CHICAGO — The growing epidemic of obesity could cause the first drop in Americans' life expectancy in modern times, according to a new study that suggests weight problems could cancel out life-extending benefits from medical advances in the coming decades.

Today's children, who are becoming obese at unprecedented rates, will suffer the greatest loss of longevity, according to the report. A national team of experts on aging and obesity compiled the estimates, led by S. Jay Olshansky, a biological demographer at the University of Illinois, Chicago.

"The take-home message is that obesity clearly needs to be considered in an entirely new light; it is far more dangerous than we ever thought," Olshansky said.

Obesity already is taking away up to nine months of life, on average, from life expectancies in the United States, the study found. That figure could reach five years or more in the first half of this century if the United States cannot reverse the current trend of rising obesity rates.

"We don't realize the situation we're putting ourselves in by pushing our children to higher and higher weights," said Olshansky, whose study was published in today's issue of the New England Journal of Medicine. "They will carry the effects of this with them throughout their lives."

A drop in life expectancy would halt centuries of steady gains in longevity and potentially shift debates over programs such as Social Security. Government projections for the program assume Americans will continue to live longer lives and draw more money from the system. Life expectancy in 2003 stood at 77.6 years — up from 77.3 in 2002, according to the Centers for Disease Control and Prevention (CDC).

But if average life expectancy stagnates, that would reduce the amount of money needed to keep entitlement programs afloat.

"It's not the way you want to fix Social Security," Olshansky said.

Other researchers welcomed the study as the first credible projection of obesity's long-term damage to society. But some doubt the losses caused by obesity will outpace expected longevity gains from new treatments and other advances.

"I expect life expectancy to keep chugging upward as it has been," said Ronald Lee, a professor in the department of demography at the University of California, Berkeley. Lee said that while the new work underlines that obesity is a serious threat, past crises such as tuberculosis and AIDS have not slowed the broader improvement in longevity.

And some researchers said the study's underlying assumptions were excessively pessimistic. The study assumed, for example, that everyone who is overweight will have health problems.

"I don't think this is based on solid, scientific ground," said Glenn Gaesser, a professor of physiology at the University of Virginia who frequently questions the effects of obesity. "It's nonsensical, really."

The new calculations are a stark contrast with Social Security Administration forecasts for slow improvement in life expectancy and with projections publicized in 2002 that said the average life expectancy could reach 100 in about 60 years. Olshansky said he hoped the new research would play a role in the discussion about changing Social Security.

James Vaupel, director of the Max Planck Institute for Demographic Research in Rostock, Germany, and a research scientist at Duke University, co-authored the 2002 forecast, based on data from developed nations including the United States. Vaupel called the new report "very one-sided" and said he doubts that obesity will negate the effects of other medical progress in improving mortality.

But the researchers said their estimates were conservative in several ways, including that they focused exclusively on adults. The true effect is likely to be much greater as obese children age and begin suffering elevated rates of diabetes, heart disease and cancer, they said.

"It's sort of like a massive tsunami heading toward the shoreline," said David Ludwig, an obesity expert at the Harvard Medical School and Children's Hospital in Boston who worked with Olshansky. "It's going to peak in a massive public-health crisis."

At least two-thirds of Americans are overweight, including about one-third who are obese. Obesity significantly increases the risk of a variety of serious health problems, including cancer, heart disease and diabetes.

What's unique about obesity is its huge and skyrocketing effect on the young, the report's authors said. Weight problems have more than quadrupled for some groups of young people since the 1970s.

Any forecast of how long people will live decades from now carries deep uncertainty. The study's leaders said that although their work previews only one possible future, it makes use of vital information lacking in other projections that have ignored the effect of obesity.

The new report is based on past studies of death rates, which have shown that people who are obese or overweight have their lives cut short by five to 20 years. Obesity can speed death by contributing to heart disease, diabetes, stroke and many other conditions. People who became obese as children tend to have especially early deaths.

Olshansky and his colleagues plugged that data about obesity's effects on individuals into an estimate of how obesity is pulling down life expectancy for the United States as a whole. They calculated that if the problems of overweight and obesity were to vanish tomorrow, that would increase overall U.S. life expectancy by four to nine months.

That may not sound like a lot, but it's greater than the effect on life expectancy of all accidents, homicide and suicide, experts said.

Even if researchers were to cure all forms of cancer, that would increase overall life expectancy by three years, less than the potential negative effect of obesity in the future.

Material from The Associated Press

and The Washington Post is included

in this report.