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The long bouts of hard exercise done by triathletes and marathon runners may be trimming years off their lives by causing tiny tears within their hearts, emerging research suggests.
Runners have occasionally dropped dead during a marathon, but now US heart specialists have assembled evidence indicating long-term problems may be building up for many more endurance athletes.
Drs James O'Keefe and Carl Lavie say in the medical journal Heart: "Chronic extreme exercise appears to cause excessive 'wear and tear' on the heart ..."
Endurance events are hugely popular. The annual Lake Taupo Cycle Challenge attracts more than 10,000 participants and the Auckland Marathon 16,000, although many enter shorter events than the 160km round-the-lake ride or the full 42.2km marathon. Others go for something even tougher, such as the 1500 who enter the Ironman New Zealand race at Taupo - a 3.8km swim, 180km ride and 42.2km run.
The US researchers cite a study in which 52,600 people were repeatedly checked for 30 years. Overall, runners had a 19 per cent lower rate of death during the study period than non-runners. But when the runners were grouped by weekly mileage, those who ran 32km-plus had the same death rate as non-runners, while those who ran 8km to 32km enjoyed a 25 per cent lower death rate than the non-runners.
The pattern is that lower mortality is associated with a moderate mileage, at a comfortable jog rather than higher speeds, and running on just several days a week, not most days.
The authors say heart micro-tears and over-stretching from excessive exercise resolve within a week, but long-term can lead to fibrosis and abnormal rhythms. Excessive exercise may also accelerate heart ageing.
Their own study of runners who had completed at least 25 marathons over 25 years found a 60 per cent increase in the amount of coronary plaque - fatty deposits in heart arteries - compared with sedentary people.
"This scarring can set the stage for dangerous heart rhythms, such as atrial fibrillation, which is increased approximately five-fold in veteran endurance athletes."
They urge people do moderate physical activity, and that vigorous exercisers limit this to 30 to 50 minutes a day.
"If one really wants to do a marathon or full-distance triathlon etc, it may be best to do just one or a few and then proceed to safer and healthier exercise patterns ... running too fast, too far, and for too many years may speed one's progress towards the finish line of life," they concluded.
However, a New Zealand sports physician, Mark Fulcher, is not convinced.
"I'm interested by this study, but it's definitely not a definitive word at this stage," said Dr Fulcher, medical director for New Zealand Football and the world championship triathlon series in Auckland in October.
"At this stage I wouldn't be counselling my patients not to run marathons ... and ironman triathlons, because for the vast majority of people it will improve their health. It will make it less likely that they are sitting around, it's got a definite psychological benefit.
"This study doesn't look at the effect of exercise on kidney function, on cancer ... and we know that exercise is better than most of the medications that we have for limiting type 2 diabetes."
- Martin Johnston of the New Zealand Herald