At a time when obesity is growing at an alarming rate, an increasing number of people are turning to gastric bypass surgery to solve their weight problem. But just how successful is gastric bypass surgery in terms of weight loss and can it really make a dramatic change to your life?

Gastric bypass has been around for more than fifty years now and, while there are of course risks as there are with any surgical procedure, in the vast majority of cases patients are more than satisfied with the results and enjoy a dramatically improved standard of living. But there is a price to pay.

Following a gastric bypass patients will need to adjust to a very different lifestyle and this can be hard unless adequate preparation is made in advance of surgery to ease patients gently into a post-operative regime.

Some changes are of course obvious. The basic principle behind gastric bypass surgery is to drastically reduce the size of the stomach and physically restrict the amount of food that the patient can eat and so patients clearly understand that the days of sitting down to a big meal are over. But other consequences of surgery are less obvious.

Even in small quantities the days of eating foods that are high in sugar or fat are also over. The consequences of eating such foods can be extremely unpleasant as the rapid absorption of these foods in the now shortened digestive tract can lead to very unpleasant feelings of faintness.

Patients also find that the dramatic change in their eating pattern also leaves them very short of water and they must adjust to constantly drinking small amounts of water throughout the day to avoid dehydration.

This fairly dramatic change in lifestyle is all well and good but just what can gastric bypass achieve in terms of weight loss?

There is of course no simple answer to this question as results will vary from person to person. As a guide however we need to start by understanding just how post-operative weight loss is measured.

The starting point is to assess just how much excess weight the patient is carrying. This is done by working out the patient's ideal weight. Measured in pounds, for a man this will be 106 plus 6 times his height in inches less 60. If that sounds complicated then here's an example. For a man 5ft 10ins tall his height in inches is 70. Deduct 60 from this and multiply the result of 10 by 6 to give you 60. Finally, add 106 and 60 together and the ideal weight for a man of 5ft 10ins is 166 pounds.

For a woman the principle is the same but this time a women's ideal weight is 100 plus 5 times her height in inches less 60.

Taking the example of our man above, if before surgery he weighs 366 pounds then his excess weight is 200 pounds. Weight loss is then measured in terms of the percentage of excess weight lost over time. So, if after 6 months he has lost 100 pounds then his weight loss will be 50 percent. In other words, at that point he will have lost 50 percent of his excess weight.

As a general guide the average patient can expect to lose about 50 percent of their excess weight within 6 months of surgery rising to 70 percent one year after surgery and to 80 percent after 2 years. For the majority of patients weight loss will not continue beyond 2 years and indeed some long-term weight gain will appear after 2 years, typically about 10 to 15 percent of the patient's excess weight.

Again, as a general rule, patients who are excessively overweight will lose a greater percentage of the excess weight (perhaps as much as 90 or 95 percent) while people who are less overweight may lose at little as 60 percent within 2 years of surgery.

It is interesting to note that patients very rarely lose 100 percent of their excess weight and thus do not achieve their ideal weight as a result of surgery. For this reason, it is sometimes said that gastric bypass cannot be said to be a complete success. The overwhelming majority of patients would not however agree with this statement.

While they may not reach their ideal weight and may have to condition themselves to a very different lifestyle following surgery, for most patients the results achieved and the improvement in their quality of life is simply unimaginable.