The concept of surgical treatment for weight loss is rooted in cancer treatment. Surgeons would 
remove large portions of the small intestine or stomach to remove tumors or cancer cells; the patients would consequently lose great amounts of body fat and weight. Doctors then began treating obesity with this procedure 40 years ago. Patients would lose pounds of body fat due to malabsorption. The idea was that the obesity patients could eat large amounts of food without gaining weight because the food would be poorly digested and move along the digestive track too quickly for caloric extraction. However, this leads to severe malnutrition and, in some cases, death. Therefore, the operation has since been altered and modified due to technological advancements and medicinal knowledge to two main kinds of weight loss surgery: Restrictive and malabsorptive surgery.
remove large portions of the small intestine or stomach to remove tumors or cancer cells; the patients would consequently lose great amounts of body fat and weight. Doctors then began treating obesity with this procedure 40 years ago. Patients would lose pounds of body fat due to malabsorption. The idea was that the obesity patients could eat large amounts of food without gaining weight because the food would be poorly digested and move along the digestive track too quickly for caloric extraction. However, this leads to severe malnutrition and, in some cases, death. Therefore, the operation has since been altered and modified due to technological advancements and medicinal knowledge to two main kinds of weight loss surgery: Restrictive and malabsorptive surgery.Restrictive Surgery: This procedure consists of making the stomach smaller by removing a section of the stomach. This limits the amount of food that the stomach can hold, causing you to feel full much quicker. Extreme care must be taken when eating; since such small amounts of food will fit into the stomach, foods with the necessary vitamins, nutrients, and supplements is crucial!
Malabsorptive Surgery: Since most of the digestion happens in the small intestine, this surgery consists of shortening the length of the small intestine and/or changing the location in which the intestine connects to the stomach. This limits the amount of food that is completely absorbed into the body.
In some cases, both of these surgeries may be done in order to maximize results. The results are obtained by eating less food, because your stomach is smaller, and/or the food doesn’t stay in the digestive track long enough to be absorbed into the body.
Sources: Web MD