Overweight people who are otherwise healthy may increase their risk of dying by intentionally losing weight, according to provocative new research.

A study of 2,957 twins in Finland found that those who were overweight who lost weight on purpose were about 86 percent more likely to die for any reason over the next 18 years compared with those whose weight remained stable.

The researchers cautioned, however, that only 268 people in the study died, a number too small to justify any firm recommendations about whether overweight people should try to lose weight.

Skeptics who think health authorities have exaggerated the risks of being overweight said the findings offer fresh support. Other researchers, however, said that the study's weaknesses undercut the findings, and that there is overwhelming evidence that being overweight increases the risk of a host of health problems.

The researchers who conducted the study said they hope the results will stimulate more research into the relative risks and benefits of losing weight.

"We think that our findings are scientifically significant because they clearly demonstrate that the relation between intentional weight loss and subsequent health effects is complex and needs much more research," said Thorkild I.A. Sorensen of Copenhagen University Hospital.

Many previous studies have found that people who lost weight appeared to have an increased risk of dying over the long term, but the prevailing wisdom has been that the weight loss in these studies was the result of illness, because people who are ill often lose weight. Still, some evidence have suggested that intentional weight loss may be unhealthy for otherwise healthy people, perhaps, for example, because losing muscle could have detrimental effects on vital organs.

To try to tease that out, Sorensen and his colleagues studied twins who had undergone detailed questioning in 1975 about various health issues, including whether they intended to try to lose weight. Over the next 18 years, those who were overweight and lost weight on purpose ended up being more likely to die than those who did not slim down, and the risk increased with the amount of weight lost, the study found. The increased risk held true even after the researchers took into consideration factors such as how much the subjects exercised, what they ate and whether they had any health problems.

"We cannot say that they should not lose weight, only that we, due to our findings, are in doubt about the possible adverse long-term effects," Sorensen said in an e-mail.

The study was published in the Public Library of Science journal PLoS Medicine, one of a new set of scientific journals that are free to the public. Sorensen said his team previously tried to publish the study in other journals, including the New England Journal of Medicine and the Journal of the American Medical Association.

"We were left with the impression that they perhaps did not like to distribute such [a] message," Sorensen wrote.

Other researchers praised the study for exploring the provocative question.

"It is one of the few studies that addresses this issue," said Katherine Flegal of the federal Centers for Disease Control and Prevention, who ignited a controversy in April when she published a study that concluded that her agency had overestimated the negative effects of obesity. "It suggests there may be more to this than people think."

The findings should counter blanket recommendations that weight loss is always beneficial, other researchers said.

"There is such a cultural assumption that weight loss is necessary that we ignore the evidence," said Linda Bacon of the University of California at Davis. "This is an important study that adds to the body of evidence that we need to question the concept of encouraging weight loss."

But other researchers said the study has a number of weaknesses. The researchers , for example, failed to fully account for the possibility that illness may have gone undetected.

"I don't think this study is in any way definitive -- far from it," said David Allison of the University of Alabama at Birmingham.

There is overwhelming evidence that losing weight reduces the risk of a host of serious health problems, including heart disease, cancer and diabetes.

"If you put it all together, you can say with certainty that weight loss in obese people is warranted and likely to be beneficial, and weight loss in overweight people is probably also healthy as long as it is accomplished by medically sound ways," Allison said.

Some researchers worried that the findings might discourage people from trying to lose weight.

"There are so many people who are struggling to lose weight, and if you give them any indication that it's harmful, that's certainly going to sap motivation," said Lawrence J. Cheskin, director of the Johns Hopkins Weight Management Center in Baltimore.