Gastric bypass surgery: Procedure, preparation and recovery | Bupa UK (2024)

Your health expert:Mr Peter Lamb, Consultant General and Upper Gastrointestinal Surgeon
Content editor review by Liz Woolf, Freelance Health Editor, June 2023.
Next review due June 2026.

You may have gastric bypass surgery to help you lose weight if you’re very overweight. The surgery makes your stomach smaller and bypasses part of your bowels (intestines). Losing weight can help to improve any weight-related health problems you have.

Gastric bypass surgery: Procedure, preparation and recovery | Bupa UK (1)

About gastric bypass surgery

In gastric bypass surgery, you have part of your stomach sectioned off and connected directly to your small bowel. Then, food you eat will bypass most of your stomach and the first part of your small bowel. You’ll feel fuller after eating smaller amounts of food. The surgery also changes hormone signals that pass between your bowel and your brain, controlling your appetite.

You may hear this operation called a ‘Roux-en-Y’. This is just the name of the method surgeons most often use to carry it out.

Gastric bypass surgery can help you lose up to two-thirds of your excess weight. You are likely to lose most weight in the first year after surgery and the rest in the following six months to a year. You should be able to keep most of this weight off permanently. But this depends on sticking to some major changes to your lifestyle after the operation. For example, you’ll need to:

  • change your diet – stop snacking and choose healthier options
  • not drink too much alcohol
  • take regular exercise

Your doctor will talk through these changes with you.

Who can have gastric bypass surgery?

Your GP may refer you to a specialist team to discuss your options. This is the weight-management service and includes a surgeon, dietitian, and psychologist. They will assess you to see if gastric bypass surgery is right for you. They may suggest weight loss surgery if you:

  • have a body mass index (BMI)of 40 or more
  • have a BMI over 35, with or without a condition that could improve if you lose weight (for example, type 2 diabetesor high blood pressure)
  • are fit enough to have an anaesthetic and surgery
  • are committed to having long-term follow-up appointments
  • commit to changing your lifestyle and diet after your operation
  • are over 18 (although children may have this surgery in exceptional circ*mstances)

You can usually only have this 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.

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 after surgery.

Your surgeon may ask you to eat a low-fat, low-carbohydrate diet for two weeks before surgery. This is to shrink your liver, which allows your surgeon better access to your stomach.

You have gastric bypass surgery under a general anaesthesia(GA), so you’ll be asleep during the operation. You won’t be able to eat or drink for some time before surgery. Your doctors will tell you what to do and it’s important to follow their advice.

You surgeon will ask you to wear compression stockings after the operation. This helps to prevent blood clots forming in the veins in your legs (deep vein thrombosis). You may also have injections of an anti-clotting medicine.

Your surgeon will discuss with you what will happen before, during and after your surgery. 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 do this by signing a consent form.

Gastric bypass surgery

A gastric bypass operation usually takes around one to two hours. But this can vary between hospitals.

Most surgeons use keyhole (laparoscopic) surgery. This means your surgeon makes several small cuts in your tummy rather than one large cut. They put small instruments through the cuts and guide them using a telescope with a camera. Sometimes your surgeon may start with keyhole surgery, but find they need to switch to open surgery.

During the operation, your surgeon staples across the top of your stomach, to form a small pouch. They then make an opening in the pouch and connect it directly to your small bowel. Food passing through will then bypass the lower part of your stomach and the first part of your small bowel. The surgeon reconnects the first part of your small bowel further down. This means the juices from your stomach, pancreas and liver can mix with the food so that you can absorb enough nutrients after your surgery.

Your surgeon will close your cuts with stitches or small metal clips and cover them with a dressing.

Bupa Weight Management Plan

The Bupa weight management plan is designed for people with a BMI over 30 (or over 27 if you have a weight related condition). The plan is designed to empower you to achieve and maintain a healthy weight in a sustainable way.

To book or to make an enquiry, call us on 03452660566

Find out more about our Weight Management Plan

Aftercare for gastric bypass surgery

After surgery, 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 helps you to recover and prevents blood clots developing in your legs. Your doctor will have made sure you have painkillers prescribed.

Your hospital will take measures to help prevent deep vein thrombosis (DVT). You may have to wear compression stockings. Or you may have an intermittent compression machine that inflates and deflates around your legs. Both of these aim to boost the blood circulation in your legs. You may also continue to have anti-clotting injections, such as heparin.

At first, you’ll have a drip inserted into a vein in your hand or arm to give you fluids. When you feel well enough, you can start to drink small amounts of water. Your nurse will then remove the drip.

Going home

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

Everyone recovers at their own rate, but it’s sensible to allow up to six weeks to recover fully from a gastric bypass operation. You usually go home on a liquid diet – your surgeon will give you advice on how to move on from this. There’s more information on diet below, in our section on lifestyle changes.

Lifestyle changes after surgery

You have to make major changes to your diet after gastric bypass surgery, as your stomach will be smaller. You have a liquid diet for the first couple of weeks after going home. Then you move on to puréed, then mashed food. By six weeks after your operation, you should be having several small solid meals a day.

It can be difficult to drink enough. You need at least one and a half litres a day to stay healthy. Drink slowly and between meals or it will fill you up too much.

You need to make sure that you get enough protein every day. If you don’t, it can affect your health. Your doctor is likely to suggest a low-fat, high protein diet.

It’s difficult to get enough vitamins and minerals from your diet after gastric bypass. So you’ll need to take a complete multivitamin supplement for the rest of your life. You may also have regular vitamin B12 injections. Your doctor will arrange 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. There you can get advice from other people who have been through the same operation.

Side-effects of gastric bypass surgery

You may have some bruising around your wounds after a gastric bypass. But this shouldn’t last more than a week or two.

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 side-effect called dumping syndrome if you eat too many sugary foods. It can make you feel sweaty, sick and lightheaded. You may also feel very tired and have tummy pain and diarrhoea. Symptoms can come on 30 minutes to an hour after you eat (early dumping), or within three hours (late dumping). It may help to:

  • eat smaller more frequent meals that are low in carbohydrates, especially sugar
  • don’t drink during meals or for half an hour afterwards
  • cut down or cut out dairy products
  • rest for half an hour after eating

Ask your dietitian or doctor for more advice.

Complications of gastric bypass surgery

There can be complications with any surgery, such as a wound or chest infection or a blood clot. To reduce the risk of these, it’s best to follow the advice of your nurse, doctor, and physiotherapist. This will include getting up and moving as soon as possible after surgery and doing breathing and leg exercises.

Gastric bypass complications can happen soon after surgery or some months later. Early complications happen within a month of surgery. They include bleeding or a leak from the stomach or bowel. This is most likely where the stomach has been stapled or your small bowel rejoined. If you have abdominal or shoulder pain, see signs of blood when you’re sick, or your poo looks black like tar, contact your surgeon or hospital.

Later complications can include the following.

  • Gallstones – this can happen in up to one in three people having this type of operation.
  • An internal hernia – part of the small bowel can get twisted at the site of the operation, causing pain.
  • Stomach ulcer – you may have medicine to reduce stomach acid to help prevent this. Ulcers are more common in smokers.
  • A stricture – this is narrowing of the join between your stomach pouch and small bowel. you may feel sick, or be sick.

If you have pain, sickness that doesn’t go away, or any other concerns after your surgery, contact your surgeon for advice.

These complications may sound frightening, but, apart from gallstones, they are uncommon, particularly after keyhole surgery. Ask your surgeon to talk them through with you. They will be able to give you more information about how likely they are in your case.

What you need to know before gastric bypass surgery

Gastric bypass surgery is a major operation with potentially serious complications. But it does also have many benefits. It’s important to discuss the surgery thoroughly with your doctor or surgeon beforehand. Then you can make an informed decision about whether to go ahead.


Your doctor may suggest gastric bypass surgery to improve your overall health. Afterwards, you are less likely to have:

  • diabetes – your doctor may suggest surgery as it can reverse type 2 diabetes in many people
  • breathing disorders such as sleep apnoea
  • a heart attack or stroke
  • back pain and joint problems such as osteoarthritis
  • high blood pressure
  • weight-related cancers, such as womb cancer

For some people, the surgery can help their mental health, with improved quality of life and less depression.


You might not lose enough weight after gastric bypass surgery, or put weight back on later. Some people find it difficult to adapt to diet advice and lifestyle changes. Your surgeon will discuss this with you and help support you.

You’ll need at least two years of medical care and check-ups after your surgery. The changes to your lifestyle, along with what and how much you eat, are life-long commitments. You will 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. This is often because they’re left with excess skin. Some people need further surgery to remove this. It’s important to have realistic expectations, so talk to your surgeon beforehand.

Alternatives to gastric bypass surgery

Other types of surgery can help you lose excess weight. These include sleeve gastrectomy and gastric band surgery. As with gastric bypass surgery, you need to make long-term changes to your diet and lifestyle.

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

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

You’ll start losing weight as soon as you’ve had gastric bypass surgery. Most people lose most of their excess weight in the first year. Weight loss can carry on after that, but will usually tail off in the second year as your weight settles.

See our section about gastric bypass surgery for more information.

Dumping syndrome is a possible side-effect of weight loss surgery. It can happen if you eat too much sugar. The stomach empties too quickly, which can cause sickness, dizziness and light-headedness.
There is more information in the section on gastric bypass surgery side-effects .

The weight you lose in gastric bypass surgery can make your skin loose, leaving you with folds of excess skin. Cosmetic surgery can remove this but it’s best to wait until your weight is stable. This could be up to two years after your bypass surgery.

More on this topic

Gastric band operation

Gastric band surgery is a common type of weight loss (bariatric) surgery for people who are very overweight (obese).

Read more

Seven tips for a healthy and well-balanced diet

A healthy, well-balanced diet involves eating foods from a variety of food groups to get the nutrients that your body needs to function.

Tips on eating more healthily

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