Catch comedian Eddie Izzard in History Channels insightful documentary, Mandela Marathons with Eddie Izzard, tonight.

London - Scientists warn that the fashion for ultramarathons, triathlons and extreme endurance bicycle races could be putting competitors' lives at risk.

The extreme events are increasingly favoured by celebrities including the comedian Eddie Izzard who last month halted a bid to run 27 marathons in 27 days in South Africa.

But while moderate exercise is of benefit, participants in extreme sports could risk doing long term damage to their hearts according to a study. Beyond an hour a day, vigorous exercise yields diminishing returns.

The warnings follow the death of Micah True, a US ultra-marathoner, who ran as far as 100 miles in a day. True was found dead of a heart attack in March while on a 12-mile training run in New Mexico. He was 58.

Under the name Caballo Blanco (White Horse) he was the hero of the 2009 best selling book Born To Run, by Christopher McDougall.

Writing in Mayo Clinic Proceedings, James O'Keefe of St Luke's Hospital, Kansas and colleagues said True may have died of cardiomyopathy, a thickening of the heart muscle that caused it to beat irregularly or pump inadequately, brought on by excessive exercise.

Dr O'Keefe has shown that extreme training can induce temporary changes in the heart.

But for some, if the injury is repeated, it can result in scarring and fibrosis - thickening of the heart tissue. In a study of marathon runners, 12 percent showed evidence of scarring of the heart and there was a “significantly higher” rate of heart disease “events” among them during the subsequent two years. - The Independent