Intensive exercise prevents the shortening of telomeres - the end pieces of DNA that prevent damage - says research published in Circulation: Journal of the American Heart Association.

As cells divide over a lifetime, the telomeres gradually get shorter, leading to cellular ageing.

By studying groups of athletes and people who do not exercise, the researchers found that those who take part in fitness training have longer telomeres, which they say is because their bodies produce the enzyme telomerase.

"This is direct evidence of an anti-ageing effect of physical exercise," said Ulrich Kaufs, lead author and professor of clinical and experimental medicine in the department of internal medicine at Saarland University in Homburg, Germany.

"Physical exercise could prevent the ageing of the cardiovascular system, reflecting this molecular principle."

Low-impact fitness training may also help stave off conditions such as osteoarthritis by protecting cartilage, findings from the University of California, San Francisco revealed this week.