Weight loss surgery - gastric bypass

About gastric bypass surgery

A gastric bypass is an operation to make your stomach smaller and to shorten the length of small intestine that your food passes through. This allows the food you eat to bypass most of your stomach and part of your small intestine – meaning that you will only be able to eat small amounts, and some of the food that you eat won't be fully digested. Gastric bypass surgery can help you lose weight if you're very overweight (morbidly obese).

Gastric bypass surgery is sometimes called a Roux-en-Y gastric bypass (named after a surgeon called Roux). This is the most common type of gastric bypass operation carried out in the UK.

Studies show that on average, people lose between two-thirds and three-quarters of their excess weight in the two years after a gastric bypass operation.

Getting gastric bypass surgery

Your doctor will only suggest surgery if you have tried to lose weight in other ways (such as with medicines, healthy eating and exercise) for at least six months and this hasn't worked. Surgery will also usually only be considered if:

  • you're morbidly obese with a body mass index (BMI) of more than 40 or you have a BMI of over 35 and also have a medical condition, such as diabetes or high blood pressure, that would be likely to improve if you lose weight
  • you’re generally fit enough to undergo surgery
  • you’re committed to losing weight and maintaining that weight loss by combining the surgery with lifestyle changes in the future, and are prepared to attend regular follow-ups

In some areas of the UK, you may still not be eligible for the procedure even if you meet these criteria, due to the high demand for weight loss surgery. If your GP thinks weight loss surgery could be beneficial to you, he or she will refer you to a bariatric surgeon (a doctor specialising in obesity surgery) to talk about your options.

What are the alternatives?

There are other types of surgery available that can help you to 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 and help you to weigh up the risks and benefits of each, to decide which is best for you.

Preparing for a gastric bypass operation

Your surgeon will explain how to prepare for your operation. For example, if you smoke you will be asked to stop, as smoking increases your risk of getting a wound infection, chest infection or blood clot, and slows your recovery. This is particularly important before any weight loss operation because being very overweight increases your risk of complications after surgery.

Your surgeon may ask you to follow a low-fat, low-sugar and low-carbohydrate diet with no alcohol for one to two weeks before your operation. This helps to shrink your liver and makes it more likely that your surgeon will be able to do the operation using keyhole (laparoscopic) surgery.

A gastric bypass operation is performed under general anaesthesia. This means you will be asleep during the operation.

You will be asked to follow fasting instructions. This means not eating or drinking, typically for about six hours beforehand. However, it’s important to follow your surgeon’s advice. At the hospital, your nurse will check your heart rate and blood pressure, and test your urine.

You may 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 heparin.

Your surgeon will discuss with you what will happen before, during and after your procedure, and any pain you might have. This is your opportunity to understand what will happen, and you can help yourself by preparing questions to ask about the risks, benefits and any alternatives to the procedure. This will help you to be informed, so you can give your consent for the procedure to go ahead, which you may be asked to do by signing a consent form.

What happens during a gastric bypass operation?

A gastric bypass operation usually takes between one and three hours.

The operation is usually done using keyhole surgery. Keyhole surgery is carried out by making small cuts in your abdomen (tummy) rather than one large cut. During a keyhole 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 incision.

Using surgical staples, your surgeon will make a pouch out of the top of your stomach, separating it from the lower part of your stomach. 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 will bypass your old stomach and the first part of your intestine. The length of small intestine that the operation bypasses can be adjusted to make sure that when you reach the right weight for your height, you will be able to eat enough to maintain that weight. Your surgeon may insert a drain, to help remove any fluid or blood that might have collected inside your abdomen.

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

What to expect afterwards

You will 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.

If you had open surgery, you may have a catheter to drain urine from your bladder into a bag. You may also have fine tubes coming from the wound which drain fluid into another bag and are usually removed after a day or two.

On the first day, you may have to wear special pads attached to an intermittent compression pump on your lower legs. The pump inflates 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 chest infections and blood clots 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.

Two to four days after your operation, you may have an X-ray to check that the stitches and staples are healing well and that there are no leaks. You may be asked to swallow a fluid (barium) that shows up on X-ray images. If the X-ray shows that the operation has been successful, you will be able to start drinking freely, including soups and fruit juice.

You will usually be able to go home two to five days after your operation. You will need to arrange for someone to drive you home.

Your surgeon may use dissolvable stitches. The length of time your dissolvable stitches will take to disappear depends on what type you have. However, for this procedure, they should usually disappear in seven to 10 days. If you have non-dissolvable stitches or clips, you will need to have them taken out. Your surgeon will tell you when and where to have them removed.

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.

Recovering from gastric bypass surgery

If you need pain relief, you can take over-the-counter painkillers such as paracetamol or ibuprofen. Always read the patient information that comes with your medicine and if you have any questions, ask your pharmacist for advice.

You will need to make major changes to the foods you eat after your operation. You will only be able to eat small amounts at each meal. To start with, stick to liquids, and then move onto puréed food. You may also need to take vitamin and mineral supplements. Your surgeon and dietician will give you all the information you need about what foods to eat and how much.

You will be asked to make other changes to your lifestyle, such as becoming more active. These changes will help you to get the most from your gastric bypass operation and will help you to lose excess weight and keep it off. Being active will also help to tone your muscles and improve your posture.

You will need to have regular blood tests after your operation. These are to check that you’re getting all of the vitamins, minerals and nutrients you need to stay healthy.

It usually takes between four and six weeks to make a full recovery from a gastric bypass operation. However, this varies between individuals, so it's important to follow your surgeon's advice.

What are the risks?

As with every procedure, there are some risks associated with gastric bypass surgery. We have not included the chance of these happening as they are specific to you and differ for every person. Ask your surgeon to explain how these risks apply to you.


Side-effects are the unwanted but mostly temporary effects you may get after having the procedure. You're likely to have some bruising, pain and swelling of the skin around your wounds.

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 too many sugary foods, 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 only until the feeling passes. See our frequently asked questions for more information about dumping.


Complications are when problems occur during or after the operation. The possible complications of any operation include an unexpected reaction to the anaesthetic, excessive bleeding or developing a blood clot, usually in a vein in your leg (DVT).

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 more at risk of developing blood clots. If you have other conditions linked to your weight, such as high blood pressure or diabetes, then these can also increase your risk of developing complications.

Complications specific to a gastric bypass operation are listed below.

  • An infection in your wound or a chest infection.
  • A blood clot in your lung (pulmonary embolism).
  • 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.
  • The opening from your stomach into your intestine can become narrowed, which makes it more difficult for food to pass through it, or your stomach can become stretched. If the opening becomes narrowed, it can usually be enlarged using a flexible telescope that you swallow called an endoscope. If you stretch the stomach pouch by overeating, this can only be repaired with surgery.
  • A lack of protein, vitamins and minerals caused by the change in the amount and type of food you’re able to eat after the operation. This can affect your general health. Vitamin and mineral supplements can help to prevent this. You may need to have injections of vitamin B12.

If you lose weight quickly, you may develop gallstones. These can be painful and you may need surgery to remove them. Your surgeon may prescribe medicine to reduce your risk of developing gallstones.

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. Inadequate weight loss is usually from not following the correct diet or taking enough exercise. If this happens, your surgeon may suggest another operation.

What foods should I eat after my gastric bypass operation?


In the first month after your operation, you will 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.


As a gastric bypass operation reduces the size of your stomach, you will only be able to eat small amounts of food at each meal.

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

After the first week, and for about three weeks after your operation, 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.
  • Initially, each meal should be up to 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.
  • Drink enough water or fluids every day. You should drink 1.5 litres in 100 to 200ml glassfuls between meals but don’t drink when you’re eating a meal, 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 half an 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 faint.

It can take up to six 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.

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


Losing a lot of weight can cause your skin to become loose. This means that you may be left with folds of excess skin after your weight loss. Cosmetic surgery can help to 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 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 you’re able to have 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. If you want to find out more about cosmetic surgery after your gastric bypass operation, discuss your options with your doctor.

Gastric bypass surgery has a side-effect called 'dumping’. What is this and what should I do if I have this?


Dumping is an unpleasant reaction to eating very sugary foods after having gastric bypass surgery. You might feel sick and faint. You can prevent dumping by eating the right foods.


After gastric bypass surgery, your stomach will be smaller and it will also empty more quickly than before.

Sugary foods enter your intestines quickly, and cause fluid to be pulled out of other tissues in your body to try and dilute it. This can be as much as 1.5 litres (three pints) and cause your blood pressure to fall.

This fluid comes from many parts of your body, including your blood, which is why you may feel tired and faint. All of this extra fluid in your intestines can be painful and make your abdomen swell. When you eat sugar, your body will also produce too much insulin, which can also cause you to feel sweaty and faint.

To help to prevent dumping, make sure you:

  • only eat small meals and stop eating before you feel full
  • rest for around 20 minutes after eating
  • eat only small amounts of carbohydrates, such as bread, potatoes and cereals
  • only eat foods that contain very little sugar or use artificial sweeteners instead
  • don’t drink with, or immediately after, eating

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

Up to half of people who have a gastric bypass operation may have the symptoms of dumping after their operation. However, the symptoms usually get better as time goes on, so that by one year after surgery, only around one in 20 people will still be affected. If you’re worried about your symptoms or the type of foods you should be eating, contact your dietitian or doctor for advice.