Bariatric surgery helps obese people lose weight and is used only after diet and exercise has not resulted in long-term weight loss. The surgery makes the stomach area smaller, so it fills up with less food. Sometimes surgery also changes digestion, so less food is absorbed.
After surgery, the person will not be able to eat very much at one time and will get full quickly. Stomach pain, vomiting, or diarrhea may occur if the person eats too much at one time or if the person eats foods that are high in fat or sugar. Getting good nutrition is important, because less food can be eaten and in some cases less nutrition is absorbed from that food.
WebMD Medical Reference from Healthwise