RUNNING EXERCISE. TOO MUCH IS BAD FOR THE HEART. EVERYTHING IN MODERATION

I have always been taught that exercise is good for the heart, and I always encourage my patients to exercise, to lose weight, help control diabetes, for cardiac and mental fitness and thought that there is no downside, I suppose short of accident and first time incidences.
Well, I may be wrong all this while.
A new study by the group from University of South Carolina, Columbia, led by Dr Duck-chul Lee presented their paper "Running and all cause mortality risk - Is more better?" at the American College of Sports Medicine 2012 Annual meeting in San Francisco recently. They had studied 52,000 participants in the Aerobic Center Longitudinal Study, through an exercise questionaire. These participants were followed up for 15 years, making this the largest longest cardiac exercise study till date. There were 2984 deaths at the end of study. Overall, there was a 19% reduction in all cause mortality, so exercise is good. But, when they looked into the details, they found that there seemed to be a U or J curve relationship, meaning that the benefits eventually becomes a risk, the more you do. For example, they found that if you jog at 7 miles per hour or less, there was a 27% reduction in mortality, where else if you jog at >8MPH, this reduction was lost. If you ran 5 days a week, there was a reduction and if you ran more than 7 days a week, this reduction is lost. If you ran 10-15 miles a week, there was a reduction in mortality, where else if you ran >25 miles a week, this reduction is lost.
The bottom line as always is, run in moderation. Too much of a good thing may also be bad.
There is some physiological basis for this. There is a paper in the Mayo Clinic Proceedings Vol 87:587 by Dr James O'Keefe, a very senior cardiologist at the Mid-America Heart Institute, studying the effects of vigorous exercise, marathon running and extreme sports on the heart. Dr O'Keefe's group found that too much exercise was associated with increase atria and ventricular fibrosis, which may lead to atria and ventricular arrhythmia. With over-vigorous exercise, there was increase calcium deposits in the heart arteries, and arterial stiffening and also LV diastolic dysfunction.
Of course, it is important to note that each of us have our exercise limits and the numbers quoted are for that population, and may not be the exact number for you. However the message is clear, do not over-exercise. I would think that keeping near where the study by Dr Lee is pointing would be a reasonable thing.
As always in life, do all things well, do it in moderation. Extremes are usually hazardous.