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The gastric bypass is predicated on two concepts: restriction and malabsorption. The concept of "restriction" is relatively straight forward. The stomach is divided with specialized surgical staplers disconnecting the upper 5% from the lower 95% of stomach. The small portion is now referred to as the "pouch"; the large portion is now referred to as the "remnant". The remnant is not routinely removed and remains in the same anatomical position that you were born with. The pouch that has been created now acts as the reservoir for ingested foods. The amount of food that the stomach can now hold has been greatly reduced. In other words, there is a restriction in the volume that your new pouch can accommodate. The actual volume of the pouch varies from surgeon to surgeon. On average, the pouch is described as being able to hold 30 cc or one ounce, approximately the size of a small egg or Dixie cup. Consider trying to push a single slice of pizza into a Dixie cup. The concept of malabsorption is slightly more complicated. The idea is explained by first grasping the basics of human gastrointestinal anatomy. The weight loss attributed to the bypass portion of the procedure can then be understood.

Normal Anatomy

When food is swallowed it traverses the esophagus before entering the stomach. The stomach acts as a reservoir and churn, breaking the larger pieces up into smaller particles. The food particles are then released from the stomach in small, controlled quantities into the first portion of the small intestine (duodenum). The duodenum is the "business end" of the small intestines even though it only compromises a very small length of the total small intestine. In the duodenum, enzymes (chemicals necessary for digestion) are secreted by the pancreas and the gallbladder that are necessary to further breakdown food particles so that they can be absorbed as the food travels the length of the small intestine. On average, the small bowel is 18-25 feet long (5-7meters). Just like people have different length fingers and toes, we all have slightly longer or shorter small intestine than the next person. As food traverses the small intestine it is absorbed into the blood stream to be used as energy, building blocks, etc. At this point the ingested food has been reduced to a liquid. The large intestine (colon), 4-7 feet long, primarily acts as a sponge, reabsorbing 90-95% of the water that remains behind. The end of line is the rectum/anus where all that cannot be used is passed out from the body.

Gastric Bypass Anatomy

The Roux-en-Y changes the gastrointestinal anatomy. The small intestine is divided one to three feet (40-90 cm) beyond the first portion of small intestine (duodenum). The side that is downstream (distal segment) is measured out an additional 4-6 feet (100-200 cm). This is called the "Roux limb" and is reconnected to the gastric pouch. In other words, food will now travel from your esophagus into your gastric pouch and then into your Roux limb. The upstream portion of small intestine is now referred to as the "biliopancreatic limb" because it carries the bile and pancreatic enzymes necessary for digestion. The end of the biliopancreatic limb is connected to the end of the Roux limb reestablishing small bowel continuity. The straight shot from the reconnected small intestine to the colon is now referred to as the "common channel". That's because this segment of small intestine will be a common mixing site for the enzymes of digestion and the food that is ingested. The end result is a "Y" shaped set of small intestinal connections.


So what is the significance of the new plumbing? The bottom line is the small intestine has been rerouted, and so has the road to be traveled by ingested food. The business center (the duodenum) continues to see the necessary enzymes secreted by the pancreas and gallbladder; however, these enzymes are not in contact with food until they reach the "common channel". Only where the food and enzymes mix can absorption of nutrients (calories) take place. This is the principle of "malabsorption"; the intestinal system has been short-circuited and therefore does not allow the complete absorption of all ingested nutrients and calories. The end result is that there is incomplete absorption of the calories that you eat.

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