Duodenal Switch Procedure
The Duodenal Switch procedure is a restrictive and malabsorptive weight loss surgery also referred to as Gastric Reduction Duodenal Switch (GRDS).
Stage 1 - Restrictive
Approximately 70% of the stomach is removed along the greater curvature, also called a vertical sleeve gastrectomy (VSG). The remaining stomach is fully functioning, banana shaped and about 3 - 5 oz in size which restricts the amount you can consume. The pylorus continues to control the stomach emptying into the small intestine; as a result patients do not experience "dumping". The upper portion of the duodenum remains in use; food digests to an absorbable consistency in the stomach before moving into the small intestine. This allows for better absorption of nutrients like vitamin B12, calcium, iron and protein when compared to gastric bypass procedures.
A benefit of removing a portion of the stomach is that it also greatly reduces the amount of ghrelin producing tissue and amount of acid in the stomach. Ghrelin is the "hunger hormone" and by reducing the amount of the hormone produced the appetite is suppressed.
The stomach will stretch over time; 9-12 months post-op it will eventually double in size and patients will be able to consume approximately 60% of what they did before surgery.
The restrictive component of the Duodenal Switch procedure is not reversible.
Stage 2 - Malabsorptive
The intestines are switched so that food from the stomach and the digestive juices travel separate paths and don't mix until they meet up towards the end of the small intestine.
The alimentary limb carries the food.
The biliopancreatic limb carries the bile and digestive juices.
The common channel, also known as the common tract or common limb, is the point from where the alimentary and biliopancreatic limbs meet in the small intestine to where they move into the large intestine. The common channel is where a DS patient's food, bile and digestive juices mix and nutrients are absorbed. Since the common channel makes up such a small portion of the small intestine dietary starches, fats and complex carbohydrates are not fully absorbed.
The most commonly quoted absorption percentages following DS are approximately 20% of fats, 60% of protein, 60% of complex carbohydrates but 100% of simple carbohydrates. The 2005 study by Gagner et al.  demonstrated that DS surgery decreased fat absorption by 81%. The Bariatric Nutrition: Suggestions for the Surgical Weight Loss Patient review reports information from a study by Slater et al. stating that "After BPD/DS procedures, the amount of protein should be increased by ~ 30% to accommodate for malabsorption, making the average protein requirement for these patients approximately 90 g/d." 
The malabsorption of fats interferes with absorption of the fat-soluble vitamins A, D, E & K. As a result all Duodenal Switch patients are required to take vitamin and mineral supplements for life. Patients who are diligent with the required supplements, have their blood work monitored regularly and adjust their supplements as needed maintain normal levels of these vitamins and minerals.
The intestinal switch portion of the Duodenal Switch procedure is fully reversible.
The following information on the metabolic effect of the Duodenal Switch procedure was provided with permission from Dr. John Husted.
In addition to the effect of dietary restriction and malabsorption, Duodenal Switch has a metabolic effect to affect weight loss and improvement in health as well. The portion that food passes through - the alimentary limb - has the ability to absorb protein and sugars. This portion of intestine also has the ability to secrete a hormone - GLP-1, or Enteroglucagon - in the presence of undigested food. Since this portion of intestine is presented to undigested food earlier on as a result of the anatomic re-arrangement induced by Duodenal Switch, secretion of GLP-1 is enhanced. Enteroglucogan (GLP-1) has the effect of suppressing the secretion of insulin in response to a carbohydrate meal, resulting in a lesser amount of ingested carbohydrates being converted to body fat.
The portion of intestine that is bypassed holds an important role as well. Enterogastrone is a hormone that is secreted by the upstream small intestine when food passes through it. This hormone has the effect of converting food to fat. When the upstream portion of the intestine is bypassed - as is the case with Duodenal Switch - enterogastrone secretion is suppressed. The effect of this bypass is that the patient's body after Duodenal Switch has less of a tendency to convert food to fat.
The metabolic effect of Duodenal Switch surgery is the reason that some patients with type 2 diabetes leave the hospital after their DS surgery with reduced needs of insulin or oral hypoglycemic medicine. It also explains why there is a 99% cure rate for type 2 diabetes following the Duodenal Switch procedure. 
You will often hear Duodenal Switch procedure patients using the term "Hess Method"  when discussing common channel length. Dr. Hess calculated the length of the alimentary limb by multiplying the total small bowel length by 40%. The remaining 60% of intestine carries the digestive juices through the biliopancreatic limb. The length of the common channel is approximately 10% of the total length of the small bowel. "Hess Method" refers to following Dr. Hess's calculation for determining the limb lengths and common channel length but often other factors are taken into consideration; like the patient's age, weight, BMI and goals. Each patient has a different length of common channel and alimentary loop designed to achieve the best results.
Not all surgeons use the "Hess Method". Some surgeon's instead use a standard measure for the common channel, 100 cm and 150 cm are often used.
Ask your surgeon how he/she determines common channel length.
Appendix and Gallbladder
The choice to remove the appendix and gallbladder during the Duodenal Switch procedure is at each surgeon's discretion. Some surgeons will not remove the appendix or a healthy gallbladder. Other surgeons routinely remove the gallbladder because there is a 30% chance of gallstones forming after rapid weight loss. [56-57] For patients whose gallbladder is not removed often medication is prescribed to help reduce the instance of gallstones from 30% to 2%.  If the surgeon chooses, the appendix is removed to avoid future confusion of questionable abdominal symptoms of appendicitis.
Staged Duodenal Switch
Duodenal Switch is sometimes performed in two stages when the surgeon determines it is too risky for the patient to undergo the whole procedure at once. This is usually a result of a patient's age, BMI, comorbidities or for a more difficult revision surgery. The surgeon will perform the stage one, restrictive component, first and once the patient has lost some weight and the other health issues clear up the stage two, intestine switch, is performed.
For more information on the Duodenal Switch procedure:
Central Valley Baratrics Graphical Comparison of Weight Loss Surgery Procedures
Central Valley Baratrics Outcomes Comparison Chart
Central Valley Baratrics Videos
Dr. Hess' Patient Brochure
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