In this procedure, stapling creates a small (15 to 20cc) stomach pouch. The remainder of the stomach is not removed, but is completely stapled shut and divided from the stomach pouch. The outlet from this newly formed pouch empties directly into the lower portion of the jejunum, thus bypassing calorie absorption. This is done by dividing the small intestine just beyond the duodenum for the purpose of bringing it up and constructing a connection with the newly formed stomach pouch. The other end is connected into the side of the Roux limb of the intestine creating the "Y" shape that gives the technique its name.
The length of either segment of the intestine can be increased to produce lower or higher levels of malabsorption.
In recent years, better clinical understanding of procedures combining restrictive and malabsorptive approaches has increased the choices of effective weight loss surgery for thousands of patients. By adding malabsorption, food is delayed in mixing with bile and pancreatic juices that aid in the absorption of nutrients. The result is an early sense of fullness, combined with a sense of satisfaction that reduces the desire to eat.
In addition to these mechanisms, there is a growing body of evidence pointing to substantial neurohormonal changes following gastric bypass surgery, that directly affect metabolism, insulin secretion and resistance, satiety and hunger. These factors explain the terrific efficacy of gastric bypass in bringing so many co-morbid conditions to resolution is a relatively short time period.