BPD removes approximately three-quarters of the stomach to produce both restriction of food intake and reduction of acid output. Leaving enough upper stomach is important to maintain proper nutrition. The small intestine is then divided with one end attached to the stomach pouch to create what is called an "alimentary limb." All the food moves through this segment, however, not much is absorbed. The bile and pancreatic juices move through the biliopancreatic limb, which is connected to the side of the intestine close to the end. This supplies digestive juices in the section of the intestine now called the common limb. The surgeon is able to vary the length of the common limb to regulate the amount of absorption of protein, fat and fat-soluble vitamins.

Biliopancreatic Diversion with "Duodenal Switch"
This procedure is a variation of BPD in which stomach removal is restricted to the outer margin, leaving a sleeve of stomach with the pylorus and the beginning of the duodenum at its end. The duodenum, the first portion of the small intestine, is divided so that pancreatic and bile drainage is bypassed. The near end of the alimentary limb is then attached to the beginning of the duodenum, while the common limb is created in the same way as described above.
 
Advantages

• These operations often result in a high degree of patient satisfaction because patients are able to eat larger meals than with a purely restrictive or standard Roux-en-Y gastric bypass procedure.
• These procedures can produce the greatest excess weight loss because they provide the highest levels of malabsorption. In one study of 125 patients, excess weight loss of 74% was achieved at one year, 78% at two years, 81% at three years, 84% at four years, and 91% at five years.
• Long-term maintenance of excess body weight loss can be successful if the patient adheres to a straightforward dietary, supplement, exercise and behavioral program.

Risks

• For all malabsorption procedures there is a period of intestinal adaptation when bowel movements can be very liquid and frequent. This condition may lessen over time, but may be a permanent lifelong occurrence.
• Abdominal bloating and malodorous stool or gas may occur.
• Close lifelong monitoring for protein malnutrition, anemia and bone disease is recommended.
• Lifelong vitamin supplements are required. It has been generally observed that if eating and vitamin supplement instructions are not rigorously followed, at least twenty-five percent of patients will develop problems that require treatment.
• Changes to the intestinal structure can result in the increased risk of gallstone formation and the need for removal of the gallbladder.
• Re-routing of bile, pancreatic and other digestive juices beyond the stomach can cause intestinal irritation and ulcers.