You may have heard about the two runners who dropped dead last month in the Philadelphia Marathon. One was 40 years old…the other just 21. Both were perfectly healthy and in great shape before the run.

It’s just further proof of what we’ve been telling you for years: that lengthy cardio is no good for you.

Two doctors have got together to put this to the clinical test…and found that long distance running increases your chances of heart attack.

And another Harvard doctor says it’s as bad for you as being hit by a truck.

Marathon Myth 2,500 Years in the Making

For mainstream media the ultimate gauge of fitness is running a marathon. Media marketing says that if you can run a marathon you’re in great shape and super healthy.

But people have been dropping dead from excessive cardio for thousands of years.

The term Marathon comes from the Greek myth about a messenger who was sent from the Battle of Marathon to Athens…to announce that the Persians had been defeated. He ran 25 miles without stopping until he got to Athens.

He had enough breath left to shout “we have won” and promptly died.

In the modern world, people die every year from running marathons. You can read more about that in our story here.

But it’s not just marathons that are the problem. Long distance cardio is plenty risky.

Take Jorge Fernandez for example. He’s a former Air Force officer: as fit and conditioned as possible. But he dropped dead after running 13 miles in a half-marathon race in San Antonio. He wasn’t the only one…48 other runners from the same race wound up in hospital too.

Co-runner Sheryl Sculley did complete the half-marathon. And she was shocked by what happened to Fernandez.

“It was very unfortunate,” she says. “He was in great shape.”

A Death Run

Because there are so many incidents like this, we’ve decided to investigate it for ourselves.

We’ve reviewed the best research from the most credible doctors…and found a clear pattern of destruction.

Dr. Eric Larose is a cardiologist Laval University in Canada. He’s been investigating the effect of long-distance training on your health. And he shocked the Canadian Cardiovascular Congress when he reported that “marathon running damage(s) your heart.”

He says it impacts the way the heart organizes itself to survive the stress of long-distance running. The heart is made up of 17 segments, and if one fails, then another picks up the slack. But overexertion for such a long period of time can cause serious problems.

“Exercise reduces cardiovascular risk by a factor three,” says Dr. Larose. But he says long distance running can increase your risk of heart attack seven fold.

He also says the risk doesn’t end on the day of the marathon. Even if you make it through the marathon alive, you can still stuffer temporary heart damage.

“It could take up to three months to completely recover,” he says.

Other top medical experts were surprised by the findings because it changes everything they’ve been told for years.

Dr. Arthur Siege is one of them. He’s Director of Internal Medicine at Harvard’s McLean Hospital in Massachusetts. And he used to run the Boston Marathon…but he doesn’t anymore.

That’s because he now believes long-distance running may be the quickest way to a heart attack.

“Your body doesn’t know whether you’ve run a marathon…or been hit by a truck,” he says.

So why is your body confused?

Dr. Siege says long bouts of exercise releases chemicals that flood your body and that leads to inflammation. He even published a study in the American Journal of Cardiology that shows distance running can raise inflammation levels and markers that are associated with heart attacks.

Putting the Theory to the Test

If you’ve been running long distances for years, you may be at even more risk.

That’s what two doctors found when they put long-distance running to the test.

Dr. Jonathon Schwartz is a health sciences expert from the University of Colorado Health Science Center in Denver. His father is Dr. Robert Schwartz and he’s a cardiology expert from Minneapolis Heart Institute and Foundation. They joined forces to study distance runners.

They looked at how much calcium plaque the runners had on the inside of their arteries. The more plaque in your arteries, the more likely you are to die of a heart attack.

The doctors found that long-term running increases calcium plaque in people’s arteries.

So why does it do that? Dr. Robert Schwartz believes there’s a clear answer.

“You have to consider that these runners may be in a constant state of inflammation, and that may be why we are seeing more plaque,” he says.

When you’re running a marathon, you get tricked into “survival mode.” You release stress hormones and a barrage of other chemicals. Then your body has an inflammatory response for protection. But instead of protecting you, it starts a vicious destructive cycle in your body.

We’ve told you in the past how inflammation is the root cause of many serious diseases and how it’s the major cause of heart disease. You can read more about that connection here.

Despite the emerging evidence, the American Heart Association (AHA) still doesn’t recognize the risks. Dr. Maria Rosa Costanzo is a spokesperson for the AHA. She doesn’t seem to know what the findings mean.

“I’m not sure you can make much from these data,” she says. “We don’t have any idea of the outcomes of these patients.”

There’s one other important question here. Why isn’t the heart holding up to this stress?

You’ve heard before that the heart is a powerful muscle. And muscles get stronger by rebuilding themselves from damage.

But the heart isn’t like other muscles. Your heart’s job is to keep up with the demands of the body. It will try to pump oxygen until it fails. You can survive another muscle failing, but not heart failure.

How You Can Strengthen Your Heart

A body of research shows the best way to exercise is with interval training. It includes short bursts of high-intensity exercise. You don’t have to do it for 45 minutes a day either. Some of the top trainers say 15 minutes of intense work will strengthen your heart and give you a better body than the treadmill.

Research also shows interval training can increase human growth hormone (HGH) naturally and boost aerobic capacity. Mix interval training with the occasional walk and that’ll give you a perfect heart-healthy exercise formula.