Regular aerobic exercise significantly retards the aging process, say researchers at Stanford University.

Doctors at Stanford's school of medicine have tracked 538 older runners for more than 20 years. The runners live longer and are much healthier than their peers who exercise little or not at all.

"If you had to pick one thing to make people healthier as they age, it would be aerobic exercise," said Dr. James Fries, the senior author of the Stanford study, which has been published in the current issue of the Archives of Internal Medicine.

When Stanford began its study of 538 runners over the age of 50 in 1984, many scientists feared the jogging craze, then in full bloom, might do seniors more harm than good, by causing a flood of orthopedic injuries.

But after 19 years, the Stanford researchers found that only 15 percent of the runners had died, compared to 34 percent of a comparably sized control group.

The runners suffered far fewer deaths from heart attacks than did the non-runners, but also suffered fewer deaths from cancer, neurological disease and infections, the researchers found.

And contrary to the prevailing assumption in 1984, the runners did not suffer osteoarthritis at a greater rate than non-runners, and did not require more knee replacements than the non-runners did, Dr. Fries said.

Both runners and non-runners suffered increasing disabilities as they aged, but the onset of disabilities for runners was, on average, 16 years later than for non-runners, Dr. Fries said.

Even though the physical health of the runners declined as they advanced from their 50s to their 70s and 80s, the gap between their health and that of the non-runners widened.

"We did not expect this," Dr. Fries said. "The health benefits of exercise are greater than we thought."

"This is good documentation of what we all know in our guts," said Dr. Vonda Wright, who directs the Performance and Research Institute for Masters Athletes at UPMC's Center for Sports Medicine. "I commend them for doing such a long-term study."

"Running is not only good for our cardiovascular system, muscle strength and bone health, it's also good for our minds," said Dr. Wright, who is training for the Pittsburgh Marathon, which will return next year after a six-year hiatus. "Any activity that prevents artery blockage in our bodies also keeps our [blood] vessels healthy in our brains. One type of stroke is due to artery blockage. And activity releases endorphins in our brains, which is great for our moods, but is also good for our brain's health."

It isn't just running that's good for you, said Dr. Edward Snell, a sports medicine specialist at Allegheny General Hospital, and a team physician for the Pittsburgh Pirates.

"Running's a good thing, and this is a running study," Dr. Snell said. "But it's aerobic activity that's extremely important. Aerobic exercise, if you do it five times a week, decreases death."

You have to exercise a lot -- at 60 to 85 percent of your maximum heart rate for 40 minutes or more five times a week -- to reduce significantly your risk of death, but "even mild activity will decrease the amount of morbidity," Dr. Snell said. "People who exercise have a better quality of life for a longer period of time."

A local example of the benefits of running is Chris Gibson, 55, who manages the Westin Workout Center at the Westin Convention Center Hotel. Mr. Gibson has run in 170 marathons, and coaches the Leukemia Society's local Team in Training program, which prepares Pittsburgh area residents to run the 26.2-mile races.

"I sure as heck don't feel like I'm 55 years old," he said. "My contemporaries seem much older physically and mentally."

Mr. Gibson runs about 50 miles a week. He's held his weight of 160 pounds (on a 5-foot-10 frame) for 30 years.

"That's one of the benefits of staying fit," he said. "Your body has a much stronger control of the intake and expenditure of calories."

The subjects of the Stanford study, now all in their 70s and 80s, have answered annual questionnaires about their ability to perform everyday activities such as walking, dressing and grooming, getting out of a chair and gripping objects. The researchers used national death records to learn which participants died, and why.

At the start of the study, runners ran an average of four hours a week. After 21 years, running declined to an average of 76 minutes per week, but they were still showing health benefits from running, the Stanford researchers said.