In a Roux-en-Y gastric bypass, the surgeon creates a small pouch at the top of the stomach. The pouch is the only part of the stomach that receives food. This greatly limits the amount that you can comfortably eat and drink at one time.
This new pouch also produces less of the appetite regulating hormones such as ghrelin, which may lessen your desire to eat.
The small intestine is then cut a short distance below the main stomach and connected to the new pouch. Food flows directly from the pouch into this part of the intestine.
The main part of the stomach, however, continues to make digestive juices. The portion of the intestine still attached to the main stomach is reattached farther down. This allows the digestive juices to flow to the small intestine.
Because food now bypasses a portion of the small intestine, fewer nutrients and calories are absorbed.