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Gastric bypass surgery


Expert reviewers, Mr Peter Lamb, Consultant General and Upper Gastrointestinal Surgeon and Yasmin Rahman, Bupa Clinics GP
Next review due March 2024

Gastric bypass surgery is an operation to help you lose weight and improve weight-related health problems if you’re very overweight. It makes your stomach smaller and bypasses part of your bowels (intestines). This changes the signals between your bowel and your brain that control your appetite, how full you feel, and your weight.

An image showing a gastric bypass

About gastric bypass surgery

Gastric bypass surgery can help you lose up to two-thirds of your excess weight within two years. And you should be able to keep most of this weight off long-term. But this will depend on committing to some changes to your lifestyle after the operation. For example, you’ll need to:


Your doctor will talk through these changes with you.

Who can have gastric bypass surgery?

Your GP may refer you to a specialist weight-management service to discuss your options. A specialist team that includes a surgeon, dietician and psychologist will assess you to see if gastric bypass surgery is the right operation for you. You may be offered 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 such as type 2 diabetes or high blood pressure which could get better if you lose weight
  • are fit enough to have an anaesthetic and surgery
  • are committed to having long-term follow-up appointments and changing your lifestyle and diet after your operation
  • are over 18 (although children are considered in exceptional circumstances)

You’ll usually only be considered for surgery if you’ve tried to lose weight in other ways and this hasn’t worked. If you don’t meet the criteria for NHS funding, you may be able to have the surgery privately.

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Preparation for gastric bypass surgery

Your hospital will let you know how to prepare for your operation.

If you smoke, you’ll be asked to stop at least six weeks before the operation. Smoking increases your risk of complications of surgery.

Your surgeon will usually ask you to eat a low-fat, low-carbohydrate diet for two weeks before surgery to shrink your liver, which will allow your surgeon better access to your stomach.

You’ll have gastric bypass surgery under general anaesthesia (GA), which means you’ll be asleep during the operation. You won’t be able to eat for about six hours before surgery, but you may be able to drink water up until two hours before. If you’re asked to follow fasting instructions, always follow your surgeon’s advice.

You’ll be asked to wear compression stockings on your legs to help prevent blood clots forming in the veins in your legs (deep vein thrombosis). You’ll also need to have an injection of an anti-clotting medicine as well as or instead of wearing compression stockings.

Your surgeon will discuss with you what will happen before, during and after your surgery at various stages. If you’re unsure about anything, don’t be afraid to ask. No question is too small. It’s important that you feel fully informed so you feel happy to give your consent for the operation to go ahead. You’ll be asked to do this by signing a consent form.

Gastric bypass surgery

A gastric bypass operation usually takes around one to two hours, although this can vary between hospitals. It’s usually done using keyhole (laparoscopic) surgery, which means your surgeon makes small cuts in your tummy rather than one large cut. They’ll use small instruments, guided by a telescope with a camera. Sometimes your surgeon may need to switch from keyhole surgery to open surgery during the procedure.

Your surgeon will use surgical staples to make a pouch out of the top of your stomach, separating it from the lower part. They’ll then make an opening in the pouch and connect it to a section of your small bowel. This means the food you eat won’t pass through the lower part of your stomach or the first part of your bowel. Your surgeon will bypass a carefully selected amount of bowel. This means that when you reach the right weight, you’ll be able to absorb enough nutrients from your food.

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

An image showing a gastric bypass

Aftercare for gastric bypass surgery

In the hospital

You’ll need to rest until the anaesthetic has worn off. But it’s important to get out of bed and move around as soon as you can. This will help you to recover and prevent blood clots developing in your legs. You may also need pain relief.

Your hospital may give you something to help prevent deep vein thrombosis (DVT). For example, you may have to wear pads attached to an intermittent compression pump on your lower legs to boost the blood flow in your legs. Or you may need to wear compression stockings to help boost your circulation and continue to have injections of an anti-clotting medicine such as heparin.

You’ll 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 a nurse will remove the drip.

Going home

You’ll usually be able to go home around two days after your operation. Ask a friend or family to drive you. Your nurse will tell you how to care for your healing wounds before you go home and will give you a date for a follow-up appointment. You may need to take painkillers for a few days after the operation.

It will probably take around three weeks to recover from a keyhole gastric bypass operation. You’ll usually go home on a liquid diet – your surgeon will give you advice on how to move on from this. For more information, see our section on lifestyle changes and our FAQ: What can I eat after surgery?

Lifestyle changes after surgery

You’ll need to make major changes to what you eat after gastric bypass surgery because the volume of your stomach will be smaller. You’ll usually need a liquid diet when you first get home. You can then move on to puréed food, mashed food, and then small solid meals during the first four to six weeks after your operation. For more information, see our FAQ: What can I eat after surgery?

It’s essential to make these changes to your diet after surgery. You also need to make sure that you get enough protein, vitamins and minerals, which can affect your health. If you don’t get enough iron, it may cause anaemia, for example. You’ll need to take vitamin and mineral supplements for the rest of your life. You may also need to have regular vitamin B12 injections and have regular blood tests to check you’re getting everything you need.

You’ll need to do more exercise and drink less alcohol too. This will help you to get the most from your gastric bypass operation, so you can lose excess weight and keep it off.

Losing weight after a gastric bypass is usually a positive thing. But you may find it hard to adapt to your lifestyle and appearance after the operation. Talk to your GP or surgeon if you’re struggling to cope. They may suggest a patient support group, where you can get advice from other people who have been through the operation too.

Side-effects of gastric bypass surgery

You may have some bruising, pain and swelling around your wounds after a gastric bypass. But this shouldn’t last too long.

You may feel sick or even be sick after eating. This is because you’ll only be able to eat a small amount of food at a time. This should get better with time as you get used to eating smaller amounts.

You may get an unpleasant sensation called dumping if you eat too many sugary foods. For more information, see our FAQ: What is dumping?

Complications of gastric bypass surgery

Complications of a gastric bypass include the following.

  • You may develop an infection in your wounds or a chest infection.
  • You may get bleeding from the operation site and need another operation to stop it.
  • You may get a leak where your intestine is re-joined to your stomach. This can be very serious and you’ll usually have an operation to repair it straightaway.
  • You may get an internal hernia. This happens when one of the two bowel loops that are created during surgery twists under the other through a newly created opening. If the twisted bowel gets trapped in this position, it may get blocked or the blood supply to it may be cut off so you’ll need surgery to repair it. If you have severe tummy pain after your surgery, it’s important to seek urgent medical attention.
  • You may get bleeding from a stomach ulcer. Ulcers are more common if you smoke, so it’s important to stop. You may need to take a medicine to reduce the amount of acid in your stomach.
  • The join between your stomach pouch and small bowel can become narrower, which will make you feel sick or even be sick.
  • If you lose weight quickly after your surgery, you may develop gallstones. If you already have gallstones, your surgeon may suggest removing your gallbladder during your gastric bypass or may prescribe medicines after your surgery.
  • You might not lose enough weight after gastric bypass surgery. There are a number of reasons why this may happen. For example, you may find it difficult to adapt to diet advice and lifestyle changes. Your surgeon will discuss this with you and help support you.

Considerations for gastric bypass surgery

Gastric bypass surgery is a major operation with potentially serious complications. It’s important to discuss the surgery with your doctor or surgeon so you can make an informed decision about whether or not to go ahead.

If you have high blood pressure or type 2 diabetes, gastric bypass surgery can improve these conditions or even make them clear up completely. Weight loss surgery may also improve joint problems such as osteoarthritis and breathing disorders such as sleep apnoea. But don’t stop taking medicines for any of these health conditions without speaking to your doctor first.

You’ll need at least two years of medical care and check-ups after gastric bypass surgery. The changes to your lifestyle, along with what and how much you eat are life-long commitments. You may also need to take vitamin and mineral supplements for the rest of your life.

Some people are disappointed with how they look after their operation, which may be because they’re left with excess skin. It’s important to have realistic expectations.

Alternatives to gastric bypass surgery

Other types of surgery can help you lose excess weight. These include sleeve gastrectomy and gastric band. Like gastric bypass surgery, you’ll need to make long-term changes to your diet and lifestyle for them to work.

Another option is to have a silicon balloon inflated in your stomach. This will make you feel full but it can only be used for short-term weight loss.

Ask your surgeon about the options available – they’ll help you decide which one is best for you.

Frequently asked questions

  • Dumping is caused by changes in the way your digestive system works after gastric bypass surgery. Up to half of people who have a gastric bypass operation get dumping but the symptoms are usually mild.

    Dumping can make you feel sweaty, sick and lightheaded. You may also feel incredibly tired and have tummy pain and diarrhoea. The symptoms happen because of changes in your circulation and your blood sugar levels. You may notice dumping symptoms 30 minutes to an hour after you eat, which is called early dumping, or within three hours (late dumping).

    Dumping may get better over time by making changes to what you eat.

    • Stick to small, regular meals that are low in carbohydrates instead of three larger meals a day.
    • Cut down on sugar.
    • Don’t drink during meals or for half an hour afterwards.
    • Cut down or cut out dairy products.

    Sometimes it can help to rest for half an hour after you eat until your symptoms go away. For advice about what you should eat to help, ask your dietitian or doctor.

  • A gastric bypass operation usually changes your appetite and how full you feel. You’ll only be able to eat a small portion of food at each meal.

    For the first day after your operation, you’ll only be able to drink water or other fluids little and often. These include skimmed or semi-skimmed milk, squash, tea and coffee, and clear soup.

    You’ll usually go home on a liquid or puréed diet before you move on to mashed food, then small solid meals during the first four to six weeks after surgery. Your hospital will give you advice, but the main rules to follow are below.

    • Puréed food should be runny, like smooth baby food. Use a blender, masher or food processor and add extra liquid such as gravy, sauces or fruit juice, to make your food smoother. At first, limit each meal to two or three tablespoons. Eat slowly and take small mouthfuls.
    • As soon as you start to feel full, stop eating. The feeling of fullness after a gastric bypass may be different – it may give you a heavy or tight feeling in your chest.
    • Drink enough water or fluids – ideally, around 2 litres of fluid spread out over the day. Drink between meals rather than when you’re eating, and don’t have fizzy drinks.

    After the first four weeks, you can gradually start to eat foods that are lumpier, but make sure you chew every mouthful well. Doughy white bread and red meats (if not minced) are often the most difficult foods to eat early on.

    In the long term, aim to eat a healthy, balanced diet to keep losing weight and get the nutrients and energy you need. Make sure you get enough protein in your diet. And only eat small amounts of carbohydrate. Don’t eat foods that contain a lot of sugar because it can cause dumping.

    It can take months to learn which foods you can eat comfortably. Your intolerance to certain foods may change over time, so don’t be afraid to revisit them at a later date.

  • The weight you lose in gastric bypass surgery can make your skin loose, leaving you with folds of excess skin. You’re most likely to notice this on your tummy, face, bottom, breasts, upper arms and thighs. This can be uncomfortable and the skin in these areas can also become irritated, painful or infected.

    Cosmetic surgery can remove extra skin. It’s usually best to wait at least a year (often longer) after your operation to make sure your weight is stable.

    Cosmetic surgery is generally only available privately. If you would like more information about cosmetic surgery after gastric bypass, including an operation called tummy tuck (abdominoplasty), speak to your doctor.



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  • Reviewed by Rachael Mayfield-Blake, Freelance Health Editor, March 2021
    Expert reviewers, Mr Peter Lamb, Consultant General and Upper Gastrointestinal Surgeon and Yasmin Rahman, Bupa Clinics GP
    Next review due March 2024

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