Regular exercise extends life by one to two years or more, a sweeping 16-year study concludes, adding scientific evidence to what has long been speculated.

The study of nearly 17,000 Harvard graduates, all men, found that those who burned at least 2,000 calories a week through exercise, including such things as walking and climbing stairs, had a 28 percent lower death rate than did their less active counterparts.

The longer lives of active people have often been explained with the argument that healthier people tended to exercise more, not the other way around. This new study makes allowances for that factor and still finds that exercisers live longer.

"There are lots of skeptics who say people are active because they are healthy," said Dr. Ralph S. Paffenbarger Jr. of Stanford University, primary author of the study. "I say you're healthy because you're active."

The findings "suggest a protective effect of exercise against all-cause mortality," he reported in Thursday's New England Journal of Medicine.

Life expectancy continued to increase up to 3,500 calories of exercise a week, at which point it leveled off and then dropped slightly, perhaps because of the inherent risks in some exercise. Walking briskly for about 20 miles, or five hours, burns about 2,000 calories. According to the National Institutes of Health, bicycling at 12 mph burns 410 calories an hour; swimming rapidly burns 500; jogging at 7 mph burns 920; and singles tennis burns 400.

The study looked at men who entered Harvard between 1916 and 1950, following their health status and life styles from 1962 to 1978. Almost 70 percent of the exercisers survived to age 80, while only about 60 percent of the nonexercisers did.