It is the news that couch potatoes have been waiting for: too much exercise is bad for your heart.
Doctors have warned that, rather than adding years to their lives, fitness freaks could be working themselves into an early grave.
They say that while regular, moderate exercise, including jogging, is beneficial, it is possible to do too much.
After reviewing various studies on the health of runners, they said: ‘A routine of moderate physical activity will add life to your years, as well as years to your life.
‘In contrast, running too fast, too far and for too many years may speed one’s progress towards the finish line of life.’
In a three-page editorial in the journal Heart, US cardiologists James O’Keefe and Carl Lavie, both keen runners, also advise that those who want to exercise at full pelt should limit themselves to 30 to 50 minutes a day. And marathons should be viewed as something to do occasionally or once in a lifetime, rather than a regular challenge.
They warn that exercising intensely for more than an hour or two can damage the heart, causing its tissue to stretch, tear and scar and raising the odds of dangerous changes in heart rhythm.
Two studies recently presented at major medical conferences back up their argument that exercise, like many other things in life, is best done in moderation.
The first, co-written by one of the editorial’s authors, tracked the health of more than 50,000 people for up to 30 years.
The 14,000 runners in the study were 19 per cent less likely to have died than the others during this time.
However, closer analysis of the results revealed the longevity benefits to be limited to those who ran between five and 20 miles a week.
In contrast, those who ran more than that did not seem to fare any better than non-runners. The same study revealed running quickly to also be of little benefit, with those who fared best doing a ‘comfortable jog’ of 6-7mph.
Similarly, running between two and five days a week was better than pounding the pavements daily. The second study, of 20,000 Danes, found slow jogging to add most to life expectancy.
The editorial, by James O’Keefe, of Saint Luke’s Mid America Heart Institute in Kansas City and Carl Lavie, of the John Ochsner Heart and Vascular Institute in New Orleans, concludes: ‘The unique and potent effects of exercise are best bestowed by moderate physical activity.
‘If one’s goal is to compete in the marathon or triathlon of the Rio Olympics in 2016, this will certainly require high-intensity exercise for hours a day. But for those whose goal is to be alive and well while watching the 2052 Olympics from the stands, then exercise and physical activity at lower intensities would be more ideal.’
Paul Thompson, a sports cardiologist and former elite marathon runner, told the Wall Street Journal the editorial’s authors had manipulated the data because they had an agenda.