03/20/2012 08:41 am ET Updated Dec 06, 2017

Can Too Much Cardio Hurt Your Heart?

Marathon running has taken off in the last few years, and for good reason. Pounding the pavement offers amazing aerobic benefits, not to mention providing a great outlet for beating stress. However, new research indicates that over the long haul, endurance exercise could end up doing more damage than good.

Running Into Problems -- The Need-To-Know
Running is often viewed as a way of protecting against cardiovascular disease, but it turns out runners might be at a higher risk for heart problems than previously thought. In one study, researchers used an imaging tool to look at the hearts of healthy endurance athletes that had taken part in multiple marathons and ultra-marathons (definitely not just weekend warriors). The results were surprising, to say the least. When compared to their non-runner counterparts, the aerobic gurus showed a high prevalence of heart stiffening. The problem with that: A stiffer heart is less efficient at pumping blood around the body -- not the reward expected after logging countless miles on the roads.

Research on furrier subjects (read: rats) showed similar stiffening of the heart muscles after intensive endurance training. The culprit seems to be deposits of collagen, better known as scarring, left behind after years of hard training. Worse than the usual battle scars, though, those on the heart don't contract, which can disrupt normal heart rhythm -- sometimes dangerously so. And if that's not enough, research also found athletes who exercised the most demonstrated decreased heart efficiency (most notably in the right ventricle, responsible for pumping blood through the lungs for uploading of oxygen) after a tough race or training bout.

The good news: In the majority of cases, the heart's efficiency returned to normal within a week of crossing the finish line without any consequences. With repeated bouts of long distance training, however, research indicates the changes may be more permanent.

Going the Distance -- The Answer/Debate
But exactly how much endurance training is too much? The majority of studies pinpoint 10 years as the time frame over which adverse effects can occur, but those years have to be filled with training sessions that would make even Lance stop and stare. Also keep in mind the athletes exhibiting stiffening were participating in at least 10 hours of intense endurance training per week and competed in extreme events like ultra-triathlons (a 3.8 km swim, 180 km cycle, then a full marathon). That's a far cry from the typical weekend 5K!

So don't part with the pavement just yet. Aerobic exercise still offers tremendous health benefits including boosting memory and brain function. Instead of logging long, slow workouts every day, consider supplementing one or two workouts a week with interval training. The short bursts provide similar cardiovascular benefits in a fraction of the time. And don't forget to pencil in those rest and recovery days. When it comes to endurance exercise, more certainly isn't always better!

Experts' Take
This article has been read and approved by Greatist Experts Dr. John Mandrola and Linda LaRue. Here's a little more on what they had to say:

John Mandrola: "The overwhelming majority [of Americans] exercise far too little. In fact, I believe the U.S. suffers from severe exercise-deficiency. That said, however, accumulating data suggest the possibility of an upper limit of what the human heart can sustain. Each study on extreme exercise has its limitations. They enroll small numbers of subjects and are almost always non-randomized. And, studying exercise is tough because of the many confounding variables: genetic make-up of individuals, the presence of underlying diseases and self-reporting of exercise amounts are just a few of many examples.

But when taken together en bloc... it looks like optimal health is born and nurtured through balance. And there's little about running (many) marathons or slogging through Ironmans that could be called balanced. Fun maybe, for some. Balanced? Heart-healthy? No way."

Linda LaRue: "It's important to take small studies or those not from top tiered research journals with a grain of salt. That said, most of the general exercising population are not ultra-marathoners. If you do happen to be that .001 percent, consult your physician if you have concerns. He or she can set up a treadmill stress thalium to properly evaluate your heart's pumping ability."

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