By Linda Carroll, NBC News
It would seem that the boom in marathon running might be just what the doctor ordered for a sedentary -- and ever plumping -- nation like ours. But some scientists are now suggesting that even as runners work to trim seconds off their best times, they might also be shaving years off their lives.
There is accumulating evidence that, over the long term, endurance sports like distance running might actually hurt the heart, an editorial in the British journal Heart argues. And that's especially true for aging runners, says the editorial's lead author Dr. James O'Keefe, head of preventive cardiology at the Mid America Heart Institute at Saint Luke's Health System.
O'Keefe points to studies that show subtle signs of heart damage in marathon runners tested right after their races have been run. And backing that up, he says, is data from the "Iron Mouse" study that found scarring in the hearts of mice forced to run long distances every day for four months.
The good news is that the mice seemed to improve after they stopped running and were allowed to return to normal rodent life.
But O'Keefe is worried that distance runners who keep at it year after year after year don't ever give their hearts a chance to heal.
"The heart pumps about 5 quarts per minute when we're sitting," he says. "When we're running it goes up to 25 to 30 quarts. The heart wasn't meant to do that for hours, day in and day out. You end up overstretching the heart and tearing muscle fibers. Up to 30 percent of those who finish marathons have elevated troponin levels, which is a marker for heart damage. That's the marker we look for to see if someone's having a heart attack - it's irrefutable evidence of heart damage."
Of course the levels of troponin found in marathon finishers aren't anywhere near as high as those associated with a heart attack. Nevertheless, O'Keefe argues that over time the damage - and the scarring it creates - adds up. And he points to MRIs of longtime marathoners that show abundant scarring all over the heart.
But the best proof, as far as O'Keefe is concerned, comes from two large studies that provided data on runners and life expectancy. One of those studies followed 52,600 people for up to three decades and included 14,000 runners and 42,000 non-runners.
The good news was that the runners as a group had a 19 percent lower risk of death as compared to the non-runners. The bad news, for runners of marathons and the like, was that those who ran over 20 to 25 miles per week ended up with the exact same risk as the couch potatoes in the study, O'Keefe says.
He argues that the best way to prolong your life is by vigorously exercising - but for no more than an hour a day. The best prescription: run at a comfortable speed for a reasonable distance.
Not so fast, says Dr. Vonda Wright, a professor of orthopedic surgery and founding director of the Performance and Research Initiative for Masters Athletes at the University of Pittsburgh Medical Center and co-author of "Fitness after 40: How to Stay Strong at Any Age."
Wright isn't convinced by O'Keefe's editorial, which she says is more like a "documented blog than an actual study."
The problem, Wright says, is that the big studies cited in the editorial don't have information on people's hearts at the outset. It's entirely possible that some of the people doing the most running also had some issues with their hearts that the running exacerbated.
Besides, Wright says, "it's not like every high level athlete who's been racing for 25 or 30 years dies of heart disease."
Wright points to her dad who just turned 73 and has been running marathons most of his life. "Until a few years ago he could beat my butt," she says.
As for the measurable increases in troponin in runners who've completed a marathon, follow-ups on these runners show troponin levels have returned to normal, Wright says. "I'd hate for this editorial to be translated in the media as a single line: "Running is bad for you and vigorous exercise can kill you," she adds.
That said, Wright does acknowledge that the heart, like any muscle in the body, can sustain over-use injury. If you find you're putting in a lot of miles and you're starting to sustain muscle injuries, you might want to have a "frank conversation" with a cardiologist, she says.
For his part, O'Keefe doesn't want people to think he's against running. He's just for moderation. Marathons are just too much of a good thing, he argues.
"A routine of moderate physical activity will add life to your years, as well as years to your life," he and his co-author concluded. "In contrast, running too far, too fast, and for too many years may speed life's progress towards the finish line of life."