Gastric bypass operations (eg.Roux-en-Y, biliopancreatic diversion, duodenal switch or fobi pouch) go further. Typically they involve a 2 stage process. First, the surgeon reduces the sizes of the stomach. Second, the first part of the small intestine is bypassed. This causes food to pass much more rapidly through the digestive tract and significantly reduces the amount of nutrients and calories that can be absorbed, which is why this type of operation is referred to as malabsorptive surgery. The point is, even if patients overeat, they will absorb fewer calories. This is why gastric bypass operations typically lead to greater weight loss than gastric stapling or banding.