Of course, there are caveats. The number of hardcore runners in the study was quite small, for one thing, consisting of barely 80 men and women. So any statistical information about death rates among that group must be viewed cautiously, as the scientists acknowledge.
And perhaps most important, the researchers did not determine how and why the runners and nonrunners had died. So it is impossible to draw any conclusions about what deleterious effects, if any, hard and prolonged exercise might have on our bodies. There could be scarring or other impacts on the heart muscle after years of strenuous exercise, the Danish scientists suggest, though that possibility remains completely speculative at the moment.
So the message of this study remains that sweaty exercise is generally healthy and desirable – but a little sweat goes a long ways. Even slow jogging counts as “vigorous exercise,” Mr. Marott said and, as this study showed, can lengthen life spans.