How does it work?

A small pouch is created using the top section of the stomach which reduces the amount of food you can eat (restriction) by about 75%. Food leaves the new stomach pouch through a new opening and bypasses the rest of the stomach and about 100-150cm of your small intestine. This means that less calories are absorbed as the food passes through your body (malabsorption).

Pre-op Liver Reducing Diet

It is recommended that a very restricted diet – one that is low in fat, sugar and starch – is followed for 1-2 WEEKS prior to surgery. By following a strict diet, your body reduces its glycogen and water stores (glycogen is a form of sugar stored in the liver and muscles for energy) causing your liver to shrink in size.This will make the surgery much easier to perform as your stomach is set behind your liver and reduces the risks of complications.

Liquid Stage (The first 2 weeks)

Following the operation you are advised to take liquids only. This will be necessary for at least 2 weeks. On the evening of the surgery, you will be able to take sips of fluid. This will be increased to half-cups on the first morning after the operation and to free fluids within a day or two. You will still only have room for a small amount of liquid at one time so drink slowly and take plenty of time between swallows. Most patients are discharged from hospital after 1- 3 days so you will still be taking liquids when you first go home. You should aim to get up to 2 litres (approximately 3½ pints) per day.

Suitable liquids include:-

  • Milk and milk shakes
  • Water
  • Fruit juice
  • No Added Sugar cordial
  • Clear / thin soups (no bits)
  • Tea / coffee
  • Low calorie hot chocolate / malted drinks
  • Oxo / bovril

Pureed Stage (For the next 2-4 weeks)

Pureed or liquidised food has the consistency of apple sauce or first stage baby food. It should be smooth with no bits. Initially it is essential that you only have 1-2 teaspoons, in total, of pureed food at each meal. If you try to eat a portion that is too large or too thick it may cause discomfort or even vomiting. As you progress through the pureed stage, you will be able to tolerate a greater range of foods and eat larger quantities

Since you are unable to eat a large amount of food at one time, it is necessary to eat 4-6 meals a day. When you feel comfortable with pureed food you can start to introduce more solid food into your diet. This is usually about 4- 6 weeks after your operation.

Here are some pureed food suggestions and sample meal plan.

Solid Stage (Progression onto normal foods from weeks 4-6)

The progression from pureed food to a normal diet moves through 3 levels, gradually introducing solid foods starting with the easiest to digest. As you tolerate each level you progress until you can eat most normal foods. When you are able to eat all solid food you should be aiming to follow a healthy eating pattern with 3 meals a day, avoiding high fat and high sugar foods and drinks.

Here are some food suggestions for level 1, level 2 and level 3.

Long Term Eating Habits

It is important that you try to eat healthily in the long term. Although initially you may lose weight even when eating high fat, high sugar meals and snacks, this will not continue. The patients who are most successful are those who change their eating habits to include low fat healthy foods such as more fruit and vegetables. These patients tend to lose weight more quickly, achieve their target weight and, more importantly, keep the weight off for life.

How Much Weight Will I Lose?

Studies show that on average, people lose between 65-75% of their excess weight in the two years after having a gastric bypass operation.

For example

If you weigh 100kgs (15 stone 10lb) and your height is 1.58m, your ideal weight is 63kg (9 stone 13lb). Therefore you have an excess of 37kg (5 stone 11lb) in weight.

With a gastric bypass you could lose 65%-75% of your excess weight (24kg - 27kg), meaning your new weight would be 76kg - 73kg (12 stone – 11 stone 7lb).

Vitamins and Minerals

After a gastric bypass, the levels of vitamins and minerals absorbed into the body are reduced. Therefore, it is recommended that you take the following supplements:-

  • Multivitamin and mineral
  • Calcium with vitamin D
  • Iron
  • Vitamin B12 injections (or tablets)

Occasionally, rapid weight loss can cause an increase in hair loss during the first six months or so. You may be advised to take zinc and selenium supplements for a few months.

What is the follow-up after a gastric bypass?

Surgical and dietetic follow-up is usually after 6 weeks, 3 months, 6 months, 12 months, 18 months and 2 years, although this changes depending on your needs. Surgery is only one piece of your lifetime commitment to weight control and altered habits. It is recommended that you ask your doctor to check your bloods at least once a year after this time for any deficiencies.

What should I do if I experience vomiting

Eating too fast, too much, or food that is too solid may cause vomiting. If it contains pieces of food, you may not have pureed your food sufficiently. Sickness can also occur if you are not eating enough – if you starve yourself you burn off fat too fast and this causes chemicals called ketones to build up in your body and make you feel sick. Remember to eat small meals at regular intervals. If you are sick, return to soups and juices for a day and then restart pureed food. Make sure you keep taking your antacid but stop taking other supplements for a week. If it continues, stay on fluids and contact your doctor, as you may need a change in medication.

What should I do if I have diarrhoea?

Bowels normally return to normal after the operation, although diarrhoea is not uncommon to start with. This usually settles as you restart solids. If it does not settle or it returns, look carefully at what you are eating. Commonly, foods high in sugar (sweets, chocolates, sweet drinks or sugar added to food), or foods high in fat (crisps, snack foods, chips, fried foods, cheese and fatty meals) can cause diarrhoea. Make sure you drink extra fluid if you do have diarrhoea until it settles, but avoid sugary drinks. If your bowel motions become fatty, yellow and difficult to flush, this is always due to eating more fat than the body is able to absorb. It will settle if your diet is adjusted.

What is dumping syndrome?

This is caused by eating too much sugary food such as chocolate. After gastric bypass there is no muscle to keep the food in the stomach pouch for very long, therefore food, especially sugary food travels quickly into the intestine. This is known as dumping and the body responds in two ways.

  1. Excessive water is absorbed back into the gut causing diarrhoea approximately 30 minutes after eating.
  2. The hormone insulin is over produced which causes your blood sugar levels to drop and symptoms of dizziness, light headedness, palpitations and sweating are experienced approximately 1-2 hours after eating.

Many individuals never experience dumping syndrome but for those who do, it usually passes after an hour or so and is most common in the earlier stages after the surgery.

What should I do if I get constipated?

This is due to drinking insufficient fluids, or drinking too much fluid that contains diuretics such as tea, coffee or alcohol. The solution is to drink more fluids, avoid tea and coffee rather than to take laxatives. If they are necessary, do not choose a senna type.


It is important to stay mobile when you first get home, and gentle exercise is encouraged. After 2 weeks you should start to build up a habit of taking regular exercise such as swimming, brisk walking or jogging, or visiting a gymnasium. This should be done slowly and increased as your body allows; ultimately you should aim for at least 30 minutes on at least five days a week. It is safe to use toning tables after 6 weeks. You will notice that your ability to exercise improves very rapidly after the surgery. Apart from speeding up the weight loss by burning off more calories, exercise reduces blood pressure and blood cholesterol and improves muscle tone, which reduces the problems of sagging skin

Food and Fluid Diaries

It is often very useful to record your food and/or fluid intake for a few days every few months to ensure you are keeping to the dietary and/or fluid recommendations. Your dietitian may also ask you to record your intake before your appointment.

You can record your food and fluid intake on this food chart.

You can record your fluid intake on this fluid chart.

Useful Links

National BMI Weight Loss website

British Obesity Surgery Patient Association

Weight Loss Surgery Information and Support

Professor Basil Ammori, Consultant Surgeon