Los Angeles ? Roberta Perry has tried it all to lose the pounds – organized diet programs, prescription pills, psychotherapy, even hypnosis.

Those efforts worked for a while for the Pennsylvania woman, but the weight inevitably crept back up. After years of yo-yo dieting, Perry realized it would take more than gimmicks to slim down.

“As much as I would like to have a magic bullet, I knew the only way to lose weight was eat less and exercise more,” said the 39-year-old public relations consultant.

Her experience is a common one. Researchers at the University of California-Los Angeles, examining 31 weight-loss studies found long-term dieting doesn’t keep the pounds off. While people can lose weight initially, many relapse and regain the weight they shed.

The findings confirm what many scientists have been saying all along: Losing weight is easy. Keeping it off is another story.

“If dieting worked, there would be a bunch of skinny people walking around,” said obesity researcher Dr. David Katz, head of Yale University’s Prevention Research Center, who did not participate in the latest study.

Since the 1970s, the ranks of overweight and obese Americans have risen with two-thirds of adults in that category. Obesity raises the risk of heart disease, diabetes and some cancers. Being overweight increases blood pressure and cholesterol levels which can lead to heart disease.

Many factors can conspire against successful weight reduction, health experts say. Diets can be boring and there’s always a temptation to return to old habits. Serial dieters may also become discouraged and give up when their weight plateaus. People who lose too much too soon don’t learn to make the overall lifestyle changes – eating healthier foods and exercising regularly – that are necessary to keep their weight stable.