In order to understand how bariatric surgery results in weight loss, it is necessary to understand how food is digested (Figure 1) After swallowing, food enters the stomach, which acts to hold the food and then allow small amounts of the food to pass further into the digestive tract. The volume of the stomach is usually between 600 - 1000 cc (20 - 30 oz.)

In the first part of the small bowel (duodenum), food comes into contact with bile, secreted by the liver as well as enzymes from the pancreas. These secretions help in the digestion and absorption of food. The small bowel is where most of the absorption of food occurs and may reach a length of 6 - 7 meters (over 20 feet). The proximal (closest to the mouth) two-fifths of the small bowel is called the jejunum and the distal (farthest from the mouth) three-fifths is called the ileum

Most bariatric procedures work by two methods - a restrictive component and a malabsorptive component

  Restrictive component - a portion of the stomach may be removed or bypassed so as to reduce the volume of the stomach. Thus, only a limited amount of food can be eaten prior to getting full

  Malabsorptive component - Bile and pancreatic secretions, which are necessary for digestion of food, are directed away from the food. These secretions reach the food several yards down the length of the small bowel, thus delaying and causing incomplete digestion and absorption of the food 

Figure 1 - Anatomy of the gastrointestinal (stomach and bowel) tract. Note that digestive juices from the pancreas and liver enter the small bowel at the duodenum. The proximal 2/5 of the small bowel is called the jejunum. The distal 3/5 is called the ileum. The small bowel empties into the colon (large bowel).

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