**4.6 Bariatric surgery**

Bariatric surgery, or Roux-en-Y gastric bypass (RYGBP), is removal of a portion of stomach and re-routing the small intestine to a small stomach pouch. It is performed on people as an efficient tool to treat obesity. After bariatric surgery huge changes occur in the GM, Proteobacteria rises and Firmicutes and Bacteroides lowers, BMI reduces by 15–32%, C-reactive protein decreases and T2DM is attenuated. However, increase in some bacteria are highly significant than the normal levels in lean controls, which means these alterations are linked with GM modification, and not body weight (**Table 2**) [112, 117, 118].
