several hundred thousand Americans participate.

But some may also remember that the author of “The Complete Book of Running,” Jim Fixx, died of a massive heart attack while running. His genetic and pre-exercise lifestyle choices notwithstanding, his untimely demise served as a sober reminder of the importance of regular medical check ups even for people who regularly engage in aerobic exercise. Marathon-related deaths made headlines in November 2007 when 28-year-old Ryan Shay died while competing in New York in the men’s marathon Olympic trials. Statistics show that for every million participants in these 26.2-mile running races, there will be four to eight deaths. The rate for triathletes is far higher – 15 out of a million, a new study shows. Almost all occurred during the swim portion, usually the first event.

As for the rise in popularity of triathlons, Virginia sports lawyer Doug Landau notes that many D.C. area races sell out overnight, with only the expensive charity slots left open, which frequently are purchased by less competitive athletes. “It’s something someone just signs up to do,” often without a medical checkup to rule out heart problems, said Dr. Kevin Harris, a cardiologist at the Minneapolis Heart Institute at Abbott Northwestern Hospital.

“Anyone that jumps into freezing cold water knows the stress on the heart,” said Dr. Lori Mosca, preventive cardiology chief at New York-Presbyterian Hospital and an American Heart Association spokeswoman. She had no role in the study but has competed in more than 100 triathlons, including the Hawaii Ironman competition. Cold water constricts blood vessels, making the heart work harder and aggravating any pre-existing problems. It also can trigger an irregular heartbeat. On top of this temperature shock is the stress of competition. “It’s quite frightening – there are hundreds of people thrashing around. You have to keep going or you’re going to drown,” Mosca said.