Conor Magee MD FRCS FRCS(Gen.Surg.)
Specialist Registrar in Oesophago-Gastric Surgery
Phoenix-Health Bariatric Fellow

The Roux-en-Y Gastric Bypass or more simply the "gastric bypass" is the most widely performed bariatric operation in the United States. In terms of complexity it is a significant step up from the gastric band, but shares certain similarities that I will explain. As I sit here writing this article it is quite obvious that explaining how the bypass works is going to be very hard going without the aid of a diagram. Now, Ken assures me that a diagram will be inserted. If not a brief trip to rygb diagram is highly recommended.

Looking at the gastric bypass diagram it is immediately obvious that much more has been done than with the gastric band. There are joins and cuts and bits moved and bits bypassed, but underlying the complicated plumbing are two simple principles:

  1. Restrict portion size (Restriction)
  2. Reduce the amount of nutrients absorbed (Malabsorption)

Portion size is restricted by creating a small stomach pouch, rather like the gastric band, at the bottom end of the gullet. This pouch (about the size of a golf ball) fills quickly with food and makes you feel full. Unlike the gastric band the stomach pouch is completely separated from the rest of the stomach. A special surgical "stapling" instrument is used to cut the stomach and staple it closed.

The second principle of malabsorption is achieved in the following manner. Normally food enters the stomach, is churned up and digestion (the breakdown of food into absorbable nutrients) is started by the action of gastric acid. This pre-digested food enters the duodenum where the real business of digestion starts. The pancreas gland and the gall bladder secrete digestive juices designed to break down fats and proteins that can then be absorbed. The gastric bypass keeps the enzyme rich pancreatic and gall bladder secretions temporarily separated from the food.

We do this by rearranging the bowel. Look at the diagram again. The small stomach pouch will not allow any churning of food and it contains much less acid than the whole stomach. This stops early digestion and absorption.  The food then passes into a loop of bowel we have created and pulled up to the stomach. We call this the "alimentary limb" and it is around 75-150cm in length.

The stomach left behind is still secreting acid, but won't have any food in it because we divided it earlier to make the stomach pouch. The stomach acid will still pass into the duodenum. The pancreas and gall bladder digestive juices are still produced, but because of the re-arranged bowel they are temporarily separated from the food. These secretions travel along the "Bilio-pancreatic" or BP limb. The BP limb is around 75-150cm long. The alimentary and BP limbs then join up to form the common channel, and it is here that food and enzyme fluid mix to allow digestion and absorption to take place. However, the malabsorptive effect is only mild, the small stomach pouch and reduction in food intake is the main mechanism of weight loss.

The gastric bypass is complicated, and don't worry if you have to keep referring back to the diagram. There are many trainee surgeons who have difficulty visualising a Roux-en-Y bypass. The term Roux-en-Y refers to Cesar Roux, a Swiss surgeon, who first described using the bowel in such a way, and also the fact that the bowel looks something like a Y-shape. In fact the Roux-en-Y is one of the most useful techniques in surgery, loved by generations of stomach surgeons!

Note there are some centres that offer a weight loss procedure called the mini-gastric bypass. This is a simpler operation than the Roux-en-Y gastric bypass, but does not have the same extensive scientific evidence to support its use. The mini bypass is not a recommended procedure in the United States or the United Kingdom. It is probably a good idea to check with your surgeon which procedure they actually do, especially if you do not see the term Roux-en-Y mentioned anywhere.

The Roux-en-Y gastric bypass generally takes one to two hours to perform. It is usually done laparoscopically (keyhole) and requires an in-patient stay of around 3 nights. Patients usually recover quickly and can be back to normal within a month.


With two different ways of producing weight loss the gastric bypass is usually quicker and more effective than the gastric band. I would expect to see patients lose 70-80% of their excess weight within two years; the majority of this weight loss tends to happen within the first 12 months (see results table). The bypass relies less on the patient than the gastric band does. It requires less out-patient visits than the gastric band does, and in many ways can be a better guarantee of weight-loss.

The gastric bypass has a number of other advantages; the biggest is in treating diabetes.  If you are diabetic, and most obese patients will have the type 2 form of diabetes; a gastric bypass can cure you. What's more, the diabetes is reversed before weight loss takes place. This is an astounding fact, one that is not widely known by the medical profession. It's worth repeating- if you have type 2 diabetes a gastric bypass operation can cure you, even before you lose weight and in some cases before you leave hospital!! The gastric band can cure diabetes but generally cure rates are lower and take longer. The reason why the gastric bypass works in diabetes is unclear but many scientists believe it is because it alters hormone signalling from the small bowel. We don't know the hormone or hormones responsible but intensive research is being done to answer this question.  Another advantage of the gastric bypass is it causes an odd condition called "dumping". Dumping is not related to functions of the more colonic end of the bowel! Dumping refers to the "dumping" of food from the stomach pouch directly into the bowel without being digested by the digestive juices. In around 80% of patients the body reacts to the dumping of the undigested food, especially sugary food, with a host of unpleasant symptoms- faintness, nausea, abdominal cramps and dizziness. Many patients feel so unwell they need to lie down for hours afterwards. Why is this an advantage, well- for the person whose weakness is sweets or full-sugar soft drinks or chocolate, having the dumping symptoms is a very effective way to stop them making these unwise food choices. For the patient who has not experienced dumping these symptoms may not sound very impressive- but take a brief look at the wlsinfo forums and see how patients actually feel!! Following a bypass patients need to make dietary adjustments similar to those who have a gastric band. Once recovered from surgery, like gastric band patients, the bypass patient should be able to eat a wide variety of foods.

So, in summary the bypass will make you lose more weight than a band, works faster than a band, can cure your diabetes before you leave hospital and will help you to stop eating sweets. Isn't it the perfect operation? Simply put, No. The big drawback is that the bypass carries more risk.


Let's look at the diagram again. Unlike the gastric band the gastric bypass involves cutting the stomach, cutting the bowel and then joining them back together. The bypass has two cuts and two joins. It is essential that these joins heal up in a safe and watertight manner. If they do not then food and digestive juices can leak out causing peritonitis, much like a burst ulcer or ruptured appendix. The patient needs immediate surgery to drain the leaks and washout the abdomen. The leaking digestive juices make the tissue around the leak very flimsy- almost like blotting paper- and very difficult to sew up, indeed attempting to sew this tissue can result in any hole being made bigger in size. Therefore, we put tubes in place to drain the leaking fluids outside the body where they cause little harm. With time the body heals the leak itself. We also insert special feeding tubes downstream of the leak to allow feeding to take place. A leak can mean staying in hospital for a number of weeks.
I apologise if this all sounds unpleasant and a bit frightening, but leaks are the big drawback of the gastric bypass. Fortunately in experienced centres a leak is a rare event, occurring in around 2-3% of cases, but along with clots on the lung is the main reason that some patients die. Death after a gastric bypass (what surgeons impersonally call peri-operative mortality) is rare, around one in 200 to one in 400 cases. Although low, this rate is still ten times more than the risk with a gastric band.  It is essential that any patient considering gastric bypass surgery asks their surgeon about his or her personal death rates, leak rates and their experience in managing leaks.

The internal plumbing of the gastric bypass also creates small internal spaces that bowel can get trapped in. These internal spaces are found at the point where the small bowel is divided and between the alimentary limb and the colon (Petersen's space). The bowel is suspended from a fatty supporting structure called the mesentery. The blood vessels supplying the bowel pass through the fatty mesentery to the bowel wall. Following a gastric bypass, fat is rapidly lost from the trunk and limbs but also internally from the mesentery. This makes the bowel more mobile and easier to enter these internal spaces, running the risk of becoming trapped. This is known as an internal hernia and can present as abdominal pain and vomiting. It requires an operation to correct, sometimes as an emergency. Many surgeons now close these spaces as part of the gastric bypass procedure.

Less serious problems can occur after a bypass. Sometimes the body's tissues heal too well and the join-ups can become tight ("stenosis"). This usually occurs at the join between the stomach pouch and bowel. Around 15% of patients develop a "stenosis" of this join-up. Patients find they can't swallow foods and vomit if they attempt to eat. A stenosis is easily treated by a telescope examination (endoscopy) using a special balloon to stretch up the join. This doesn't require a general anaesthetic and can be done as a day-case procedure. The stomach helps with the absorption of vitamin B12 and over a third of patients need a vitamin B12 injection at some point. Serious nutritional complications are rare after a bypass but regular blood tests can identify any nutritional problem early.

Many patients are delighted by the rapid weight loss that the bypass can give, but rapid weight loss (for whatever reason and with any bariatric operation) can cause problems such as gallstones, hair-loss and loose skin.

Please remember that, like the gastric band, the gastric bypass is a tool. If used correctly it is very effective. But, a bypass can be abused- you can gain weight after a bypass- around 10% of patients will regain weight at some point. Patients who push their portion sizes, or those who eat lots of chocolate or sweets (those who do not dump) can overpower the bypass leading to weight gain. A gastric bypass can be modified or reversed but this is a highly complex undertaking with significant risks. You generally only get one shot at weight loss surgery- work with it and it will change your life, if you don't then options are few and dangerous.

Is the gastric bypass right for me?

For most obese patients, the choice of surgery is between a band and a bypass. The band is safer but late complications are relatively common. The bypass on the other hand "front loads" the risk. Early complications are uncommon but serious, with later complications occurring much less frequently.

Counselling a patient who is unsure about a band or bypass can be difficult. Ultimately it is about risk and benefit. The gastric bypass may be a better choice for the severe diabetic with a BMI of 50, and   the gastric bypass may be a better choice for the patient with a BMI of 36 who has high blood pressure. Remember, that if you have an excess weight of 40kg the gastric band will reduce that by 22kg; a gastric bypass will reduce it by 30kg. That means for all that extra risk you may lose an extra 8kg. How much is that extra 8kg worth to you? However, if you have 100kg of excess weight the gastric bypass will lose you an extra 20kg, that's over 3 stone on top of what the band will give you. That may be worth the extra risk, a risk that is still relatively low. The gastric bypass has a large body of scientific opinion supporting its use, mainly from the United States. A recent review suggested the bypass should remain the primary procedure of choice in the United States, but note that there was a lack of large trials comparing the band and bypass rendering these findings debateable at the least.