I'm convinced that too much exercise is unhealthy for the heart. The debate for a long time has been, "what defines too much exercise?" I don't think we've answered that question fully yet, but we're seeing more and more data to suggest where we may need to draw that line. Most of these research studies are being presented at cardiology conferences such as ACC, AHA, and international events that attract cardiologists, cardiac electrophysiologists, and other heart specialists.
A recent story on Runner's World titled, "New Studies: More Exercise Could Produce Bad Health Outcomes," summarizes some recent clinical research studies including a Swedish study that tracked 44,000 men for 12 years to see if extreme endurance running increases the risk for atrial fibrillation. A different study was a German study of 1038 patients who had enrolled in cardiac rehab programs because they had heart disease.
The bottom line is that both exercise intensity and duration play a key factor in reducing or increasing the overall health of our hearts. Too much of a good thing can be bad for us. So, what level of exercise intensity and duration is detrimental? How much running (or bicycling, or swimming) is too much that it causes harm to our heart?