After analyzing the effects of 31 studies on the longterm effects of a number of diets, scientists in the U.S. say not only do they not work but they can also put a person's health at risk.

The researchers say dieting often leads to long-term weight gain and although many lose weight within the first six months, within five years most are heavier than when they first started dieting.

The study involved thousands of dieters and the researchers reached the conclusion that most would have been better off not bothering in the first place.

It seems that as a rule most people on diets lose 5 to 10 per cent of their starting weight in the first six months but ultimately up to two-thirds regain more weight than they lost.

The study did not name any diets in particular, but in one 50% of dieters weighed more than 4.99kg (11lbs) over their starting weight five years after the diet.

Traci Mann, professor of psychology at the University of California, Los Angeles and lead author of the study, says most of them would have been better off not going on the diet at all and their bodies would have been excused the wear and tear of losing weight and gaining it all back.

Professor Mann says diets do not lead to sustained weight loss or health benefits for the majority of people.

The researchers caution that the habit of repeatedly losing and gaining weight is suspected to be linked to health problems such as heart disease and stroke.

Professor Mann says they found that the majority of people regained all the weight, plus some more and only a small minority of participant managed to sustained the weight loss.

Complete weight regain was found in the majority which accounted for 80 per cent of the dieters who were followed for more than two years.

This estimate is thought to be conservative and the real figure is probably far higher.

Professor Mann is of the opinion that eating in moderation and exercising regularly was by far the best strategy to take.

The researchers say in their report that 93 percent of patients achieved different periods of insulin independence and treatment-related toxicity was low, with no mortality.

The only severe adverse effects were pneumonia in one patient and endocrine dysfunction in two others.

The team believe the treatment re-set the immune systems of the patients, stopping, at least temporarily, the onslaught on the pancreatic islet cells and allowing some of them to regenerate and produce insulin on their own.

As yet the research is in the preliminary stage and the researchers have no hard evidence as they have not tested this by examining the pancreases of the patients.

However 14 patients became insulin-free, one for nearly three years, four for two years and others for at least several months.

Other research is being carried out in this field and experts are optimistic that cell transplants will in future be used to treat diabetes.

The report is published in the Journal of the American Medical Association.