Gastric bypass surgery is an operation that shrinks the stomach to reduce the calories a patient takes and it makes the patient lose weight. The stomach is disconnected on the top part and made a small porch first. Then, a loop of the small intestine is brought up and connected to this porch. In that way, the surgeon reroutes the GI (gastrointestinal) tract. Consequently, It is known that surgeons bypass about 95% of the stomach in gastric bypass surgery.