Everyone will lose some weight on a diet if they eat fewer calories than they use, but there is a group of people who will rapidly regain all their lost weight when they stop their diet. They usually then overshoot their starting weight. It is almost impossible for these people to maintain adequate weight loss with dieting alone.

Some mechanical measures to enforce dieting - such as jaw wiring and inflating balloons in the stomach - can work for a short period, but afterwards most people regain their lost weight.

A number of different types of operations have been developed to treat obesity over the years. These basically fall into two groups. The first group of operations bypass most of the small intestine (the top part of the gut) to stop food from being absorbed. This type of operation is rarely used now because it can cause failure of the liver. The second group of operations is designed to reduce the capacity of the stomach by stapling across it, so that less food is eaten. In Leeds, we were unhappy with the results of simple 'stomach stapling' because many people regained their lost weight after a few years, or had considerable problems with vomiting.

We have therefore adopted the 'gastric bypass procedure'. This combines reducing the size of the stomach with a perfectly safe degree of bypass of part of the small intestine. This operation gives better results than standard stomach stapling, and does not carry the risk of liver disease seen with pure bypass procedures. We calculate the length of gut that is bypassed very carefully, and there have been no liver problems in any of our patients using this procedure. Also, people who undergo the gastric bypass operation are less likely to regain their weight later than with other types of stapling operations.

An initial consultation is carried out to see if a person is considered suitable. If they are, additional tests may be needed to make sure that there is no glandular cause for the obesity. We usually find that these tests (such as thyroid and adrenal gland function tests) have been carried out in the past.

Some people will require a separate overnight stay in hospital to monitor their breathing while they are asleep, if there is concern that they may suffer from obstructive sleep apnoea. If a person has obstructive sleep apnoea, it does not mean that they cannot have the operation, but they will need special monitoring afterwards.

Patients are admitted to hospital on the day before the surgery takes place and have preoperative blood tests and X-rays.

There is a high incidence of gallstones in obese people, and there is a risk of forming gallstones after the operation even if they were not present before. For this reason patients are routinely offered a 'cholecystectomy' (removal of the gall bladder) at the time of their operation. The gall bladder is an unnecessary organ, and gallstones can cause pain and sometimes damage the pancreas and liver. We have found that taking out the gall bladder during the operation does not slow down recovery at all, and permanently takes away the risk of gallstones developing.

Two to four days after the operation, a special X-ray is taken to ensure that the internal stitching and stapling is functioning properly. After this, the patient then starts drinking and eating - initially a pureed diet. Most people stay in hospital for six days after the operation. Solid food is then reintroduced step by step with careful supervision over the next few weeks.

After the gastric bypass operation, weight is lost through three separate mechanisms, which work in combination with one another:

1. The stapling part of the operation reduces the capacity of the stomach, which cuts down the size of meals which can be eaten. After the procedure, people still feel hungry and enjoy their food, but feel full after a much smaller meal than before the surgery. Also the operation is designed so that this smaller stomach empties more slowly than the normal stomach. This means that the person does not start to feel hungry sooner, despite having eaten a smaller meal.

2. Normally, the stomach empties its contents into the top of the gut - where special enzymes digest the food. After this procedure, the stomach's contents pass straight to a point further down the gut, then travel some distance before enzymes digest the food. This cuts down the number of calories that are absorbed by the body.

3. After the operation, when someone eats too much sugar (such as a large amount of chocolate), the body is tricked by the sugar into releasing too much of a hormone called 'insulin'. This causes blood sugar level to fall, which makes the person feel light-headed and queasy. This leads to a genuine distaste for the wrong types of food and acts as a built-in mechanism to prevent people over-eating the wrong types of food.

These three effects - smaller meals, poorer absorption of calories, and less craving for sugary foods - produce a large weight loss.

Patients can take more exercise after the procedure, for example because they can breathe more easily, and have less pain in their back, knees and hips. Also, self esteem improves as weight falls, so that people recover their confidence and enjoy going out again and no longer feel self-conscious about visiting the local swimming pool or jogging in the park. Taking more exercise helps burn more calories, leading to extra weight loss.

Most of the weight loss will occur in the first two years after the procedure. As a rule, most people will lose two-thirds to three-quarters of their excess weight during this time, largely in the first year. It is possible to reach ideal body weight; this usually requires an extra effort towards the end of the two years after the procedure. Studies have shown that there is little tendency to regain excess weight later, as long as patients remain sensible about their diet.

As well as weight loss itself, the medical complications of obesity also improve and may clear up completely. Blood pressure and blood cholesterol levels get better, and the risk of heart disease falls to normal. Symptoms such as incontinence and arthritis also improve rapidly. Also, many of our diabetic patients have come off all their diabetic drugs.

Patients are independent and can move around when they leave hospital. Most people can drive after two weeks and go back to work after three to four weeks.

After the procedure, pureed food is given first, then solid food is gradually started in stages. Some people are eating a variety of solid food after two to three weeks, others take things more slowly. After six to eight weeks most patients are back to normal food, but eating smaller amounts than before. It is important to drink enough fluid, since we normally get a large proportion of our water from food, and smaller meals mean that more water will have to be drunk rather than eaten. Patients stay closely in touch with our dietician during this period, and come back regularly to the clinic for check-ups.

After the operation, a sensible, healthy diet is important. Small meals satisfy the hunger, but nibbling all day and snacking on high calorie foods should be avoided. Meals should be taken at fixed meal times, and once a person feels full, they should not come back to try to finish their meal off an hour later. We keep in touch with patients long term and help make sure that their diet is suitable for them.

We routinely prescribe vitamin and mineral supplements after the operation, together with a tablet to reduce the amount of acid produced by the new stomach.