Duodenal Switch Procedure
The Duodenal Switch procedure is a restrictive and malabsorptive weight loss surgery. Also, referred to as Gastric Reduction Duodenal Switch (GRDS).
Duodenal Switch Anatomy
1 – Restrictive ComponentApproximately 70% of the stomach is removed along the greater curvature. This is 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, part of stomach, 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. Compared to gastric bypass procedures, this allows for better absorption of nutrients like vitamin B12, calcium, iron and protein. A benefit of removing a portion of the stomach is it 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 the hormone produces, 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 50% of what they did before surgery. The restrictive component of the Duodenal Switch procedure is not reversible.
2 – Malabsorptive ComponentThe intestines are re-routed so that food from the stomach, alimentary limb, and the digestive juices, biliopancreatic limb, that are needed for the absorption of fat and proteins can travel separate paths; this is so they don't mix until they meet up towards the end of the small intestine.
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 move into the large intestine. The common channel is where a DS patient's food, bile and digestive juices mix; the majority of the fat, protein and the associated nutrients are absorbed. Since the common channel makes up such a small portion of the small intestine the dietary starches, fats and complex carbohydrates may 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 must be diligent with the required supplements and have blood work monitored regularly. Also, they must adjust their supplements as needed to maintain normal levels of these vitamins and minerals.
The intestinal switch portion of the Duodenal Switch procedure is fully reversible.
There is no dispute that Duodenal Switch produces the best-sustained weight loss, and has the best resolution rate for most comorbidities of all weight loss surgical procedures. These results cannot be simply explained by the restrictive and the malabsorptive component of DS. The general consensus is: that there are a number of hormonal regulatory changes involving Ghrelin, GLP-1, Enteroglucagon, Insulin, and others hormones (enzymes) that regulate the complex endocrine pathways.
Many 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 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. “Length Measurement in Duodenal Switch” is discussed and explained in detail at http://www.dssurgery.com/procedures/streaming-media.php
Not all surgeons use the "Hess Method", but some surgeon's use a standard measure for the common channel of 100 cm and 150 cm, which is often used.
Ask a surgeon how he/she determines common channel length.
Appendix and Gallbladder
Some surgeons will remove the appendix because there is almost no extra effort that is placed in removal of the appendix, since the small bowel is measured from the junction of the colon and the appendix.
Few surgeons routinely remove the gallbladder because there is a 30-50% 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%.  One other compelling reason why the gallbladder should be removed is because of the inaccessibility of the biliary tree by a specialized endoscopic technique ERCP. The medication that is also prescribed has frequent side effects that limit their use and compliance with patients.
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 stage one, restrictive component, then when the patient has lost some weight and the other health issues clear up then stage two, intestine switch, is performed.
For more information on the Duodenal Switch procedure:
Central Valley Bariatrics Graphical Comparison of Weight Loss Surgery Procedures
Central Valley Bariatrics Outcomes Comparison Chart
Central Valley Bariatrics Videos