Background

Changes of gut microbiome (GM) including low microbial gene richness (MGR) combined with compositional and functional alterations can promote obesity. Roux-en-Y gastric bypass (RYGB) is an effective procedure for achieving major and sustained weight loss and metabolic improvement in severe or morbid obesity. This review presents the evolution of GM after RYGB, one of the most widely used bariatric surgery interventions.

Methods

A systematic search of literature was conducted using the search terms severe obesity, morbid obesity, GM, dysbiosis, RYGB, diet, prebiotics, probiotics, and weight loss.

Results

Subjects with severe or morbid obesity have dysbiosis with important decrease in MGR. Low MGR is strongly correlated with high adiposity, inflammation, insulin resistance, and type 2 diabetes. Through multiple mechanisms including reduced stomach size, changes in gastric pH, alteration in bile acid metabolism, modifications of gut hormones, and malabsorption, RYGB significantly reshapes GM (e.g., increase in MGR, increase in Bacteroidetes and Proteobacteria phyla, and decrease in Firmicutes phylum). Concomitant diet and medications can impact GM outcome after RYGB. GM improvements caused by RYGB influence the extent of weight loss. There is no full correction of MGR with RYGB, even after 5 years, and additional treatments with diet, prebiotics, and probiotics may be necessary to optimize the results.

Conclusions

Severe or morbid obesity is frequently associated with low MGR and compositional and functional alterations of GM. RYGB significantly improves GM richness, composition, and functionality, and promotes a drastic weight loss. However, MGR may remain permanently low after RYGB requiring additional medical treatments.