Effects And Mechanisms Of Gut Microbiota In High-fiber Diet-induced Alleviation Of Type 2 Diabetes

Recent studies have shown that gut microbiota is associated with human health;there are close interactions between host metabolism and immunity and gut microbiota.The dysbiotic gut microbiota may lead to a variety of diseases,such as metabolic diseases.Diet represents the most important factor that affects gut microbiota.High fiber diet has been shown to improve type 2 diabetes mellitus(T2DM)but the mechanism remains elusive.Dietary fiber can enrich the short-chain fatty acid(SCFA)-producing bacteria and display potent regulatory effects on gut microbiota.However,how will the gut microbiota in patients with T2 DM respond to a high-fiber diet and through which mechanisms the improvement of T2 DM is achieved still warrant further studies.In our laboratory,obesity and metabolic dyndrome was significantly improved by regulating the gut microbiota using a diet rich in dietary fiber(based on whole grains,traditional medicinal foods and prebiotics,the WTP diet).To investigate the relationship of gut microbiota with T2 DM,we enrolled T2 DM subjects and randomly assigned them to two groups,the control group(Group U,in which the subjects received acarbose for a total of 3 months)and the intervention group(Group W,in which the subjects received acarbose plus the WTP diet for a total of 3 months).We found improvement in glucose and lipid metabolism of subjects in both groups,with the improvement in group W being more pronounced.GC-MS showed that the fecal acetate levels trended to increase in both groups after intervention;however,the fecal butyrate level was significantly increased only in group W during the intervention.Following the intervention,the secretions of such gut hormones as GLP-1(Glucagon-like protein 1)and PYY(peptide YY)were significantly elevated only in group W.In addition,the two interventions improved the inflammations of subjects in both groups significantly,with the improvement in group W being more pronounced.Besides,we found significantly decreased fecal concentrations of indole and H2 S following the intervention.Dietary fiber shows beneficial effects on host metabolism possibly by regulating gut bacteria and their metabolites.Analysis of the structure of gut microbiota using 16 S rRNA gene sequencing revealed that the diversity of gut microbiota was reduced in both groups after the intervention.There was no significant difference with respect to the structure of gut microbiota of subjects between the two groups prior to intervention;significant changes were noted at 28 days after intervention as compared with the baseline in both groups,and moreover,the structure was not significantly changed between 28 days and 84 days in two groups.During the intervention,significant differences were observed in the structure of gut microbiota among the two groups.Using the Random Forest model,key OTUs with differences were separated for each group both pre-and post-intervention and those in the two groups post-intervention.According to the covariation of OTUs abundance pre-vs.post-intervention,the correlation coefficient of these key OTUs was calculated using the SparCC to establish the network and divide the Co-Abundant Group(CAG).The CAG2 abundance values of OTUs in Bifidobacterium and Lactobacillus were increased after the two interventions,and were negatively correlated with the disordered phenotype of glucose and lipid metabolism.On the other hand,the abundance of CAG16 primarily containing potential opportunistic pathogen Escherichia/Shigella and CAG17 and CAG18 primarily containing the OTUs of Bacteroides were significantly decreased following the two interventions,and were significantly positively correlated with the indicators of glucose and lipid metabolisms.In the feces of subjects in group W following the intervention,the abundance of CAG6 primarily containing OTUs in Roseburia and Lachnospiraceae rich in butyrateproducing bacteria was significantly higher than in group U,which might partly explain the higher level of fecal butyrate in group W than in group U following the intervention.To further investigate the causative relationship between the changes in gut microbiota induced by dietary fiber with the improvement of glycometabolism of T2 DM,we selected one subject from group U and group W,respectively and transplanted their pre-and post-intervention gut micorbiota to germ-free mice.The gut microbiota of recipient mice was close to that of the donor mice,indicating that the gut microbiota of donor mice was effectively transplanted to the germ-free mice.The α-diversity of gut microbiota of mice receiving the postintervention microbiota of the subject in W group(MW-Post group)and those receiving the post-intervention microbiota of the subject in U group(MU-Post group)was significantly lower than in mice receiving baseline microbiota of subjects in W and U group(MW-Pre group and MU-Pre group).The Oral Glucose Tolerance Test(OGTT)showed that Glucose_AUCs in the MW-Post group and the MU-Post group were significantly lower than in the MW-Pre group and the MU-Pre group,and that Glucose_AUC in the MW-Post group was the lowest in all groups.The results of above clinical trials and animal experiments showed that dietary fiber-modulated gut microbiota causatively contributed to the improvement of clinical indicators of patients.Further to that,RT-qPCR was performed to analyze the gene expression of tissue function in gnotobiotic mice involved in glycometabolism regulation together with the correlation analysis with the metabolic indicators of glycometabolism.It showed that the expression of Gcg(encoding proglucagon,the gene from which GLP-1 is derived)in colons of mice in the MW-Post group was significantly lower than in the MW-Pre group,and that the expression of Gcg of all mice was significantly positively correlated with Glucose_AUC,for which it was considered to be associated with the GLP-1 resistance in mice in the MW-Pre group and the significantly higher GLP-1 sensitivity in mice in the MW-Post group than in the MW-Pre group.The expression of Sqr(encoding sulfide: quinone oxidoreductase)in the colon of mice in the MW-Post group tent to be higher than in the MW-Pre group,and the expression of Sqr was negatively correlated with Glucose_AUC,indicating that the gut microbiota of mice in the MW-Post group could promote the oxidative degradation of H2 S in the colon.The expression of Slc2a4(encoding the GLUT4 protein)in the parauterine fat pad of mice in the MW-Post group was significantly higher than in the MW-Pre group,and the expression of Slc2a4 in the para-uterine fat pad was negatively correlated with Glucose_AUC,indicating that the gut microbiota following the intervention of high-fiber diet could improve host glycometabolism by increasing the uptake of blood glucose by adipocytes.Finally,the majority of key OTUs as selected by the Random Forest model were closely correlated with the expressions of Gcg and Sqr in the colon,Slc2a4 in the para-uterine fat pad and Glucose_AUC.To sum up,the current study,through the high-fiber diet intervention of clinical trials and microbiota transplantation experiments for T2 DM,indicated that the intervention could enrich the SCFA-producing bacteria,increase the level of SCFAs in the intestines,induce the intestinal cells to secret more GLP-1,increase the concentration of blood insulin,and reduce the inflammations and H2 S level,thereby improving T2 DM.On the other hand,the microbiota transplantation experiment also showed that dietary fiber may promote the regulation of degradation of H2 S in the colon through the regulation of the gut microbiota and improve host glycometabolism by inducing the uptake of glucose by adipocytes.This study will provide insights into the mechanisms of gut microbiota in the initiation,development and improvement of T2 DM,supporting the improvement of the gut microbiota-targeted dietary intervention.