Specifically, obesity raised the risk of death from heart disease, diabetes and kidney disease, and several cancers previously linked with excess weight, including breast, colon and pancreatic cancer.
But being merely overweight — having a BMI between 25 and 30 — did not increase the risk of dying from heart disease or any kind of cancer.
Also surprising was that overweight people were up to about 40 percent less likely than normal-weight people to die from several other causes including emphysema, pneumonia, injuries and various infections. The age group that seemed to benefit most from a little extra padding were people aged 25 to 59; older overweight people had reduced risks for these diseases, too.
Why extra fat isn't always deadly and might even help people survive some illnesses is unclear and in fact disputed by many health experts.
But University of South Carolina obesity researcher Steven Blair, who says people can be fat and fit, is a believer. He called the report a careful and plausible analysis, and said Americans have been whipped into a ''near hysteria'' by hype over the nation's obesity epidemic.
While the epidemic is real, the number of deaths attributed to it and to being overweight has been exaggerated, Blair said.
People should focus instead on healthful eating and exercise, and stop obsessing about carrying a few extra pounds or becoming supermodel thin, Blair said.
He says his hefty grandmother used to justify her extra padding, saying, '''That way I have protection in case I get sick.' Maybe there is something to that.''
A little extra weight might provide ''additional nutritional reserves'' that could help people battle certain diseases, Flegal said.
Dr. Robert Eckel, a spokesman for the American Heart Association, argued that the results may be misleading. For example, diabetes and heart disease often occur together and both often afflict overweight people. So when diabetes is listed as a cause of death, heart disease could have contributed, he said.
Eckel also said the study results might reflect aggressive efforts to treat high blood pressure and cholesterol or other conditions that can lead to fatal heart attacks. Those conditions often occur in overweight people and can be costly and debilitating even if they aren't always deadly, he said.
Obesity researcher Barry Popkin of the University of North Carolina in Chapel Hill, agreed, noting that the study ''is about death. This is not about health and sickness.''
It doesn't address whether cancer and heart disease occur more often in overweight people — something that has been suggested by other research.
Dr. Michael Thun of the American Cancer Society noted that staying slim tops a recent list of recommendations for preventing cancer in a report from the World Cancer Research Fund and American Institute for Cancer Research. The report was based on a review of more than 7,000 studies.
The CDC report ''definitely won't be the last word,'' Thun said.
Manson, the Harvard researcher, cautioned that extra pounds can lead to obesity so people shouldn't be complacent about being overweight.
Laurie Slocum, who went from a size 20 to a size 12 after joining Weight Watchers two years ago, says the study won't turn her into a slacker.
A 47-year-old banker from Durand, Ill., she lost more than 60 pounds and still has a few to go. Thanks to dieting and exercise, her blood pressure has dropped from ''the stroke zone'' to normal. She said she feels too good now to use the new findings as an excuse to indulge.
''It's not going to change anything I'm doing,'' Slocum said. ''The number on the scale isn't my goal ... it's a healthy lifestyle.''