A treadmill test given to different age groups showed that as people aged, their aerobic capacity - the amount of oxygen consumed while exercising - declined at higher rates with each passing decade whether they exercised or not.
The researchers knew the rate of decline would worsen with age, but they were surprised by the magnitude, said Dr. Jerome L. Fleg, a cardiologist who is lead author of the study and a medical officer at the National Heart, Lung and Blood Institute in Maryland.
"I guess we were a little disappointed that regular exercise didn't make a difference in the rate of decline," he said.
However, he pointed out that those who exercise still end up ahead because their aerobic capacity was higher to begin with.
"If I start higher, I'm going to end higher," Fleg said. "Having a higher aerobic capacity translates into being more fit."
For the study published in Monday's online edition of the American Heart Association journal Circulation, researchers analyzed treadmill tests from 435 men and 375 women ages 21 to 87 taking part in the Baltimore Longitudinal Study of Aging.
The aerobic capacity was measured in the tests about every four years for a median of 7.9 years.
During their 20s and 30s, the volunteers' aerobic capacity declined at a rate of 3 percent to 6 percent per decade. Those in their 70s and beyond faced a decline of more than 20 percent per decade, the researchers found.
Fleg said that participants filled out a detailed questionnaire on physical activities and they were divided into groups depending on the strenuousness of their exercise. He said that those who exercised more strenuously had a higher aerobic capacity than those who didn't.
"People who have low aerobic capacity may not even be able to make their bed," said Dr. Nieca Goldberg, a cardiologist and chief of Women's Cardiac Care at Lenox Hill Hospital in New York City.
"There is a decline with aging, but older people should be in programs to improve their regular function capacity because it will increase their independence," Goldberg said. "It'll offset some of the decline. What I really don't want people to take away from this study is 'don't bother."'
Researchers said that one limitation of the study was that participants were healthy and able to walk on a treadmill, making it hard to make comparisons with less fit people in poorer health.
Dr. Chuck McCauley, of the cardiology department at the Marshfield Clinic in Marshfield, Wis., said that clinically speaking, he's noticed that people who exercise throughout their lives seem to do a lot better physically at handling the daily activities of life as they head into older age.
"Those who just have sat their entire life seem to age a little bit prematurely," he said.
By Jamie Stengle