The life expectancy of Americans, which has been increasing for more than two centuries, could soon level off or even decline because of the surge in obesity among the country’s young, according to a group of researchers who study aging and obesity.

The projection, to be published today in the New England Journal of Medicine, estimated that if there were no adult obesity, the current average lifespan of about 77 1/2 years would be increased by four to nine months.

Because rates of obesity in childhood have at least doubled in the last several decades, they added, obesity could reduce life expectancy by two to five years sometime in the first half of this century. Premature deaths caused by obesity are likely to surpass any gains that would push lifespan upward, such as improved medical care, they said.

S. Jay Olshansky, senior author of the paper and a professor of public health at the University of Illinois at Chicago, emphasized that an average loss of two to five years is similar in magnitude to the impact of all cancers on the U.S. lifespan.

Several population researchers, however, said they felt the paper made some assumptions that might not be true, resulting in an overly pessimistic projection.

“It describes one possible scenario — not the most credible scenario that exists, but a possibility,” said Roland Sturm, a senior economist at Rand Corp., a Santa Monica, Calif., think tank.

The primary authors of the paper were Olshansky, obesity researcher Dr. David Ludwig of Children’s Hospital Boston and David Allison, an obesity researcher at the University of Alabama.

The scientists used national statistics, gathered from 1988 to 1994, on the rates of obesity among U.S. adults.

They then used other national data to estimate the increased risk of death associated with different degrees of plumpness in different age groups, genders and ethnic groups.

Finally, they related the estimated risks of premature death from obesity to the makeup of the population today.

If anything, the predictions are an underestimate, Ludwig said. Doctors have never seen this rate of obesity-associated diabetes in children, he said, and do not know how having the disorder from so young an age will affect life expectancy.

Sturm and other critics said the study failed to take into account likely advances in medicine — including those to prevent and treat obesity.