Marathon running appears to cause heart damage. 12% of asymptomatic marathon runners had evidence of myocardial damage. Only 4% of the sedentary controls had damage.

This Breuckmann study repeatedly found about a 2.5 to 1 increased incidence of heart attack from marathon running over sedentarism. The number of marathons run was an independent and significant predictor of the likelihood of myocardial damage.

http://www.paleonu.com/panu-weblog/2009/11/1/cardio-causes-h...