Bariatric surgeons first began to recognize the potential for surgical weight loss while performing operations that required the removal of large segments of a patient's stomach and intestine. After performing such weight loss surgeries, doctors noticed that in many cases patients were unable to maintain their pre-surgical weight. With further study, surgeons were able to recommend similar modifications that could be safely used to produce weight loss in morbidly obese patients. Over the last decade these surgical weight loss procedures have been continually refined in order to improve results and minimize risks. Today's bariatric surgeons have access to a substantial body of clinical data to help them determine which weight loss surgeries should be used and why. 

Today, the American Society for Bariatric Surgery describes two basic approaches that weight loss surgery takes to achieve change:

1. Restrictive weight loss procedures that decrease food intake.
2. Malabsorptive weight loss procedures that alter digestion, thus causing the food to be poorly digested and incompletely absorbed so that it is eliminated in the stool.

Restrictive Weight Loss Procedures
The theory behind restrictive weight loss procedures is simple. When you feel full, you are more likely to have reduced feelings of hunger and will no longer feel deprived. The result is that you are likely to eat less. Restrictive weight loss surgery works by reducing the amount of food consumed at one time. It does not, however, interfere with the normal absorption (digestion) of food. In a cooperative and compliant patient, the reduced stomach capacity, along with behavioral changes, can result in consistently lower caloric intake and consistent weight loss. 

During recovery, restrictive weight loss surgery patients must adhere to the strict specific dietary guidelines and restrictions their bariatric surgeon prescribes. When the time comes to resume eating "regular" food, the patient must learn to adapt to a new way of eating. At each meal, they are restricted to consuming approximately 1/2 to a full cup of food before feeling uncomfortably full. Patients who see the best results from a restrictive weight loss procedure are those who learn to eat slowly, eat less, and avoid drinking too many fluids, particularly carbonated beverages. The effectiveness of a restrictive procedure is reduced by constant snacking or by drinking high-calorie, high-fat liquids. Failure to achieve the expected level of weight loss is usually the result of a restrictive weight loss surgery patient failing to comply with the recommended dietary and behavior modifications, such as increased exercise and regular support group attendance.

Malabsorptive Weight Loss Procedures
Malabsorptive weight loss procedures were developed to work in conjunction with restrictive procedures. Some of these weight loss surgery techniques involve a bypass of the small intestine (gastric bypass surgical procedure), thus limiting the absorption of calories. On balance, malabsorptive weight loss procedures or malabsorptive/restrictive procedures have resulted in an overall increase in the loss of excess weight. The risk of complications and side effects generally increases with the lengthening of the small intestine bypass. You and your bariatric surgeon must determine the risks and benefits over your lifetime with the type of weight loss surgery you choose 

Basically, weight loss surgery procedures fall into three categories:

• Restrictive weight loss procedures make the stomach smaller to limit the amount of food intake.
• Malabsorptive weight loss procedures reduce the amount of intestine that comes in contact with food so that the body absorbs fewer calories.
• Combination operations take advantage of both restriction and malabsorption.

Gastroinestinal Tract
To better understand how weight loss surgery works, it is important to understand how your gastrointestinal tract functions. As the food you consume moves through the tract, various digestive juices and enzymes are introduced at specific stages that allow absorption of nutrients. Food material that is not absorbed is then prepared for elimination. A simplified description of the gastrointestinal tract appears below. Your Bariatric surgeon can provide a more detailed description to help you better understand how weight loss surgery works. 

1. The esophagus is a long muscular tube, which moves food from the mouth to the stomach.
2. The abdomen contains all of the digestive organs.
3. The stomach, situated at the top of the abdomen, normally holds just over 3 pints (about 1500 ml) of food from a single meal. Here the food is mixed with an acid that is produced to assist in digestion. In the stomach, acid and other digestive juices are added to the ingested food to facilitate breakdown of complex proteins, fats and carbohydrates into small, more absorbable units.
4. A valve at the entrance to the stomach from the esophagus allows the food to enter while keeping the acid-laden food from "refluxing" back into the esophagus, causing damage and pain.
5. The pylorus is a small round muscle located at the outlet of the stomach and the entrance to the duodenum (the first section of the small intestine). It closes the stomach outlet while food is being digested into a smaller, more easily absorbed form. When food is properly digested, the pylorus opens and allows the contents of the stomach into the duodenum.
6. The small intestine is about 15 to 20 feet long (4.5 to 6 meters) and is where the majority of the absorption of the nutrients from food takes place. The small intestine is made up of three sections: the duodenum, the jejunum and the ileum.
7. The duodenum is the first section of the small intestine and is where the food is mixed with bile produced by the liver and with other juices from the pancreas. This is where much of the iron and calcium is absorbed.
8. The jejunum is the middle part of the small intestine extending from the duodenum to the ileum; it is responsible for digestion.
9. The last segment of the intestine, the ileum, is where the absorption of fat-soluble vitamins A, D, E and K and other nutrients are absorbed.
10. Another valve separates the small and large intestines to keep bacteria-laden colon contents from coming back into the small intestine.
11. In the large intestines, excess fluids are absorbed and a firm stool is formed.