinal flora.For instance, water extracts of Plukenetia volubilis leaves restored pancreas injury, including the improvement in injured pancreas islets and pancreatic β-cells in T1DM mice [185].Polysaccharides from red kidney bean also showed pancreas-protective effects in T2DM mice [186].Mulberry fruit polysaccharides healed damage to the liver, kidneys and pan- The metabolites of the gut microbiota enter the blood and interact with receptors on muscle, liver, pancreas, and other organs, influencing their functions [179].The microbiotaliver axis, microbiota-brain axis, microbiota-kidney, microbiota-lung, and other microbiotaorgan axes have been studies [180][181][182][183][184]. For example, one study found that hepatic markers were associated with Ruminococcaceae family [153].The regulation effects and communication with other tissue and organs of the gut microbiota has aroused researchers' interest.
Polysaccharides can play protective roles by regulating intestinal flora.For instance, water extracts of Plukenetia volubilis leaves restored pancreas injury, including the improvement in injured pancreas islets and pancreatic β-cells in T1DM mice [185].Polysaccharides from red kidney bean also showed pancreas-protective effects in T2DM mice [186].Mulberry fruit polysaccharides healed damage to the liver, kidneys and pancreas of obese diabetic mice [187].

Structure-Activity Relationship of the Anti-Diabetic Effects of Polysaccharides through Regulating Gut Microbiota
As a kind of biological macromolecule, polysaccharides have complex structures.The differences in the structure of polysaccharides, such as monosaccharide composition, molecular weight, types of glycosidic linkages and other intrinsic physicochemical characteristics, can profoundly influence their anti-diabetic effects [188].Their structural characteristics are closely related to the fermentability of polysaccharides by the gut microbiota [189].The structure-activity relationship of the anti-diabetic effects of polysaccharides through regulating the gut microbiota is summarized in the following section.

Monosaccharide Composition
Monosaccharide composition is an essential factor influencing the anti-diabetic effects of polysaccharides.In general, the hypoglycemic activity of polysaccharides increases with the complexity of monosaccharide composition [190].For example, polysaccharides from four legume species-mung bean, azuki bean, pea and cowpea-with different monosaccharide composition show different hypoglycemic activity.Azuki bean polysaccharides with more complex monosaccharide types, including arabinose, galactose, glucose, xylose, mannose, and galacturonic acid (unique monosaccharide among the four polysaccharides), exhibit the best hypoglycemic effects in STZ-induced diabetic mice.What is more, polysaccharides from mung bean and cowpea with the same monosaccharide type but in different proportions had different hypoglycemic activity, indicating that galactose and glucose might be two important monosaccharides in the anti-diabetic process [175].Similarly, polysaccharides from four Gastrodia elata plants with the same monosaccharide type but in different proportions showed different hypoglycemic activity.G. elata Bl. f. elata polysaccharides possessed the highest hypoglycemic activity with the lowest glucose content and highest xylose content, which indicated that xylose was also an important kind of monosaccharide participating in the anti-diabetic process [191].

Molecular Weight
Molecular weight significantly influences the anti-diabetic and gut microbiota regulation activity of polysaccharides [192].Typically, polysaccharides have stronger anti-diabetic activity when their molecular weight is lower [76].For example, blackberry polysaccharides with lower molecular weight were more easily broken down and consumed by the gut microbiota and exhibited better prebiotic effects [193].Pectin from artichoke and citrus promoted the growth of Bifidobacterium and Lactobacillus, and the enzymatically modified pectin with lower molecular weight showed higher activity [194].Rhamnogalacturonan-I (RG-I)-enriched pectin (WRP) from citrus and its depolymerized fraction (DWRP) with lower molecular weight showed different gut microbiota regulation function.WRP increased the abundance of Ruminococcaceae, while the abundance of prebiotics like Bifidobacterium and Lactobacillus were significantly by DWRP [195].
There are also some polysaccharides with higher or medium molecular weight showing higher hypoglycemic activity.For instance, Chinese yam polysaccharides HSY-I (>50 kDa) and HSY-II (10 to 50 kDa) exhibited hypoglycemic activity by digestion resistance or β-insulin cell repair.HSY-III (<10 kDa) had no hypoglycemic effect [196].Peach gum polysaccharides (PGPs) promoted the growth of Bacteroides and Parabacteroides.PGPs with high molecular weight played the most significant role in the production of SCFAs [197].Konjac glucomannan with medium molecular weight displayed higher hypoglycemic effects and regulated the gut microbiota by modulating gut microbiota composition and improving gut microbiota-related metabolites [145].
Laminaria japonica polysaccharides with higher (HLJP) and lower (LLJP) molecular weight showed different gut microbiota and metabolite regulation function.The function of HLJP focused more on the regulation of SCFAs and metabolites, while that of LLJP focused more on the proliferation probiotic of gut microbiota like Akkermansiaceae [198].
Numerous polysaccharide characteristics, such as viscosity and advanced structure, are correlated with molecular weight.More research is needed to determine the relationship between polysaccharide molecular weight and anti-diabetic activity.

Types of Glycosidic Linkage
The orientation and position of glycosidic bonds affect the anti-diabetic activity of polysaccharides.For example, fucoidan from Ascophyllum nodosum with alternating (1→3) and (1→4)-α-l-fucose showed better α-glucosidase inhibitory activity than that with repeated (1→3)-α-l-fucose from Fucus vesiculosus, while the inhibitory effect on α-amylase was opposite to α-glucosidase [199].The glycosidic linkage also influenced the production of SCFAs.The fermentation of polysaccharides with diglucose α-(1→1) structure is positively related to the production of butyrate and negatively related to the production of acetate.Fermentation of polysaccharides with diglucose β-(1→4) structure produces more butyrate, propionate and butyrate than diglucose β-(1→4) structure [200].As for gut regulation, fucogalactan sulfate from Laminaria japonica, with more branched sugar residues like (1→2, 3, 4)-linked β-D-ManpA and sulfate ester groups, showed better effects in terms of the proliferation of beneficial bacteria and production of SCFAs and other metabolites than mannogluconic acid [201].

Conclusions and Prospects
The gut microbiota is closely related to DM and is becoming a new target in the treatment of DM.In this paper, we discussed the anti-diabetic effects of polysaccharides through regulation of the gut microbiota.Most polysaccharides cannot be digested by the human body directly, but are fermented by the gut microbiota to produce SCFAs and other metabolites.The interaction between polysaccharides and the gut microbiota can result in the formation of different metabolites and remodeling of the microbiota.The proposal of the gut-organ axis also provides a model for the understanding and treatment of human diseases.However, due to the complexity of the gut microbiota and the uncertainty of polysaccharide structure, the exact interaction between polysaccharides and the gut microbiota still needs further study.It is necessary to give a more detailed description of polysaccharide structure so that we can gain a clearer picture of how polysaccharides influence the gut microbiota and improve DM.The regulatory functions of the gut microbiota, including the metabolism process and the metabolites themselves and their interactions with other organs or tissue types, also need further study.
sources of polysaccharides and the structure of some examples are shown in Figure 1.The anti-diabetic effects of polysaccharides from different sources and their regulation effects on the gut microbiota are summarized in the following.

Figure 1 .
Figure 1.Sources of polysaccharides and the structures of some examples [48-55].

Figure 1 .
Figure 1.Sources of polysaccharides and the structures of some examples [48-55].

Pharmaceuticals 2024 , 27 Figure 2 .
Figure 2. Mechanisms of polysaccharides: (a) repairing gut barrier, (b) changing gut microbiota composition and metabolites, (c) regulating anti-inflammatory activity and immune function, and (d) regulating signal pathways.The up-pointing or right-pointing arrow (red or green) means increase, and the down-pointing arrow (red) means decrease.√ means improvement.

Figure 2 .
Figure 2. Mechanisms of polysaccharides: (a) repairing gut barrier, (b) changing gut microbiota composition and metabolites, (c) regulating anti-inflammatory activity and immune function, and (d) regulating signal pathways.The up-pointing or right-pointing arrow (red or green) means increase, and the down-pointing arrow (red) means decrease.√means improvement.

Table 1 .
Interactions between plant polysaccharides and the gut microbiota in diabetic mellitus.

Table 2 .
Interactions between fungi polysaccharides and gut microbiota in diabetic mellitus.

Table 3 .
Interactions between marine polysaccharides and gut microbiota in diabetic mellitus.