> I wonder how much of this is pre-existing heart conditions that went undiagnosed versus running _causing_ heart damage. The sample size might be too small, b

> I wonder how much of this is pre-existing heart conditions that went undiagnosed versus running _causing_ heart damage. The sample size might be too small, but I'd love to see a comparison of heart-failure events among ultra runners to other very intense exercise. Conditions like Marfan Syndrome are obvious candidates for these deaths: it's often undiagnosed, it significantly increases risk for sudden heart failures like aortic dissection (especially in younger people), and it produces a progressive risk with intense exercise (by weakening heart tissues which then stretch under stress). Which, oddly, is both pre-existing and exercise-induced; running wouldn't do the same thing without the condition, but a sufferer who didn't run as hard might never have a problem. Directly attributing the problem to conditions like that is probably impossible; there are a bunch of them and there's no guarantee they'll be diagnosed or even caught at autopsy. But a cross-sport comparison might turn up interesting data - breath-restricting sports like swimming and scuba diving are higher risk than running.