Dieters, despair. A U.S. study has found that diets don’t work in the long run with about two-thirds of dieters putting back the weight they lost - and more - within four to five years.

Researchers from the University of California, Los Angeles, found in the short run dieting was successful, as people could initially lose five to 10 percent of their weight on any number of diets in the first six months of dieting.

“But after this honeymoon period, the weight comes back - and often with more. Sustained weight loss was found only in a small minority of participants,” said Traci Mann, UCLA associate professor of psychology and the study’s lead author.

“I know this sounds quite bleak. We’ve already had some complaints that this will dissuade people from even going on a diet and stop people from trying.”

The study, reported in the April issue of the journal American Psychologist, is based on the analysis of 31 long-term studies on dieting. It comes amid rising obesity rates in the United States, where one third of adults are now obese.

Mann said she and her co-authors wanted to find out what happened to people on diets in the long run so they analyzed every study that followed people on diets for two to five years.

The majority of people - up to two thirds - regained all the weight they had lost, plus more.

“We concluded most of them would have been better off not going on the diet at all. Their weight would be pretty much the same, and their bodies would not suffer the wear and tear from losing weight and gaining it all back,” said Mann.

“Diets do not lead to sustained weight loss or health benefits for the majority of people.”

Mann said the people who did manage to keep their weight down tended to exercise regularly.

Co-author Janet Tomiyama, a UCLA graduate student of psychology, said several studies indicated that dieting was actually a consistent predictor of future weight gain.

One study found that both men and women who participated in formal weight-loss programs gained significantly more weight over a two-year period than those who had not participated in a weight-loss program.

In several studies, people in control groups who did not diet were not much worse off, and in many cases were better off, than those who did diet, she said.

So if dieting doesn’t work, what does?

“Eating in moderation is a good idea for everybody, and so is regular exercise,” said Mann, who is planning to study whether exercise is the key factor leading to sustained weight loss.