But 40 years ago, Americans could expect to live slightly longer than Europeans, said the study jointly conducted by researchers from University of Southern California (USC), the Harvard School of Public Health and the RAND Corp., a non-profit think tank.

　　In addition to Western Europeans, Americans also die younger than the residents of most other developed nations, according to the study appearing in the July issue of Social Science & Medicine.

　　The life-expectancy disparity, which begins around the age of 50, stems from higher levels of middle-age obesity and obesity-related chronic diseases, such as hypertension and diabetes, said the study.

　　In the first half of the last century, average life expectancy increased by saving more babies, said author Dana Goldman, director of the Schaeffer Center for Health Policy and Economics at the USC.

　　"But now it is reduction in mortality among the elderly, rather than the young, that propels increases in life expectancy," he said. "The question is whether 'being American' is an independent mortality risk factor."

　　If 50-year-old U.S. adults could be as healthy as Europeans, it could save Medicare and Medicaid 632 billion dollars by 2050, the study said.

　　Though the transition to better health initially raises expenditures, the researchers estimate that by 2050 healthcare savings from health improvements among the middle age could total more than 1.1 trillion dollars.

　　"The international life expectancy gap appears much easier to explain than gaps within countries: there is no American-specific effect on longevity beyond differences in disease at age 50," said Darius Lakdawalla, an associate professor in the USC School of Policy, Planning and Development.