Bill Wenmark has run 104 marathons and personally taught more than 4,000 first-time marathoners, with another class of graduates ready to lace up their running shoes for next weekend's Grandma's Marathon. He isn't about to be scared away from either endeavor.

But Wenmark, founder of the Twin Cities-based ALARC running club, is watching with increasing angst as the medical community voices new concerns about marathon running — along with other so-called "extreme fitness" events such as Iron Man triathlons, boot-camp inspired obstacle courses and long-distance bike and ski racing. The concerns have nothing to do with joint and bone injuries.

Some doctors now say marathoning can cause heart damage.

The evidence is far from conclusive, causing Wenmark, and other experts to warn that the warning itself is potentially dangerous because it could be used as an excuse for not exercising.

"People who are anti-exercise are going to start with the 'I told you sos,' " he said. "If you have someone who wants to run and a spouse who's not supportive, you're going to hear: 'See, I told you so.' "

The worries about endurance exercising come as participation is booming. The number of marathoners jumped 50 percent between 2000 and 2010, according to figures gathered by Johns Hopkins University. While there are no statistics on how many of those marathoners also participate in other endurance sports, the turnout for other such events is surging.

And now it's a different ilk of endurance athlete, said Dr. Bradley Bart, chief of cardiology medicine at Hennepin County Medical Center in Minneapolis.

"If we go back 10 years, the people running marathons trained ardently for that," he said. "Now, with [endurance sports] becoming so popular, we're seeing all these weekend warriors with much less training. They're going to have a higher risk than a well-conditioned, well-trained person doing the same race."

The reports raising questions about marathoning started appearing in medical journals last summer but have only recently caught the attention of the mainstream media, where headlines have been blaring. Take this one in the London Telegraph: "Too many marathons can kill."

The research is not nearly that definitive. A study published in the journal Mayo Clinic Proceedings cited "emerging data" suggesting that long-term endurance exercise "may induce" cardiovascular irregularities. In a nutshell, the study said that endurance athletes experience temporary changes in their hearts that, in most cases, return to normal within a week of the race. However, for a few of the racers, permanent scarring, irregular heartbeats and calcium build-ups occur.

The report, which was not prepared by Mayo personnel, included a major caveat in its opening paragraph. "This concept is still hypothetical and there is some inconsistency in the reported findings," the authors said. "Furthermore, lifelong vigorous exercisers generally have low mortality rates and excellent [cardiovascular] functional capacity."

Dr. Tom Allison, director of sports testing and sports cardiology at the Mayo Clinic, said the studies are intriguing but not definitive.

"They're based on observational data, and studies based on observational data always have their issues," he said. "We're still missing that large, randomized study that follows people for 10 or 15 years."

The heart of the issue

Allison, who was a nationally ranked marathoner as a young man, said that the studies make sense from a theoretical standpoint.

"A typical heart pumps five liters of blood a minute when you're at rest," he said. "When you exercise hard, it will pump 15 to 20 liters a minute, and the heart stretches to accommodate the increased volume. But it's reversible. When you slow down, the volume decreases. And once you stop, it returns to normal."

He added: "If you do this for long periods of time — two, three or four hours straight — for 20 years, there's a chance that the change in the structure of the heart could become permanent."

One of the problems he has with the studies is that they have focused only on athletes.

"There could be a reporting bias," he said. "If you're running, you're much more likely to be aware of a change in your heart rhythm than if you're sitting on a couch, watching TV. Runners check their pulse and monitor their performance [in terms of their speed]. If my performance drops, I'm going to notice it. But if I'm just sitting, I'm not going to notice anything, and that means that I'm not going to report it."

Bart, who participates in 30-mile cross-country ski races, said there still are some basic questions that the marathon studies need to address.

"We know that, in general, marathon runners live longer," he said. "What we don't know is how much of that is self-selection. Are they healthier because they exercise more, or do they exercise more because they are healthier?"

To run or not to run

Bart shares other experts' concerns that the studies can be twisted into an argument against exercising. "The bottom line here is that exercise is good for you," he said.

He does recommend that anyone considering any sort of endurance sport check with their doctor first. Plus, one of the most important things to exercise is common sense, he said.

"All things in moderation," he said. "Listen to your body."

Allison seconded that theme.

"There does seem to be some evidence [in the studies] that obligatory running — feeling the need to run every day no matter what — increases risk," he said. "Never giving your body a rest doesn't make a lot of sense. The body needs time to recover from stress."

Wenmark, whose ALARC organization is the nation's largest marathon club, insists that he's not endorsing endurance sports as an exercise regimen for everyone.

"I don't think everyone should run marathons," he said, pointing out that there are also joint and muscle issues that make marathoning a bad idea for some people. "But I think there's a lot of misinformation in these reports."

He's worried that such information could cause an overreaction in the other direction.

"It's the yin and yang," he said. "This is the yin. A year from now, we might get the yang and there will be studies saying that everybody should run a marathon. I'll take just as much umbrage about that."