Cardiologists have a lifesaving warning about your heart: Too much running can permanently damage it - and may even kill you! That’s the word from world-famous cardiologist Dr. James O’Keefe, the head of preventative cardiology at the Mid America Heart Institute.

Dr. O’Keefe recently issued a bombshell report in the medical world, warning that excessive running can cause arrhythmias, heart scarring and artery damage – putting you on the fast track to an early death. Now, Dr. O’Keefe says that running for an hour or less a day is safe – and improves your health. But a growing number of people are hooked on trendy ultra-running, where they run for hours a day. And a lot of people are jumping on the marathon train, and competing in dozens of marathons a year. But more and more studies show that can be dangerous.

Researchers recently forced mice to run long distances every day for four months. At the end of the study, every single one had damaged hearts. In another study, researchers tested runners’ hearts right after they completed a marathon. And they all suffered major heart damage.

So, why is too much running deadly? Dr. O’Keefe says that our heart was designed to work intensely for short periods at a time – not hours a day. In fact, our heart pumps about five quarters of blood per minute when we’re sitting. But when we run, it shoots up to six times that! And that constant pumping stretches our heart and tears muscle fibers, putting us at risk of developing everything from arrhythmias to permanent scarring.

So, stick to running for an hour a day. And if you’re training for a marathon or long race, give your body a few weeks of rest post-race, so your heart has time to recover.