Conventional wisdom says you can pretty much eat whatever you want if you’re an avid runner, as your body is burning off all of the excess calories. But is that really true? New research now suggests that avid runners need to pay attention to their diets as well when it comes to the potential for heart disease.

As a 10-mile-a-day runner, Dave McGillivray thought he could eat whatever he wanted without worrying about his heart. “I figured if the furnace was hot enough, it would burn everything,” said McGillivray, who is 59.

But a diagnosis six months ago of coronary artery disease shocked McGillivray, a finisher of 130 marathons and several Ironman-distance triathlons. Suddenly he regretted including a chocolate-chip-cookie recipe in his memoir about endurance athletics.

“My first reaction was, I was embarrassed,” he said.

As race director of the Boston Marathon, McGillivray is a high-profile exhibit in a growing medical case against the devil-may-care diets of many marathoners. Their high-mileage habit tends to lower their weight, blood pressure, heart rate and cholesterol levels, leading them (and sometimes their doctors) to assume their cardiac health is robust regardless of diet.

“‘I will run it off’—that attitude clearly prevails among the marathoners themselves, almost sometimes to an arrogance,” said Paul Thompson, a veteran marathoner who is chief of cardiology at Hartford Hospital.

A growing body of research shows the error of that thinking. A study published in the current edition of Missouri Medicine 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. A British Medical Journal study published this year compared the carotid arteries of 42 Boston Marathon qualifiers with their much-less active spouses. “We hypothesized that the runners would have a more favourable atherosclerotic risk profile,” says the article. As it turned out, that hypothesis was wrong.

Many assumed that extreme-endurance sports could help prevent heart disease, but now the research suggests this extreme activity may actually cause problems.

We should be careful of course to jump to conclusions after several studies, but certainly this raises questions and challenges old assumptions.

It also brings us back to some common sense notions that moderation in diet and excercise can be the best combination. Pushing anything to the limit – whether its your diet or your activities, can lead to risks.