Exercise is a good thing, except when it is bad thing, which can be simply too much of the good thing. Gretchen Reynolds writes in the Well column about research published in the Journal of Applied Physiology that compared the heart health of a group of extreme endurance athletes (men who had run more than 100 marathons and others like them) with the heart health of a group of younger endurance athletes and a group of less extreme amateur athletes. “None of the younger athletes or the older nonathletes had fibrosis in their hearts,” Reynolds writes. “But half of the older lifelong athletes showed some heart muscle scarring. The affected men were, in each case, those who’d trained the longest and hardest. Spending more years exercising strenuously or completing more marathon or ultramarathon races was, in this study, associated with a greater likelihood of heart damage.”
And because, as it is with rats so it is with humans, Reynolds cites a study in which researchers prodded young, healthy male rats to run at an intense pace, day after day, for three months, -equivalent of about 10 years in human terms. At the end of the training period, heart scans showed that most of the rodents had developed diffuse scarring and some structural changes, similar to the changes seen in the human endurance athletes. A control group of unexercised rats had developed no such remodeling of their hearts.