Most people who have spent a lot of time in the gym, on the trail or in the pool have heard of someone who keeled over dead, or near dead, while exercising.

The early years of the running boom had the shocking example of Jim Fixx, who died while running at age 52 in 1984, seven years after the publication of his popular “The Complete Book of Running.” (An autopsy revealed severe coronary artery disease.) More recently, ultramarathoner Micah True, a hero of Christopher McDougall’s 2009 book, “Born to Run,” died on a run at age 58; at autopsy, his heart was grossly enlarged. And there’s the original marathoner, ancient Greece’s Pheidippides, who is said to have run 25 miles from Marathon to Athens, yelled “Nike!” (“Victory!”) and collapsed dead.

Can exercise kill you? For an unlucky few, the answer is yes. Should that scare people away from such endurance sports as marathons, triathlons, bike races and open-water swims? Most experts would say, “Definitely not.”

“I don’t think there is danger out there. There’s no study that shows that endurance athletes live shorter lives,” said Paul D. Thompson, chief of cardiology at Connecticut’s Hartford Hospital, who has studied the effects of extreme exercise.

That said, there are some interesting asterisks.

There’s little doubt that when a person exercises vigorously — hard enough to raise the pulse and respiratory rates — he or she has a transiently higher risk of sudden death than when sitting at rest. For joggers in Rhode Island from 1975 to 1980, it was one death for every 792,000 hours of exercise. In a chain of health clubs with 3 million members, it was one death for every 2.57 million workouts. A study of 10.9 million participants in half- and full marathons over a 10-year period found one cardiac arrest for every 200,000 participants. (About a quarter of the victims survived).

While such events are impossible to predict, one can guard against them by . . . exercise. The “habitually sedentary” are 50 times as likely to die during vigorous exercise as are people who exercise that way more than five times a week.

The overwhelming evidence, however, is that physical fitness brings longevity. A study of 15,000 former Olympians from nine countries found that they lived, on average, three years longer than the general population. And 2,600 elite Finnish athletes lived six years longer than army recruits, who themselves were a fit group.

Nevertheless, a few studies suggest there is a J-shaped curve in the relationship between exercise and mortality, with the most extreme exercisers at greater risk than the somewhat less fanatical. (It should be called a “backwards-J-shaped curve,” as that’s how it looks.) In most studies, however, the trend is not statistically significant.

Other studies have found that some marathoners leak cardiac enzymes (indicative of damage to heart cells) after races. Some endurance athletes have unexpectedly large amounts of calcium in their coronary arteries. Some studies show a higher incidence of atrial fibrillation in endurance athletes compared with less-extreme exercisers.

These findings baffle researchers. There may be a subset of people for whom endurance training is hazardous, and one day it may be possible to identify them. Until then, experts warn against making too much of the studies.

“Until we have stronger evidence that this is a harmful practice, we cannot tell people to stop doing high-level exercise,” said Erin D. Michos, a 41-year-old preventive cardiologist at Johns Hopkins University.





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