You probably know it best as “gastric bypass,” but there are actually several types of bariatric surgery. The Roux-en-Y gastric bypass, which is the most well-known, is where a surgeon essentially attaches the very top of the stomach to the small intestines, thereby bypassing the main stomach cavity. It works primarily by controlling how much you can eat. The new, tiny stomach holds less, so patients feel satiated with a much smaller amount of food. Think about how little you want to eat when you’re super full, then imagine if you felt that way after eating half a bowl of cereal. You’d take in way fewer calories.