MARATHON runners and fitness fanatics risk shortening their lives by exercising too much, a new study has warned.
Cardiologists in the US found that runners who exercise strenuously for more than three hours a week may unwittingly damage their health and reduce their life expectancy.
They said those who stick to moderate workouts – amounting to two to three hours a week – benefit more than people who do either extreme exercise routines or no exercise at all.
Experts from the Cardiovascular Research Institute in Pennsylvania announced their findings after studying more than 3,800 male and female runners, with an average age of 46.
But Irish fitness experts said runners should not be unduly put off by the findings.
They said Irish people should maintain a balanced approach to their training regimes and use common sense.
The US researchers took into account the medication that participants used, and whether they had high blood pressure, high cholesterol, and if they smoked.
Almost 70pc of those who took part in the study said they ran more than 20 miles a week.
The scientists concluded that the medical factors and smoking habits could not be used to explain why people who ran the furthest had the shortest lifespans.
However, the institute's co-director, cardiologist Martin Matsumara, said people should not just stop running.
"Runners in general enjoy longer and better health," he said.
Although researchers could not pinpoint the optimal dose of running for health and longevity, they said they believed the results proved that too much running was harmful to the health of the heart.
Dr James O'Keefe, director of preventive cardiology at the Mid-American Heart Institute in Kansas City, said the results could be explained by the "wear and tear" inflicted on people's bodies when they do a lot of exercise.
Dr O'Keefe, who reviewed the research, advised joggers to aim for about two-and-a-half hours of slow to moderate paced running a week.
He added: "If you want to do a marathon, run (just) one and cross it off your bucket list."
Irish fitness expert Karl Henry said: "Too much of anything is bad for you and that includes too much long-distance and endurance running. The heart will grow with too much training. Striking a balance is important.
"Using 'down time' for more passive exercise like pilates or yoga will help to counter-balance too much long-distance sport. I train for marathons and I'm interested in ultra-marathons, so it is important to have down time.
"Everybody is different and it depends how one's own body is built. If it feels wrong, you shouldn't be doing it," said the personal fitness trainer.
But he added that for each new medical study, there is probably another study saying "the very opposite".
And Frank Greally, editor of 'Irish Runner' magazine, said that ever since the running boom began in the 1980s there have been those who have issued similar warnings.
"These warnings are as old as running itself. Just like new nutrition reports that tell us we're eating the wrong stuff.
"I never recommend that anyone takes up a running programme without first getting a thorough medical examination, regardless of their age," he said.
"Running brings undeniable great benefits to people's lives, physically, socially, and for mental health too. People view it as bringing life to their years rather than adding years to their lives."