The Gastric Bypass is the mixed Bariatric surgical procedure that is most frequently performed for the surgical treatment of obesity.

Mason first performed this procedure in 1966. The technique has undergone several modifications. A gastrectomy is no longer performed and instead the stomach is divided creating a small pouch and leaving the rest of the stomach isolated (remnant stomach). Later the small intestine is cut at the jejunum dividing the digestive system in a bilio-pancreatic limb (transporting gastric fluid, bile, and pancreatic fluid) and alimentary limb (transporting the ingested food from the new gastric pouch to the small intestine) both limbs are then connected, forming a common limb where the nutrients begin to be absorbed.

The length of the common limb modifies the absorption of nutrients, therefore the shorter the common limb is the greater the mal absorption.