Recent evidence has suggested that the gut bacteria mediates a number of the beneficial effects of bariatric surgery. Small studies have demonstrated changes in the composition and diversity of the gut microbiota after RYGB and SG in humans. One study also confirmed long-term microbial changes for RYGB. However, comparative trials have been small (less than 15 participants per treatment group) and important differences between specific bacterial populations have not been well elucidated. Furthermore, no human study has examined the differences in bacterial composition following RYGB and SG in relation to their metabolic consequences.