Andrew Marr, the 53-year-old BBC TV presenter and journalist, made a guest appearance on his own show yesterday, just four months after a life-threatening stroke, which he attributed to "heavily overworking" and an intensive session on a rowing machine.

While he was on air, a 23-year-old man collapsed and died at the 16th mile of the Brighton marathon. He is thought to have had a cardiac arrest, although it is too early to know the specifics. His death brings back memories of Claire Squires, 30, who collapsed and died in the final stretch of the London marathon last year. The coroner later ruled that she died of cardiac failure caused by extreme exertion and complicated by DMAA toxicity from a nutritional supplement which has now been banned.

These incidents raise many questions about the dangers of extreme exertion: how much exercise is good for you? Do these tragic events only affect individuals who are predisposed to heart disease or stroke anyway? How can we tell if we are one of them?

Marr said he had followed the advice to "take very intensive exercise in short bursts – and that's the way to health … I went on a rowing machine and gave it everything I had, and had a strange feeling afterwards – a blinding headache, and flashes of light – served out the family meal, went to bed, [then] woke up the next morning lying on the floor unable to move". Marr, who is making a good recovery, said his advice would be to be wary of rowing machines, or at least of being too enthusiastic on them. He didn't warn off working out too hard, but perhaps that's implied, too.

But what of the current enthusiasm for short bursts of ultra-intensive exercise? A large-scale trial of 300 volunteers, led by Jamie Timmons, professor of systems biology at the University of Birmingham, is underway to assess whether just three minutes of high-intensity interval training (HIT) a week yields the same benefits as hours pounding the streets or in the gym. Previous results have been encouraging, but the long-term safety of the technique has yet to be proven.

The NHS offers much more cautious advice about exercise. It suggests at least 150 minutes a week of moderate-intensity activity, such as fast walking or cycling, and muscle-strengthening exercise, like weights, gardening or yoga, twice a week. But over 60% of men and 70% of women in the UK say they don't even follow the modest NHS guidelines. Every marathon death or Marr story is grist to the mill of the sedentary and idle.

Although moderate exercise is undoubtedly better than no exercise, experts are beginning to warn that excessive, sustained and unsupervised exercise can cause problems. American cardiologists and keen runners James O'Keefe and Carl Lavie, writing in the journal Heart last year, recommended limiting workouts to 30-50 minutes a day. Marathons or triathlons should only be done occasionally, they said, adding that intense exercise for more than an hour or two can stretch, tear and scar the heart and cause rhythm abnormalities. They cited research that tracked the heart health of 50,000 people over 30 years: the 14,000 runners in the study were likely to live longer than non-exercisers, but only if they ran between five and 20 miles a week, not more.

The question remains why some people can undertake extreme exercise and be fine, and others will suffer unexpected and sometimes fatal heart attacks and strokes. Anyone with a family history of sudden, unexplained deaths should seek expert medical advice before taking up extreme exercise. And if you experience chest pain when exercising, it pays to get that checked out. Unfortunately, there is no way of giving you a cast-iron guarantee that your heart will be fine. Professional footballers are subjected to detailed medical testing, but when Bolton Wanderers player Fabrice Muamba nearly died of a cardiac arrest while playing, none of the tests he'd undergone had hinted that he had a problem.

The overall message is clear: some exercise is better than none, but moderate exercise is better than extreme exertion.