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Gastric bypass surgery

Gastric bypass is one of the three most common types of weight loss (bariatric) surgery carried out in the UK. It’s sometimes called a Roux-en-Y gastric bypass.

Gastric bypass works by making your stomach smaller and bypassing part of your intestines (bowels), so that you absorb fewer calories. It can help you lose weight if you’re very overweight (morbidly obese).

An image showing a gastric bypass

How does gastric bypass work?

A gastric bypass allows the food you eat to avoid most of your stomach and part of your small intestine. As a result, you’ll only be able to eat small amounts of food and your body will only be able to absorb some of the calories.

The procedure can help you lose up to 65 per cent (two-thirds) of your excess weight, within two years. And you should be able to maintain your weight loss for up to 10 years. However, this will depend on your diet and the amount of exercise you do, after the operation.

Who can have gastric bypass surgery?

You may be considered for weight loss surgery if you:

  • have a body mass index (BMI) of 40 or more
  • have a BMI of 35 to 40 and a condition, like type 2 diabetes or high blood pressure, which could be improved by losing weight

You’ll usually only be considered for surgery if you’ve tried to lose weight in other ways for at least six months and this hasn’t worked. You may be offered a gastric bypass straight away if you have a BMI of more than 50.

As well as being above a certain BMI, you must be fit enough to have anaesthesia and surgery, to be considered for a gastric bypass. You must also be committed to attend long-term follow-ups and willing to follow your doctors’ instructions regarding your lifestyle and diet after your operation.

Like all types of weight loss surgery, a gastric bypass is generally not recommended for children and young people. So, you must be over 16 years old to be able to have it.

You may still not be able to have the procedure even if you meet all the criteria above, due to the high demand for weight loss surgery. If your GP thinks that you may benefit from surgery, they may refer you to a specialist weight management service to discuss the different options available to you. If you don’t fulfil the criteria for NHS funding, you may be able to consider private treatment.

Deciding on gastric bypass surgery

Gastric bypass surgery is a major operation, which has both benefits and risks. It’s important to discuss these with your doctor or bariatric surgeon so that you can make an informed decision about whether to have the procedure. You can help yourself by preparing questions to ask about the risks, benefits and any alternatives to the procedure. It’s a good idea to bring your partner or family member with you when you talk to your surgeon, so that they are also aware of the pros and cons of the operation.


If your BMI is over 30, a gastric bypass can help you lose weight more effectively than other, non-surgical methods. The procedure also gives you the best chance of keeping off the excess weight in the long term.

If you have high blood pressure or type 2 diabetes, gastric bypass surgery can improve these conditions. Up to six in 10 people may even find their diabetes goes into remission (your sugar levels go back to normal). Weight loss surgery can also improve joint problems and breathing disorders, such as sleep apnoea.


Having gastric bypass surgery means you’ll need at least two years of medical care and check-ups. You’ll also need to change your lifestyle, and what and how much you eat, which you may find difficult to adapt to. You may need to take vitamin and mineral supplements for the rest of your life.

Some people are disappointed with some aspects of their appearance after the operation. For instance, if they have excess skin, or if they had unrealistic expectations from the operation.

Serious complications may occur after gastric bypass surgery. These can happen both during and after the operation, and can mean more treatment or another operation, to correct the problems. Some of the complications can be life-threatening. See the section Complications for more information.

Preparing for a gastric bypass operation

Before the operation

Your surgeon will let you know if there’s anything you need to do to prepare for your operation.

If you smoke, you’ll be asked to stop, at least eight weeks before the operation and permanently afterwards, as smoking increases your risk of getting complications.

Your surgeon may also ask you to follow a low-fat, low-carbohydrate diet for one week before your operation. This helps shrink your liver, making it more likely that your surgeon will be able to do your operation using keyhole (laparoscopic) surgery.

You may want to make some preparations for when you return home, before you go in for your operation. If you live alone, it may be a good idea to arrange for a relative or friend to help you during the first few days after your operation.

On the day

You’ll be admitted to hospital either the night before your gastric bypass operation, or on the same day. The operation is performed under general anaesthesia, which means you’ll be asleep during the operation.

You’ll be asked to follow fasting instructions before your operation. This means you won’t be able to eat or drink, typically for about six hours beforehand. At the hospital, your nurse will check your heart rate and blood pressure, and test your urine.

You will be asked to wear compression stockings to help prevent blood clots forming in the veins in your legs. You may also be given an injection of an anticlotting medicine called low molecular weight heparin (LMWH).

Your surgeon or nurse will ask you to sign a form to confirm that you understand the procedure and accept the risks involved.

What are the alternatives to a gastric bypass operation?

There are other types of surgery available that can help you lose excess weight by making your stomach smaller. These include gastric band and sleeve gastrectomy. Your surgeon will talk to you about the options available. This will help you to weigh up the risks and benefits of each, and decide which is best for you.

What happens during a gastric bypass operation?

We’ve given a general overview here of what you can expect. But the exact details about the procedure may be different depending on your circumstances.

A gastric bypass operation usually takes between one and three hours and it’s usually done using keyhole surgery.

Keyhole surgery is carried out by making small cuts in your abdomen (tummy) rather than one large cut. During the operation, your surgeon uses small instruments, guided by a special telescope with a camera, to perform the procedure.

If you’re very overweight or have had surgery to your abdomen before, then your surgeon may recommend having an open operation, which is done through a single, larger cut. Sometimes it may be necessary to convert from keyhole surgery to open surgery during the procedure, if your surgeon thinks this will be safer once they’ve started.

Using surgical staples, your surgeon will make a pouch out of the top of your stomach, separating it from the lower part. Your surgeon will make an opening in the pouch and connect it to a section of your small intestine. This means the food you eat won’t pass through the lower part of your old stomach or the first part of your bowel.

The length of intestine the operation bypasses is set so that when you reach the right weight, you’ll be able to absorb enough nutrients from your food to maintain that weight.

Your surgeon will close your cuts with stitches or small metal clips, and place a dressing over your wounds.

What to expect afterwards

In the hospital

You’ll need to rest until the effects of the anaesthetic have passed. You may need pain relief to help with any discomfort as the anaesthetic wears off.

On the first day, you may have to wear special pads attached to an intermittent compression pump on your lower legs. The pump inflates and deflates the pads and encourages blood flow in your legs, helping to prevent deep vein thrombosis (DVT). You may also need to wear compression stockings to help maintain your circulation.

You will be encouraged to get out of bed and move around, as this helps prevent both chest infections and blood clots developing in your legs. You may have a nasogastric tube fitted (a tube inserted into your nose that goes down to your stomach) to keep your stomach pouch empty. This is usually removed the following day.

You will have a drip put into a vein in your hand or arm to give you fluids. If you feel well enough, you can start to drink small amounts of water soon after your operation, and the drip will be removed.

Going home

You’ll usually be able to go home one to three days after your operation. You’ll need to arrange for someone to drive you home. Your nurse will give you advice about caring for your healing wounds before you go home and give you a date for a follow-up appointment. You may need pain relief for a few days after the operation.

If your surgeon used dissolvable stitches, they’ll be under your skin so you won’t see them. If you have non-dissolvable stitches or clips, you’ll need to have them taken out. Your surgeon will tell you when and where to have them removed.

It usually takes between four and six weeks to make a full recovery from a gastric bypass operation. This will vary from person to person, so talk to your surgeon about what’s right for you.

Lifestyle changes after surgery

You will need to make major changes to the foods you eat after your operation. You’ll only be able to eat small amounts at each meal – starting with liquids and purées, before moving on to solid food. Your surgeon or dietitian will give you all the information you need about what foods to eat and how much.

The changes to your diet that you’ll need to make after the operation can mean that you don’t get enough protein, or certain vitamins and minerals. For example, iron deficiency can cause anaemia after gastric bypass surgery. Your surgeon is likely to recommend you take vitamin and mineral supplements. You may also need to have regular injections of vitamin B12. You will need to have regular blood tests to check that you’re getting all of the vitamins, minerals and nutrients you need to stay healthy.

You’ll also be asked to make other changes to your lifestyle, such as doing more physical activity. These changes will help you to get the most from your gastric bypass operation, so that you can lose excess weight and keep it off. Being physically active will also help tone your muscles and improve your posture.

Losing weight after a gastric bypass is usually a positive thing. But some people have difficulty adapting to the changes in lifestyle and in their appearance that come from having the operation. Talk to your GP or surgeon if you have these feelings or are concerned about something, as there may be things they can suggest to help you. Your doctor may also suggest a patient support group, which can help to support or advise you.  

Side-effects of gastric bypass surgery

You’re likely to have some bruising, pain and swelling of the skin around your wounds after a gastric bypass.

You may feel sick or vomit after eating, because the amount you’re able to eat is a lot less than before the operation. This should get better as you change your eating habits.

If you eat foods rich in sugar or fat, you can get an unpleasant sensation called dumping. It can make you feel sick or faint, or give you abdominal pain and diarrhoea. Eating small meals and eating little or no sugary food will help to ease these symptoms. If you feel unwell after eating sugary foods, lie down and drink fluids until the feeling passes.

See section on dumping below for more information.

Complications of gastric bypass surgery

Complications are problems that occur during or after the operation. The possible complications of any operation include reactions to the anaesthetic, and blood clots in your legs (DVT) or lungs (embolism).

Being very overweight increases your risk of complications following any operation. You’re more likely to have complications from having a general anaesthetic and are at greater risk of developing blood clots. If you have other conditions linked to your weight, such as high blood pressure or diabetes, these can increase your risk of developing complications.

Additional complications you may get following a gastric bypass operation are listed below.

  • You may develop an infection in your wound or a chest infection.
  • You may get bleeding from a stomach ulcer. You’re more likely to get this if you smoke.
  • The join between your stomach pouch and small intestine can become narrower, making you feel sick and vomit.
  • You may get a leak at the place where your intestine is re-joined to your stomach. This can be very serious and you will usually have an operation to repair it straight away.
  • If you lose weight quickly, you may develop gallstones. These can be painful and you may need surgery to remove them. Your surgeon may suggest removing your gallbladder during your gastric bypass operation, or prescribe you medication after your surgery, to prevent this from happening.

During your operation, your surgeon may come across an unexpected problem and may need to convert the keyhole operation to an open procedure. This won’t affect how well the operation works, but it may take a longer time for you to recover afterwards.

Not everyone loses enough weight after gastric bypass surgery. The most common reason for not losing enough weight is not following the rules about diet and exercise, or drinking too much alcohol.

FAQ: Gastric bypass surgery has a side-effect called ‘dumping’. What is this, and what should I do if I have it?

Dumping is an unpleasant reaction to eating very sugary or fatty foods after having gastric bypass surgery. You might feel sick and lightheaded. You may also have abdominal (tummy) pain and diarrhoea. You can prevent dumping by eating the right foods and taking other simple precautions.

After gastric bypass surgery, your stomach will be smaller and will empty more quickly than before so foods enter your intestines quickly. If the foods are sugary, fluid is pulled from other tissues in your body (including your blood) to dilute the concentration of sugar. This can cause your blood pressure to fall so you may feel tired and faint. All the extra fluid in your intestines can be painful and make your abdomen swell. In addition, eating too much sugary food can cause you to release too much insulin, and this can cause your blood sugar to fall.

To help prevent dumping, make sure you do the following.

  • Stick to small, regular meals that are high in protein (lean meat, fish, lentils and eggs) but low in carbohydrates (bread, potatoes and cereals).
  • Rest for around 20 minutes after eating.
  • Eat only foods that contain very little sugar, or use artificial sweeteners.
  • Don’t drink with, or immediately after, eating.

If you have symptoms of dumping, lie down and stick to small amounts of fluid until the symptoms have passed.

Up to half of people who have a gastric bypass operation may have symptoms of dumping after their operation. However, the symptoms aren’t severe in most people. If you’re worried about your symptoms or the type of food you should be eating, contact your dietitian or doctor for advice.

FAQ: What foods should I eat after my gastric bypass operation?

In the first month after your operation, you’ll need to eat small amounts of puréed foods, while your stomach is healing. After that, you can gradually start to eat solid foods. Eating small amounts of healthy foods that are low in fat and sugar and high in protein will help you to lose weight and keep it off.

A gastric bypass operation reduces the size of your stomach, so you’ll only be able to eat small amounts of food at each meal.

For the first week after your operation, you’ll only be able to drink fluids. Fluids can include water, skimmed milk, hot drinks such as tea and coffee, thin soups and fruit juices.

For the next three weeks, the food you eat should be puréed. This is because the opening out of your stomach will be quite swollen and solid food won’t be able to pass through it. You may be given specific dietary advice, but the main rules to follow are listed below.

  • Your food should be runny, like smooth baby food. You can use a blender, masher or food processor to do this. Adding extra liquid, such as gravy, sauces or fruit juice to your food can help to make it smoother. At first, each meal should no more than five or six tablespoons in amount. Eat slowly and take small mouthfuls.
  • As soon as you start to feel full, stop eating. The feeling of fullness you get after a gastric bypass operation may be different from the feeling you had before. A full stomach may give you a heavy or tight feeling in your chest.
  • Make sure you drink enough water or fluids every day. You should drink 1.5 litres of fluid over the day (up to 200ml at one time). Drink between meals rather than when you’re eating, and don’t drink fizzy drinks.

After the first four weeks, you can gradually start eating foods that are lumpier, but make sure you chew them well.

In the long term, you’re aiming to eat a healthy, balanced diet. This will help you to keep losing weight and to get all the nutrients and energy you need to stay healthy. Make sure you eat three balanced meals a day, eat slowly and stop when you feel full. Try not to drink anything in the half hour before your meals or for an hour afterwards, as the fluid will make you feel full.

Ensure that you eat adequate amounts of protein such as meat, fish, pulses and eggs, and only small amounts of carbohydrate such as potatoes, pasta, rice and bread. You should eat four to five portions of fruit and vegetables a day. Choose low-fat foods, such as skimmed milk and low-fat cheese.

Don’t eat foods that contain a lot of sugar. This can cause dumping, which can make you feel sick and cause other unpleasant symptoms.

It can take months after your operation for you to learn which foods you can eat, and to feel confident and comfortable eating. There may be days when you feel sick after food and other days when you don’t.

Talk to your surgeon or dietitian for more information about the foods you can eat after your gastric bypass operation.

FAQ: Will I need surgery to remove excess skin after weight loss?

Losing a lot of weight can cause your skin to become loose. This means that you may be left with folds of excess skin. Cosmetic surgery can help reshape the parts of your body where you have extra skin.

Most people who have a gastric bypass operation will lose a lot of weight. On average, people lose up to about three-quarters of their excess weight in the two years after surgery.

This amount of weight loss can cause your skin to become loose. Once it has been stretched, skin loses its tightness. This means that you may be left with folds of excess skin after your weight loss. The places where this is most common are on your abdomen (tummy), face, bottom, breasts, upper arms and thighs.

You may find this extra skin uncomfortable and embarrassing. The skin in these areas can also become irritated, painful or infected. Cosmetic surgery can help to reshape the parts of your body where you have extra skin.

It’s usual to wait for between 12 and 18 months after your operation before having any cosmetic surgery. This is to make sure that your weight is stable and isn’t likely to change significantly.

Cosmetic surgery isn’t suitable for everyone and can cause further health problems. Surgery purely for cosmetic reasons (to improve your appearance) is only available privately. If you want to find out more about cosmetic surgery after your gastric bypass operation, discuss your options with your doctor.


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