Physiology of the Duodenal Switch

Anatomy and Physiology of the Duodenal Switch (DS)

March 30, 2016

The most important job of a Duodenal Switch (DS) patient is to make sure that you not only eat well with the proper amounts of protein and fluids but to take the vitally important vitamins and supplements after surgery. This will ensure not only excellent weight loss, reduction of co-morbidities, improvement in your social and mental wellbeing, but maintenance of your overall nutrition. This article is intended to educate you about the anatomy and physiology before and after you have a DS.

Normal Gastrointestinal Anatomy and Physiology


When you eat, food and fluids pass through your mouth, into the esophagus and then empties into your stomach. The stomach is composed of multiple areas which have different jobs. The fundus which is at the very top of the stomach is where gasses that you swallow or that are produced from carbonated drinks collect. It is also where most of your Ghrelin is produced.  Ghrelin is a gastrointestinal hormone which causes hunger.

The next portion of the stomach is the body where most of the acid and some digestive hormones are produced. Digestion of food is started there. The antrum is the last part of the stomach. Gastrin which stimulates acid production is produced here.

Intrinsic factor, which is needed to absorb Vitamin B 12 is produced throughout the stomach. At the end of the stomach, the pylorus controls stomach emptying. It will prevent concentrated foods and fluids from leaving the stomach until they are more dilute, thereby preventing dumping.

The small intestine is broken down into three parts—the duodenum, the jejunum, and the ileum. The first part is the duodenum. Bile from the liver and gallbladder as well as digestive enzymes from the pancreas enter the duodenum. Bile is needed for fat absorption.

The pancreatic enzymes are needed to break down complex foods into smaller components so that they may be absorbed in the small intestine. The bile and pancreatic enzymes are alkaline and neutralize the highly acidic fluid that is produced in the stomach to help prevent injury to the intestine from the acidic fluid. The stomach is resistant to injury from acid in its normal state while the small intestine in not.

The jejunum and ileum are the next two parts of the small intestine. They make up anywhere from about 400 to 700 cm in length. They have millions of small finger-like projections into the lumen which increase the surface area of the intestine to optimize absorption. It is here that almost all of the nutrients from what you eat are absorbed.

The large intestine or colon is the last part of the intestine. Liquefied non-absorbed contents from the small intestine enter the colon. The main job of the colon is to absorb water and electrolytes so that its contents become more solid prior to excretion.

Altered Anatomy and Physiology of the Duodenal Switch (DS)


The lateral part of the stomach is removed leaving a narrow stomach tube which widens a little at the antrum. The fundus is removed reducing the production of Ghrelin. This decreases a patient's appetite and may cause nausea initially. The space for gas accumulation is also eliminated and you will more readily become uncomfortable and burp up this gas.

All of the other parts of the stomach are still present, so that digestion, acid and hormone production as well as intrinsic factor production is maintained. The stomach size is however much smaller and you will get full quite quickly as compared to before. There is also a relative reduction in the production of intrinsic factor and acid which will also adjust digestion of food and absorption of Vitamin B12. The pyloric muscle is preserved and the physiology of emptying of the stomach is maintained.

The duodenum is transected after the pylorus and before the area where bile and pancreatic enzymes enter the duodenum. Some of the absorptions which occur in the duodenum are maintained but as most of the duodenum is bypassed, much of this absorption is limited.

The ileum is transected about 250 cm proximal to the large intestine and anastomosed (connected) to the duodenum allowing ingested food to pass from the stomach to the beginning of the duodenum directly into the ileum, bypassing about 60% of the small bowel.

The biliopancreatic limb which starts as a blind pouch of the duodenum just proximal to where the bile and pancreatic enzymes enter the duodenum then connects to the alimentary limb (food limb) forming the common channel where the bile and pancreatic enzymes start mixing with the food. This occurs usually about 100 cm before the small intestine enters the colon.

As a result, the length of the intestine where absorption can take place is reduced by 60% and the area where fats and complex carbohydrates and some proteins are absorbed is reduced by about 80%. Therefore, some of these foods will pass undigested into the colon. If one eats a diet high in fat, then a significant amount of fat will enter the colon producing looser and more odorous bowel movements. If one eats a diet high in complex carbohydrates, then a significant amount of these will enter the colon. Here, billions of bacteria will metabolize these carbohydrates producing gas which will eventually be expelled.

All in all, the DS is intended to limit your fat and carbohydrate absorption. The combination of intake reduction and reduced absorption is why the DS has the best long-term weight loss of any of the weight loss operations.

Physiology of the Duodenal Switch


David F. Greenbaum, MD, FACS is the past director at LMCBC and currently at Virtua Health Systems, Virtua Memorial Hospital in Mt Holly, NJ. Dr. Greenbaum is a member of the American Society for Metabolic and Bariatric Surgery (ASMBS), a past chairman of the Public and Professional Committee and a member of several other committees at the ASMBS.

Read more articles from Dr. Greenbaum!


Barbara Metcalf RN, CBN, entered bariatrics in 1995 with Dr. Robert A Rabkin and Dr. Ara Keshishian in 2000, where the DS was the primary procedure. She has published in “The Benefits of Exercise with Bariatric Surgery,” pioneered the first “Walk From Obesity” in 1999 and helped to develop the national “Walk From Obesity.” She has been on faculty for ASMBS since 2000 teaching, “Motivating Patients with Exercise,” “Facilitating Group Support Meetings” and “DS Nursing.” She retired in 2013 but continues to volunteer for Drs. Rabkin and Keshishian & online for patients with questions on the DS.