“Normally, the two responses don’t happen at the same time, but when they do, the heart can go into abnormal rhythms, which can cause sudden cardiac death,” Mike Tipton, who runs the Extreme Environments Laboratory at the University of Portsmouth, wrote in a commentary for the British Journal of Sports Medicine.

Runners should maintain their pace or slow down in the last mile and not sprint unless they have trained for it, the International Marathon Medical Directors Association said in 2010 in response to race-related sudden deaths.

Running appears to lower mortality risk when an athlete doesn’t cover more than 20 miles a week, exceed 5 to 7 mph or run more than two to five times a week, researchers at the Ochsner Health System in New Orleans and the University of South Carolina found last year.

More than 2 million Americans participate in long-distance running races each year, a number that has doubled since 2000. Even though the risk of death from running marathons is small, increased participation has resulted in a higher incidence of sudden death at the events, according to a study published last year in the New England Journal of Medicine. Of 11 million long-distance runners, 59 suffered cardiac arrest, 51 of them men.

By middle age, most people have developed some underlying early-stage vessel disease, such as hardening or plaque buildup in their coronary arteries, said Kade Davison, who teaches clinical exercise science at the University of South Australia in Adelaide.

“If anyone is going to have a cardiac event, they’re far more likely to have one during exercise,” said Davison. A person is seven times as likely to have a heart incident while exercising as they are while at rest, he said.

Tears of the heart

Intense exercise for periods longer than one to two hours can cause overstretching and tiny tears of heart tissue, said James O’Keefe, a sports cardiologist and head of preventative cardiology at the Mid America Heart Institute in Kansas City, Mo. Such repeated injury over years can cause irregular heart rhythms, increased inflammation, scarring and stiffening of the arteries, he said.

Athletic overachievers tend to think that more is better, though “moderation is almost always best,” O’Keefe said.

As a precaution, getting a computerized tomography, or CT, scan of the heart to look for calcified plaque is a good way for endurance athletes to check if their workouts are putting their heart at risk, O’Keefe said. Yet there is no agreement on what the best strategy is for testing.

“The throwaway line is to consult your doctor to make sure you’re fit to race,” La Gerche said. But “in healthy asymptomatic people, there is no good test to see if someone is at risk of sudden death.”

World Triathlon, owner of the sport’s Ironman-branded events, made changes to the swim portion of select races after an increase in competitor deaths in recent years, the company said last month.

Some events no longer feature a mass swim start. Athletes at those races will either enter the water in a continuous stream through an access point, with their time starting when they cross a timing mat, or in staggered waves.

O’Keefe advises his patients, especially those older than 45, to run no more than 20 miles a week, spread out over three to four days.

“That’s not to say you can’t get problems when you’re under 45,” O’Keefe said. “But you’re much more susceptible when you’re over 45 because it just takes longer for your body to recover, and when you hammer it day in and day out, it just takes a toll on your body.”