How does gastric surgery promote weight loss?

Gastrointestinal surgery for obesity, also called bariatric surgery, alters the digestive process. The operations promote weight loss by closing off parts of the stomach to make it smaller. Operations that only reduce stomach size are known as restrictive operations because they restrict the amount of food the stomach can hold. Some operations combine stomach restriction with a partial bypass of the small intestine. These procedures create a direct connection from the stomach to the lower segment of the small intestine, literally bypassing portions of the digestive tract that absorb calories and nutrients. These are known as malabsorptive operations.