Extreme exercise and heart health

"Each time the suggestion that too much exercise is harmful makes the news 'it invariably rockets around the cybersphere powered by schadenfreude.'"

As this article in the New Yorker concludes, the jury is still out on how much exercise is too much. However, this medical study of 18000 runners in Copenhagen, "published in theAmerican Journal of Epidemiology, confidently concluded that, when compared to non-joggers, runners lived, on average, five to six years longer." 

The evidence suggests that there may be a diminishing return to vigorous exercise, beyond about 45 minutes per day.

And when it come to more extreme forms of exercise, like the marathon, preparation is key: "the degree of detectable damage was lowest, or completely absent, among those who had trained by running more than forty-five miles a week, as opposed to those who ran thirty-five or fewer."