If we were to tell you that studies now show that running anything beyond 20 miles a week could kill you, you’d cut back from your 50-, 60-, 80-mile-a-week habit straightaway, right? Yeah. Sure you would.

For years, athletes, particularly those of you known to scientists as EEs (Extreme or Endurance Exercisers)—marathoners, ultrarunners, triathletes—have tended to eat whatever you damn well please, under the assumption that all calories being more or less equal, if you’re burning 3,500 a day, you can have that bacon double cheeseburger and the vanilla shake no problemo. Turns out, not only has that been magical thinking dietetically, but even more disheartening, all those extra miles may have been doing you more harm than good—if your only goal is to live as long a life as possible.

And it may not even have everything to do with what you’re eating or how much. (Though it certainly has an effect.) The issue may be in how hard you’re pushing yourself. “People on the far level of exertion may be putting themselves at risk for mortality,” says Dr. Paul D. Thompson, an 11-time Boston Marathon finisher and director of cardiology at Connecticut’s Hartford Hospital.

What got lost in all the hype over last month’s Wall Street Journal story, “Why Runners Can’t Eat Whatever They Want,” was the real issue buried inside it: that the atherosclerotic risk EEs may be subjecting themselves to has less to do with diet and more to do with what they’re overdoing. The theory is, says Dr. Thompson, “If something’s good for you, more is better. So, the more exercise, the less heart disease.” Well, everybody’s susceptible—no matter how much you exercise.

Like an ultramarathoner’s gonna cut back on their mileage. Then again, maybe they should—if longevity is their primary concern (it isn’t). Picking up on Ralph Paffenbarger’s landmark Harvard Alumni Health Study, which took a longitudinal look at the exercise routines and rates of cardiovascular disease of 17,000 Harvard alum, other researchers have shown that “the benefit of exercise becomes less and less as you exercise more,” says Dr. Thompson. “To the point where there may be no benefit at all.

What’s misleading about the Missouri Medicine study cited in the WSJ, wherein researchers found that 50 men who had run at least one marathon a year for 25 years had higher levels of coronary-artery plaque than a control group of sedentary men, was that these EEs likely “had, for one, a very unhealthy diet,” says Dr. Aaron Baggish, associate director for the Cardiovascular Performance Program at Boston’s Massachusetts General Hospital. “They also led an unhealthy life in their 20s and 30s. And they were at high risk for hereditary heart disease before they even started running.”

As contradictory as that may appear—which is worse, eating whatever you want because you exercise all you want, or simply exercising all you want?—it’s likely evidence for conducting a study on EEs who have no significant cardio issues prior to late-age exercising and who’ve always eaten a balanced diet.

And lost in all the schadenfreude among the sedentary, who finally got to thumb their noses at the EEs, is that, while exercise is good for you (and way better than sitting on your ass and praising yourself for it), “Marathon running puts extreme stress on the body,” says Dr. David Katz, director of Yale University’s Yale-Griffin Prevention Research Center. “That’s exercising too much.”

“The difference between running 75 miles a week and 100 is not that significant,” adds Dr. Thompson. “When you go out to these extreme levels of health, you’re doing it for something other than just exercise.”

Which, for many an EE, is exactly the point.

“The take-home message is that exercise is good for you,” says Dr. Thompson, who adds that another ongoing study of Tour de France riders shows that they’re living longer and seemingly without much damage to their hearts. “It could be there’s less benefit once you get over some threshold. We just don’t know yet what that threshold is.”