The gut microbiota of Mongolian hosts has distinctive characteristics due to their meat-and dairy-oriented daily diets and unique genotype.The aim of the present study was to investigate the effect of switching from the typical high protein and fat Mongolian diets to carbohydrate-rich meals composed principally of wheat,rice and naked oats on the host gut microbiota within three weeks.Our study took the advantage of the long sequence reads produced by the PacBio single molecule real-time sequencing technology (SMRT) to enable the profiling of subjects gut microbiota communities along the diet intervention to the species precision.We found that the bacterial richness and diversity decreased apparently along the diet intervention.During the diet intervention,the gut microbiota composition displayed no significant difference at phylum level (with major phyla of Firmicutes,Bacteroidetes,Tenericutes and Proteobacteria).The relative abundances of some genera such as Bacteroidetes,Faecalibacterium,Roseburia,Alistipes,Streptococcus,and Oscillospira were significantly altered after the diet switching started.Notably,significant changes were also observed in the proportions of the species Bacteroides dorei,Bacteroides fragilis,Bacteroides thetaiotaomicron,Ruminococcus albus,Ruminococcus faecis,Roseburia faecis and Eubacterium ventriosum.These results have demonstrated that diet and host gut microbiota are closely linked.