About two years ago, a group of federal researchers reported that overweight people have a lower death rate than people who are normal weight, underweight or obese. Now, investigating further, they found out which diseases are more likely to lead to death in each weight group.

Linking, for the first time, causes of death to specific weights, they report that overweight people have a lower death rate because they are much less likely to die from a grab bag of diseases that includes Alzheimer's and Parkinson's, infections and lung disease. And that lower risk is not counteracted by increased risks of dying from any other disease, including cancer, diabetes or heart disease.

As a consequence, the group from the Centers for Disease Control and Prevention and the National Cancer Institute reports, there were more than 100,000 fewer deaths among the overweight in 2004, the most recent year for which data were available, than would have expected if those people had been of normal weight.

Their paper is published today in the Journal of the American Medical Association.

The researchers also confirmed that obese people and people whose weights are below normal have higher death rates than people of normal weight. But, when they asked why, they found that the reasons were different for the different weight categories.

Some who studied the relation between weight and health said the nation might want to reconsider what are ideal weights.

''If we use the criteria of mortality, then the term 'overweight' is a misnomer,'' said Daniel McGee, professor of statistics at Florida State University.

''I believe the data,'' said Dr. Elizabeth Barrett-Connor, a professor of family and preventive medicine at the University of California, San Diego. A body mass index of 25 to 30, the so-called overweight range, ''may be optimal,'' she said.

Others said there were plenty of reasons that being overweight was not desirable.

''Health extends far beyond mortality rates,'' said Dr. JoAnn Manson, chief of preventive medicine at Brigham and Women's Hospital in Boston.

Dr. Manson added that other studies, including ones at Harvard, found that being obese or overweight increased a person's risk for any of a number of diseases, including diabetes, heart disease and several forms of cancer. And, she added, excess weight makes it more difficult to move about and impairs the quality of life.

''That's the big picture in terms of health outcomes,'' Dr. Manson said. ''That's what the public needs to look at.''

Researchers generally divide weight into four categories -- normal, underweight, overweight and obese -- based on the body mass index, which is a measure of body fat based on height and weight. A woman who is 5 foot 4, for instance, would be considered at normal weight at 130, underweight at 107 pounds, overweight at 150 pounds and obese at 180.

In this study, those with normal weight were considered the baseline and others were compared to them.

The federal researchers, led by Katherine Flegal, of the Centers for Disease Control and Prevention, said the big picture they found was surprisingly complex. The higher death rate in obese people, as might be expected, was almost entirely driven by a higher death rate from heart disease.

But, contrary to expectations, the obese did not have an increased risk of dying from cancer. They were slightly more likely than people of normal weights to die of a handful of cancers that are thought to be related to excess weight -- cancers of the colon, breast, esophagus, uterus, ovary, kidney and pancreas. Yet they had a lower risk of dying from other cancers, including lung cancer. In the end, the increases and decreases in cancer risks balanced out.