This page contains technical details about how the guts are rearranged to make various distal bypass procedures—the Distal RNY, the Duodenal Switch, and the BPD.  The plot is quite simple—they all start as distal procedures and we rearrange the small bowel the same way for all of them.  It is what we do to the stomach during the operation that makes the difference.

Distal vs. proximal

The length of the bypassed segment determines whether it is called distal or proximal.  If you bypass 10-15 percent of bowel it is proximal. – more than 50 percent, it is distal.

The difference between a proximal and a distal bypass depends on where you cut the small bowel.  As you can see from figure A, if you cut it at point A it is distal, and you cut it at point B it is proximal.  The first step in any distal procedure is the same:

All distal procedures follow the steps described below

the small bowel is cut
First the small bowel is cut 200 centimeters from the colon (or about 40 percent of the length of the small bowel) at point A.

First the small bowel is cut


This leaves two limbs such as the Roux en Y (RNY) surgery. The one limb will be attached to this small bowel.

Here the limb is reattached


Here the limb is reattached—forming a “T”,although we like to say it is a “Y”, where that Y is is called the common channel. In this example the common channel is made about 100 cm long.

Figures A, B, and C represent the steps which are done for the duodenal switch, the BPD, and the distal RNY surgery.  Technically all of these operations are a long limb RNY surgery.  The differences between them is what we do to the stomach.

Duodenal Switch

The most common distal procedure is the duodenal switch. To do the DS you need to do everything in Figures A, B and C.

the long limb Roux en Y Bypass


The first step in the duodenal switch is to make the long limb Roux en Y Bypass (see figures A, B, and C, above).  The difference with the DS is what you do to the stomach.

With the DS we cut a portion of the stomach


With the DS we cut a portion of the stomach and send that to pathology.  We also make a cut an inch from the pylorus.

attachment of the long-limb of the Roux en Y


The old stomach is sent away.  The attachment of the long-limb of the Roux en Y is to the first part of the duodenum.  The other side of the duodenum is sewn closed.

duodenal switch final product


Now you attach everything and this is the final product.

So, as you can see, the duodenal switch is merely a Roux en Y bypass to the first portion of the duodenum.

Bilio-pancreatic Diversion

The bilio-pancreatic diversion has been largely replaced by the duodenal switch.  There are still places that do this operation, so we put it here for completeness.

the first step to making the BPD is to make the long limb RNY bypass


Again the first step to making the BPD is to make the long limb RNY bypass (Figures A, B, and C, above). The difference is that with a BPD, you make the common channel shorter (50 cm instead of 100 cm). The other difference is what you do to the stomach (see below).

cut the stomach in two places

You cut the stomach in two places. The upper stomach is about 250 cc or 8 to 9 ounces.

lower stomach is completely removed

The lower stomach is completely removed and the duodenal stump is sewn over. The RNY will be attached to the large pouch.

Bilio-pancreatic Diversion final product

Unlike the DS, the common channel for the BPD is 50 cm, or 20 inches. Again, the BPD has largely been replaced by its “upgraded” model, the duodenal switch, which allows a more normal digestion with the pylorus in place, doing its job.

The BPD and the standard RNY surgery do appear similar, however the differences are worth noting. The BPD has a 9-ounce pouch and the lower stomach is completely removed. With the standard gastric bypass there is a one-half to one-ounce pouch and no stomach is removed.

Roux en Y Gastric Bypass

divide the stomach into a one ounce upper pouch

To make a RNY surgery we divide the stomach into a one ounce upper pouch.

two parts of the stomach


With the two parts of the stomach we will attach the limb of the RNY to the upper pouch.

final product distal gastric bypass - RNY

This final product is a distal RNY.  Again, the common channel is 100 cm. In this configuration, not only do the duodenal contents but also some of the stomach juices go through the duodenum (but not food).

 Proximal Roux en Y Gastric Bypass

The typical “proximal” gastric surgery has a Roux en Y limb of 50 to 60 centimeters, and it is reconnected about 15 centimeters from the ligament of Treitz (average of 75 cm of bowel not seeing both food and duodenal juices). This means that the common channel, where duodenal contents and stomach contents meet, is about 525 cm. In terms of “malabsorption,” this is essentially none. If you go from a 75 cm bypass to 150 cm there is a noticeable difference in patient outcomes (or a common channel of 450 cm).

Proximal Roux en Y Gastric Bypass


This is a proximal bypass, where the afferent or bioliopancreatic limb is 15 cm and the enteric limb is 60 cm, for a total bypass of 75 cm. The common channel is 525 cm long.

The effect of Malabsorption in bypass surgery

Longer limb bypasses tend to have more weight loss. We call these procedures “malabsorptive,” because there is less intestine to absorb food.  The distal surgeries used today (DS, distal RNY) only produce “moderate malabsorption.”  Why is it that patients with higher BMI’s (>55) tend to lose more weight with distal procedures?  The answer is simply by limiting calories.  By having a shorter common channel, you limit the amount of calories that are absorbed. Primarily, you are limiting the absorption of fat.

This does not mean that higher BMI patients should have distal procedures.  The primary mode of weight loss for all surgeries is restricting the amount of food you can eat.  The small bowel becomes more efficient at digestion as time goes on.  So, no matter how short your common channel is, over the course of years it will absorb more fat and calories.

Proximal RNYDistal RNYDuodenal SwitchBPD
Stomach size30 cc30 cc120-180 cc250 cc
Stomach size in ounces1/2 to 1 ounce1/2 to 1 ounce4-8 ounces8-9 ounces
Common channel500 cm100-400 cm100 cm50-100 cm
Common channel in inches200 inches40 to 160 inches40 inches20-40 inches
Biliopancreatic limb length in cm  (not including duodenum)100 cm or less than 15% of the distance between the Ligament of Trietz and the colonFor some insurance purposes, anything longer than 100 cm bypassed is considered a distal bypass400 cm or about 60% of the distance between the ligament of Trietz and the colon400 cm or about 60% of the distance between the ligament of Trietz and the colon