The federal analysis failed to exclude smokers and people who were already ill. 'That can lead to serious underestimates of mortality linked to overweight and obesity.'

But in interviews, the federal researchers said that they stood by their conclusions.

The study under attack was published last month by researchers at the Centers for Disease Control and Prevention and the National Cancer Institute.

It concluded that people who are overweight but not obese have a lower death risk than people of normal weight.

The scientists also reported that being very thin increased the risk of death, even if the thinness was longstanding and not a result of illness.

Death Risk from Obesity Increased Continually

In a seminar and news conference Thursday in Boston, the critics said their own studies found that the death risk from obesity increased continually over the full range of body weight.

Citing the Nurses' Health Study, which has followed 120,331 women since 1976, Dr. Frank Hu, an associate professor of nutrition and epidemiology at Harvard, noted that as body mass index increases, "the death rate increases dramatically."

He and a colleague, Dr. JoAnn Manson, said the federal analysis had failed to exclude smokers and people who were already ill.

"That can lead to serious underestimates of mortality linked to overweight and obesity," she said.

No Mortality Risk from Being Overweight

But Dr. Katherine Flegal of the CDC, lead author of the contested paper, said she and her colleagues had analyzed their data in a variety of ways.

They looked at the results both with and without people who had chronic diseases, and current or former smokers.

The results always came out the same: There was no mortality risk from being overweight and little with obesity, unless the people were extremely obese.