In bariatric surgery, or weight-loss surgery the capacity of your stomach is reduced and/or the small intestine is shortened. As a result the capacity of your stomach is significantly reduced and your intestine absorbs fewer calories. Decrease in capacity of your stomach makes you feel full after eating significantly less at a meal (about 90% less).
The four main types of weight-loss surgery are:
Gastric bypass (GBP): In this surgery a large section of your stomach is sealed off. This significantly reduces the capacity of your stomach. The remaining part of the stomach is attached to the middle of your small intestine. Hence the part of the intestine which absorbs most calories (duodenum) is bypassed. It is the most effective type of weight-loss surgery.
Adjustable gastric band (AGB): In this surgery an inflatable band is placed around the upper part of your stomach. The band is gradually tightened over a period of few months. This divides your stomach two parts---a smaller upper pouch and a larger lower pouch. The narrow passage between the smaller upper pouch and the larger lower pouch limits the amount of food you can eat. Gastric band surgery is simpler and has fewer complications as compared to gastric bypass surgery. But it is much less effective than gastric bypass surgery.
Vertical-banded gastroplasty (VBG): In Vertical-banded gastroplasty or stomach stapling, your stomach is divided into a smaller upper pouch and a larger lower pouch. The small upper pouch limits the amount of food you can eat and your total calorie intake. However VBG is not adjustable like adjustable gastric band
Biliopancreatic diversion with duodenal switch (BPDDS): This procedure is done in extremely obese people (with a BMI of 50 or greater). A large part of the stomach is removed and the small intestine is rerouted to let the pancreatic juices enter the colon rather than the small intestine (duodenum). After this surgery your risk of nutritional deficiencies is increased.