The risk of dying in a triathlon is almost double that of dying in a marathon, finds a new study on the subject.
The triathlon races involve three phases - swimming, cycling and running, which can be of varying lengths from the "sprint" to the punishing "iron man" involving a 2.4-mile swim, 112-mile bike ride and 26.2-mile run - the same as a marathon.
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In the US, 13 of the 14 deaths occurred during the first portion of the swim phase.
Doctors believe that that the stress of jumping into cold water under competitive conditions could cause vulnerable people to hyperventilate or suffer a heart attack.n Britain, two deaths have occurred in recent triathlons, one during the bike race and one involving a competitor who died after the event, while asleep.
Both the deceased had heart abnormalities.
Kevin Harris, a cardiologist at the Minneapolis Heart Institute of Abbott Northwestern hospital said the risk was not large, but was also "not inconsequential", reports the Telegraph.
However, Gary O'Donovan, an exercise physiologist at the University of Exeter strongly disagrees with the idea.
"It is sedentary living that is killing people, not exercise. The last thing we need is a suggestion that exercise is bad for you," he added.