How Weight Loss Surgery Reduces Weight

The Gastrointestinal 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 gastrointestinal 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 doctor 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. The colon may absorb protein, when necessary.

The two main reasons why surgery works:

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

Restrictive Procedures

The theory 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 restrictive weight loss procedure, the surgeon creates a smaller upper stomach pouch. The pouch has a capacity of approximately 1/2 to 1 oz. (15 to 30 ml) in the lap band or 100-150 cc with the sleeve gastretomy. 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, you must adhere to the strict specific dietary guidelines and restrictions your Bariatric surgeon prescribes. When the time comes to resume eating "regular" food, you must learn to adapt to a new way of eating. At each meal, you 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. Failure to follow these guidelines can result in nausea, dry heaving and heartburn. Over time, the pouch can stretch. 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 patient failing to comply with the recommended dietary and behavior modifications, such as increased exercise and regular support group attendance.

Malabsorptive Procedures that Alter Digestion

It can be said that some of the restrictive approaches discussed above have not always achieved the excess weight loss surgeons and patients anticipated. For this reason, weight loss procedures that alter digestion, known as malabsorptive procedures, were developed to work in conjunction with restrictive approaches. Some of these techniques involve a bypass of the small intestine, thus limiting the absorption of calories. On balance, malabsorptive or malabsorptive/restrictive procedures have resulted in an overall increase in the loss of excess weight. 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 operations fall into three categories: 

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