As challenges go, the tri-athlon is hard to beat. A gruelling combination of three sports rolled into one — swimming, cycling and running — it is, to many, the ultimate fitness goal. It’s also the fastest-growing sport in the UK.

There are 350 triathlon clubs and 600 triathlon events held around the country each year. About 12,000 people will take part in the London Triathlon in August, making it the largest event of its kind in the world.

But, after a spate of deaths in the event, experts are beginning to question whether the demands of the sport are simply too great for the average person.

In a recent study presented at the American College of Cardiology Scientific Sessions conference researchers reported how the risk of dying in a triathlon is nearly double the risk of dying in a marathon.

Dr Kevin Harris, of the Minneapolis Heart Institute Foundation, who led the study, says that it is a “not inconsequential” risk of 1.5 per 100,000 participants. Harris, who studied 2,846 triathlons in the US, found that there were 14 deaths among the 922,810 competitors. Of these, 13 happened during the swim (one man died after falling from his bike) but none during the run. Harris says that the average age of those who died was 43.

So why does the swim leg hold such increased potential for disaster? The most plausible theory is that the open-water swim triggers a particular type of cardiac arrhythmia caused by the genetic condition long QT syndrome.

Dr Michael Ackerman, a cardiologist at the Windland Smith Rice Sudden Death Laboratory, at the Mayo Clinic, Minnesota, who has been studying the triathlon link, says that about 1 in 2,000 people is born with a condition that can cause a blip in the heart’s wiring, with long QT syndrome being the most common.

“In someone with long QT syndrome, the heart is sluggish between beats and these delayed intervals can give rise to skipped beats,” Ackerman says. “When that happens, the heart’s electrical system can go so badly off-course that it leads to a potentially fatal arrhythmia.” In his studies Ackerman has identified several types of long QT syndrome, one of which is particularly affected by swimming.

Some researchers think that part of the problem is that triathlon swims are held in open water that is often much colder than the pools in which the competitors train. They suggest that the surge in adrenalin during the swim — the first part of a triathlon — could be problematic. Certainly, the swim is usually the most chaotic, with starters jostling for position, possibly causing some to panic.

Dr Ese Stacey, the medical director of the London Triathlon, says that the risk must be kept in perspective. “There is a risk of sudden death from an underlying heart problem if you walk down the street,” she says. “And the risk in triathlons is no greater than that.”

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