How the Gastric Bypass Works

For the Gastric Bypass, the stomach and intestine are cut in the manner shown in Diagram #1, below.

The lower part of the intestine (darker colored in the diagram) is moved up and attached to the small upper portion of the stomach, labeled “P” for pouch.

In the body this move is only four or five inches, as these organs lie very close together inside our abdomen.

The upper portion of the intestine is reattached to the lower portion of the intestine approximately 100cm downstream from where it was originally attached.

There is roughly 400cm (12 feet) of small intestine, and 200cm (6 feet) of large intestine in our bodies. All of this intestine is within our abdominal cavity, curled around itself in an organized way so that it rarely gets tangled up. When surgeons do cancer operations, they routinely cut out a section of intestine, and then attach the two cut ends back together. A similar technique is used to reattach the stomach and intestine for the gastric bypass.

As you can see below, a gastric bypass results in bypassing most of the stomach, and a short portion of the intestine.

The stomach functions mainly to grind up, dissolve, and digest the food we eat.

Most nutrition is absorbed in the intestine, all 400 cm of it, and some nutrition is absorbed in the large intestine. Very little nutrition is absorbed in the stomach.

In the final diagram, below, you can see the stomach and intestine reattached in final form.

Food goes first into the small stomach, called a pouch, “P,” and then travels immediately into the intestine where absorption of nutrition takes place.

Since the small stomach pouch cannot do a very good job of grinding up, dissolving and digesting food, patients with this operation must chew food well. The enzymes people have in their mouth are extremely effective at digesting food, so adequate chewing will improve nutrition.

The shaded portion of the esophagus, the stomach, and the intestine is where the food you eat travels. The blue arrows indicate food, and the green arrows show that acid and enzymes from the stomach, as well as bile from your liver, and pancreatic juices from your pancreas, all still travel through the bypassed stomach and intestine, and then mix back in with the food you have eaten. The fluids coming from your bypassed stomach and intestine, liver and pancreas are all needed for adequate digestion of nutrition. The bypassed stomach and intestine continue to live normally, making important fluids for digestion and absorption. If there is a reason to reconnect the stomach and intestine back to the original anatomy, as was present before the gastric bypass, it can be done, and food will generally go back to being digested and absorbed in the way it was before your bariatric surgery. Of course, you will very likely regain some of the weight you lost.

No organs are removed during a standard gastric bypass operation.


One last topic I need to discuss is the phenomenon of dumping. Dumping is very misunderstood by most patients as well as by many physicians. The fact is, dumping is a very useful effect of the gastric bypass. I call dumping an “effect” and not a “side effect” because I consider it to be one of the most useful effects of this operation.

Dumping occurs when certain types of foods, primarily food with concentrated sugar (e.g. ice cream), enters the intestine before it’s been digested by the enzymes present in the stomach and first part of the intestine. Since, as you can see in the above diagram, the stomach and first part of the intestine are bypassed, the intestine that is attached to the gastric pouch will “see” the food before it mixes and digests with the enzymes coming from the pancreas and liver. When the small intestine experiences concentrated sweets before they have been properly digested, the intestine will “dump.”

Dumping describes the phenomenon of the intestine reacting to a caustic substance, in this case undigested sweets or sugars, and trying to flush this caustic substance through. To flush it through it will flood itself with water and wash the contents through to the large intestine. This is often accompanied by a feeling of indigestion or cramping. The large intestine then reacts to all this fluid coming downstream too fast, and if there is too much, it will get rid of this fluid; in other words, you will have diarrhea. Remember, this occurs almost exclusively as a result of eating concentrated sweets.

I consider this a very useful phenomenon. Remember, people who undergo gastric bypass are people who have been battling obesity for years. Eating concentrated sweets is hardly a good weight-control maneuver, so experiencing adverse effects when you eat more than a couple of bites of a sugar-filled dessert is not a bad thing.

In my experience, most patients experience dumping only in the mild form. However, if a person were to eat a large amount of sugar after a gastric bypass, he could develop so much fluid in his intestine that he becomes light headed, and could faint, so in the extreme form this could be dangerous.

The only patients that I’ve had who complain about dumping after gastric bypass are patients that don’t get it. Yes, there is individual variation in how severe dumping is after gastric bypass, with some patients experiencing severe dumping and others not getting it at all. Patients who find they do not get dumping at all have come in to me and complained; they sometimes ask, “How do I keep from eating sweets?” and I tell them they have to avoid sweets on their own.

Historically, gastric dumping existed before the gastric bypass operation. It was observed in patients that had operations for ulcers, or for cancer, where a portion of the stomach was removed, and the stomach and intestine were reconnected with the Roux-en-Y configuration seen above. Dumping also occurs, although rarely, in patients with normal anatomy. When the stomach empties its contents into the intestine too fast, the intestine can react, and dump.

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