"Yes, it is almost certain that the risks of these various diseases will rise as obesity rises in the population, but you also have to assume that the medical sciences will get better at treating some of these complications," said Dr. Rudolph L. Leibel, an obesity researcher at Columbia University. "Certainly doing that is going to end up costing more, but it may not end up stripping months or years off life."

An editorial in the same issue of The New England Journal, written by Dr. Samuel H. Preston, a demographer at the University of Pennsylvania, raises similar questions. It suggests that the predictions of decreased life expectancy might be "excessively gloomy," given potential advances in medicine and genetic engineering, and the reduction of harmful behaviors like smoking.

Dr. Preston concludes, however, that "the rising prevalence and severity of obesity are capable of offsetting the array of positive influences on longevity."

The report's lead author, Dr. S. Jay Olshansky, a professor of public health at the University of Illinois, Chicago, said he considered the report's projections of reduced life expectancy to be "very conservative, and I think the negative effect is probably greater than we have shown."

"Hopefully, we can fix obesity so that our projections are wrong," Dr. Olshansky added. "But we're seeing such large increases in obesity in the last couple of decades that it's hard to imagine that we're going to be able to work fast enough."

Estimating the number of obesity-related deaths is controversial, too. Last November, the Centers for Disease Control and Prevention said its earlier estimate that 400,000 people die annually from obesity was inflated. A revised, lower estimate is expected soon. The New England Journal report uses an estimate of 300,000 deaths, which some experts contend is still too high.

The report projected life expectancy by calculating how much longer people would live if "everyone who is currently obese were to lose enough weight to maintain an optimal" body-mass index, a measure of the relationship between a person's height and weight. The authors believe it is more accurate than other projections.

The report comes at a time when the country is embroiled in a debate over Social Security. While the report's authors say they started their research long before the current debate, they write that "the U.S. population may be inadvertently saving Social Security by becoming more obese" and dying sooner, but that "this 'benefit' will occur at the expense of the economy in the form of lost productivity before citizens reach retirement and large increases in Medicare costs associated with obesity and its complications."