Older people who go running several times a week walk with the same vigour as a typical 19 or 20-year-old, a study has shown.
But those who only exercise by walking are likely to tire more easily, researchers found.
"The bottom line is that running keeps you younger, at least in terms of energy efficiency," said Prof Rodger Kram of the University of Colorado at Boulder.
The study involved 30 healthy volunteers with an average age of 69 who either ran or walked regularly for exercise.
They were asked to walk on a treadmill at three different speeds - 1.6mph, 2.8mph and 3.9mph.
During the training sessions, measurements were taken of their oxygen consumption and exhaled carbon dioxide.
"We found older adults who regularly participate in highly aerobic activities have a lower metabolic cost of walking than older, sedentary adults and also lower than seniors who regularly walk for exercise," said Prof Justus Ortega of Humboldt University in California.
"It's been known for a long time that as people age, their maximum aerobic capacity, or 'horsepower', declines, and that is true for runners as well.
"What's new here is we found that old runners maintain their fuel economy."
Volunteers who ran regularly expended the same energy as a normal 19 or 20-year-old.
However, the amount of energy used by non-runners for walking was typical of older, sedentary adults - about 22pc higher than it was for 20-somethings.
"It was quite surprising to find that older adults who regularly run for exercise are better walkers than older adults who regularly walk for exercise," said Owen Beck, another member of the University of Colorado team.
"The take-home message of the study is that consistently running for exercise seems to slow down the ageing process and allows older individuals to move more easily, improving their independence and quality of life."