Why do you leave part of the stomach behind during a gastric bypass?

During a gastric bypass, the lower (distal) larger part of the stomach is left in place because of the stomach still produces acid and pepsin to help digestion of the food. These digestive juices travel down the duodenum and into the jejunum to mix with food. Removing the lower part of the stomach raises the possibility of complications without proven benefit

In rare cases, if for any reason a patient needs to have the operation reversed, the stomach is still there and can be hooked up again. This remnant stomach can also be used to help feeding if an unrelated serious medical problem should occur.