Experts have released strong new evidence that physically fit people live longer and even moderate exercise can improve health substantially.

An eight-year study involving 13,344 Americans, the largest study if its kind, found those in top form live longer than people out of shape, researchers reported Thursday in the Journal of the American Medical Association.Compared to the most-fit men, the least-fit men in the study had more than triple the death risk while the least-fit women had more than quadruple the risk, researchers from the Institute for Aerobics Research in Dallas said.

The group that appeared to benefit the most was the least-fit. The greatest difference in death rates was between the least fit and the moderately fit groups. The least-fit were about twice as likely to die as the moderately fit.

"Getting up off our butts and moving around a little bit offers a substantial benefit. You don't have to be a marathon runner," said Steven Blair, who led the study.

"Mr. and Mrs. Average, if they would just get up and turn off the television and do moderate amounts of exercise . . . I think they would get some important health benefits," he said.

Moderate exercise, such as walking briskly for about a half-hour to an hour every day, is probably enough for unfit people to achieve moderate fitness, he said.

Between one-third and one-fifth of Americans would probably fall into the least-fit category, he said.

Physical fitness appeared to reduce the death rate primarily by lowering the chances of developing heart disease or cancer, the researchers said.

The most-fit were between eight and nine times less likely to die of cardiovascular disease than the least-fit while the most-fit were between four and 16 times less likely to die from cancer, the researchers found.