By FIONA MacRAE

Last updated at 12:32 10 April 2007

The world's largest study of weight loss has shown that diets do not work for the vast majority of slimmers and may even put lives at risk.

More than two-thirds pile the pounds straight back on, raising the danger of heart attack, stroke and diabetes.

Indeed most dieters end up heavier than they did to start with, the researchers found.

They warn this type of yo-yo behaviour is linked to a host of health problems. And they say the strain that repeated weight loss and gain places on the body means most people would have been better off not dieting at all.

The findings follow other research that shows the UK is in the grip of a dieting frenzy, with one in four Britons at any one time trying to lose weight.

The average woman is estimated to lose and gain 251/2 stone during her lifetime - putting on 151/2 stone for the ten stone she loses through dieting. Last night, the U.S. scientists behind the latest research - the most thorough and comprehensive analysis of its kind - said that dieting simply does not work.

The University of California researchers analysed the results of more than 30 studies involving thousands of slimmers.

Although the overview did not name specific weight loss plans, popular diets in recent years include the low carbohydrate, high protein Atkins diet and the GI diet, which is rich in slow-burning wholegrain carbohydrates.

Pooling the results of the various studies clearly showed that while people do lose weight initially, most quickly put all the pounds back on.

In fact, most people end up weighing more than they did to begin with. Researcher Dr Traci Mann said: "You can initially lost 5 to 10 per cent of your weight on any number of diets.

"But after this honeymoon period, the weight comes back. We found that the majority of people regained all the weight, plus more. Sustained weight loss was found only in a small minority of participants, while complete weight regain was found in the majority."

Dr Mann's research showed that up to two-thirds of dieters put on all the weight they lose - and more - over a four to five-year period. Half of those taking part in one study were more than 11lb heavier five years later, while dieters taking part in another study actually ended up heavier than other volunteers who hadn't tried to lose weight.

A four-year study into the health of 19,000 men revealed that most of those who put on weight had dieted in the years before the start of the study.

Bleak as these figures seem, the true picture could be even worse, as it is thought that most people lie about their weight - and don't like to tell researchers that their weight has started to creep up again.

The analysis, published in the journal American Psychologist, concluded dieters may actually be damaging their health.

Research has shown the repeated rapid weight gain and loss associated with dieting can double the risk of death from heart disease, including heart attacks, and the risk of premature death in general.

Such yo-yo weight loss has also been linked to stroke and diabetes and shown to suppress the immune system, making the body more vulnerable to infection.

Dr Mann said: "We decided to dig up and analyse every study that followed people on diets for two to five years. We concluded most of them would have been better off not going on the diet at all.

"Their weight would have been pretty much the same, and their bodies would not suffer the wear and tear from losing weight and gaining it all back.

"The benefits of dieting are simply too small and the potential harms of dieting are too large for it to be recommended as a safe and effective treatment for obesity."

The psychologist, who advises would-be slimmers to swap calorie-controlled diets for a balanced diet coupled with regular exercise, added: "Exercise may well be the key factor leading to sustained weight loss.

Studies consistently find that people who report the most exercise also have the most weight loss."

The finding comes as Britain fights a rising tide of obesity.

A growing reliance on fast food and time-saving technology has led to the UK developing the worst weight problem in Europe, with almost a quarter of adults classed as obese.

Last night, British experts said that fad diets do not work and that the key to maintaining a healthy weight is making gradual, long-term changes.

Dr Beckie Lang, of the Association for the Study of Obesity, said: "Maintaining a healthy weight isn't about going on a diet and coming off a diet when you reach your target weight. It is about adopting skills that change your eating habits for life."

• CASE STUDY

Mother-of-two Chris Siveter is a self-confessed serial dieter. Over the last four decades her weight has fluctuated between 11 stone and 17 stone and her dress size between 12 and 22.

She first started dieting after putting on three stone during pregnancy.

The pounds fell off and she fitted into size 14 clothes again for the first time in years.

But her resolve did not last and she piled the weight back on.

She tried countless diets including a grapefruit diet, the Atkins diet and even a Cabbage Soup diet but all failed after she went on "sweet rampages".

In 2003 her weight soared to its highest point - 17 stone.

She has now taken up aerobics and joined Slimming World and at 58, is 11 stone and a size 12 - hoping her days of yo-yo dieting are over.