For men competing in triathlons past the age of 40, the grueling slog to the finish line could be their last.

As the average age of competitors in endurance sports rises, a spate of deaths during races or intense workouts highlights the risks of excessive strain on the heart through vigorous exercise in middle age. Among the recent casualties: Michael McClintock, senior managing director of Macquarie Group Ltd. (MGQ) and a triathlete, who died at age 55 of cardiac arrest earlier this month after training.

The 40-to-60-year age bracket, often referred to as middle aged men in Lycra, or Mamils, now holds 32 percent of the membership in USA Triathlon, the sport's official governing body in the U.S. More fitness conscious than previous generations, their numbers in competitive races are swelling, along with their risk of cardiac arrest. Triathlons, the most robust of endurance races requiring swimming, biking and running, are also believed to be the most risky.

"People need to understand that they're not necessarily gaining more health by doing more exercise," said David Prior, a cardiologist and associate professor of medicine at the University of Melbourne. "The attributes to push through the barriers and push through the pain are common in competitive sport, but that's also dangerous when it comes to ignoring warning signs."

While benefits of exercise are well-known, researchers now suspect that there may be a point at which exertion becomes dangerous, especially for middle aged men who, because of gender and changes that accompany aging, are more susceptible to cardiac arrest caused by vigorous exercise.

David Fuller's view For some people, exercise is generally regarded as playful therapy, with the benefit of being good for one's health. I always enjoyed endurance sports, mainly running, swimming and cycling during much of my life to date. I was lucky because the splendidly challenging cycling trips continued into my early 60s, without any problems.

Had I not stopped, I almost certainly would not have recognised the heart symptoms that caused me problems a few years later. We are all different, in terms of our responses to medical treatments, but I struggled with statins and other heart medications into my 70s. Ultimately, the accumulative downsides for me were not worth it, so I went off all medications last November. However, my LDL cholesterol level also rose, despite a sensible diet and my intake of various supplements, including fish oils.

Fortunately, a cardiologist with homeopathic interests proscribed Ezetrol, a non statin drug to lower LDL cholesterol, produced by Schering-Plough. This immediately reduced the LDL cholesterol and I have yet to experience any unpleasant side effects from this product. In fact, all the problems that I experienced with statins have gradually gone away. I have a home gym machine which I use almost daily and my strength has gone up considerably since I came off statins. Life is full of uncertainties so I could keel over tomorrow, although hopefully not. The important point is that I feel great, which is an unexpected bonus. I also take a blood thinner called Clopidogrel.

I mention all this because a number of subscribers have had somewhat similar symptoms. I wish you all well. Fortunately, we live at a time when more life extending treatments are available, and some of them can help us. My medical regret is that I stayed with a treatment that was not helpful, under the mistaken belief that I had no other choice. That was not true, as more of you will have discovered as well.