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."
Triathlete shrugs off US report
Jo Carrel won't be put off endurance sport by research suggesting it may be hurting her heart and shortening her life.
"I haven't heard of those sorts of problems with endurance running and stuff before," said the 31-year-old ironman triathlete, physiotherapist and podiatrist.
The only serious heart trouble she knew of associated with endurance sports was the occasional athlete who collapsed and died during a race from an undetected congenital heart disorder.
She did not expect she would be influenced by the American research, "because I think there are plenty of other things that could kill you".
And there were many benefits to health and wellbeing from running, swimming and cycling.
"Probably a lot of it is feeling good about yourself. Exercise releases endorphins, which makes you feel happy. It makes life better - and I like the people I train with."
Carrel rates her current ironman performance as "okay". In 2009, she came second in her age group, in 10 hours, 23 minutes in the Ironman New Zealand race.
Her weekly training regimen involves three swims of up to 80 minutes, cycling 10 to 12 hours and running for three to five hours, which may seem a lot to non-athletes, but she rates it as just "a reasonable amount".
She felt "pretty wrecked" after an ironman event, which was usually limited to sore muscles for a few days, although a couple of times she had caught a cold straight after a race.
"I think you can run yourself down."
Who lives longest?
Runners v couch potatoes
1st Runners who cover 8 to 32km a week
2nd= Runners who cover more than 32km a week AND people who didn't run at all.
