There are two basic types of bariatric surgeries for weight loss: restriction procedures and malabsorption procedures. Restriction procedures reduce the size of the stomach through the use of a gastric band, staples, or both, and do not interfere with the normal digestion process. Malabsorption procedures, on the other hand, reduce the size of the stomach and bypass duodenum, the first segment of the small intestine, and sometimes the whole of the jejunum (the mid-section of the intestines). Some bariatric procedures combine the two.

The Roux-en-Y gastric bypass surgery is widely accepted as the most effective surgical weight loss treatment available. During this procedure, the stomach is divided into two sections, reducing the size of the new pouch 90 percent, from approximately two quarts to one or two ounces. This drastic reduction limits the new stomach pouchs ability to hold food, causing the patient to feel full after eating only a small amount of food. This also causes the food to bypass part of the digestive system, reducing the amount of calories the body absorbs.

A more extensive gastric bypass procedure calls for the removal of portions of the stomach. A small pouch remains, which is then is connected directly to the final segment of the small intestine, completely bypassing the duodenum and jejunum. Although this surgery can result in substantial weight loss, it is seldom used due to the high risk of nutritional deficiencies.

Other options include adjustable gastric banding, where the stomach is encircled with an inflatable band allows only a small portion of the stomach to be used for holding food. Patients typically achieve less weight loss with this procedure because no intestine is bypassed and there is no malabsorption, but the absence of stapling makes it the least invasive and lowers the risks involved.