This operation, the most frequently performed in the United States, combines the benefits of restriction and malabsorption. In this procedure a small part of the upper stomach is isolated to provide a small chamber. Food travels from this pouch into a segment of small intestine; digestive enzymes move through a different area of the small intestine. The food and digestive enzymes meet together farther down, bypassing about 15-20% of the small intestine.

Read more about gastric bypass surgery.

The small intestine is the area that absorbs the majority of nutrients -- proteins, carbohydrates, and fats. Malabsorption means to reduce the amount of nutrients taken in by the intestine, in order to get fewer calories from the food that is eaten. This operation reduces the stomach in order to reduce ulcers. Food travels into a segment of small intestine; digestive enzymes move through a different area of the small intestine. The food and digestive enzymes meet together farther down, bypassing about 90% of the small intestine's area. This operation relies on separating food from the enzymes and juices needed to break it down and absorb it.

Read more about duodenal switch surgery.

This operation is designed to limit the amount or volume of food that a person can eat at one time. When solid food is eaten, it stretches the small compartment of the stomach. This sends a signal of "satisfaction" or "fullness" to the brain. By listening to this signal, one stops eating sooner, which means one eats less food. Therefore the total calories consumed in a day is reduced, thus causing weight loss.

Read more about LapBand surgery.