Regular intensive exercise can slow the heart to risky levels, scientists warn.
The heart rate naturally slows down as we get older. If we then take on too much exercise - such as running for ever longer distances or training for a marathon - it could cause us to collapse or find ourselves needing an electronic pacemaker.
Professor Mark Boyett of Manchester University said: "Exercise undoubtedly has enormous benefits for health but do things in moderation."
The heart researcher issued his advice after studying mice who were put through their paces on an uphill treadmill.
He found their heart rate fell over time, just like human runners. He also showed the slowing down was due to changes in the heart's own pacemaker - a clump of cells that makes the organ beat.
The finding could help explain why heart rate drops in those who do endurance sports, including marathon running, long-distance cycling and triathlons.
In most people, the heart beats around 70 times at rest. However, in the super fit, a low heart rate is common and Olympic champion cyclist Sir Chris Hoy is said to have a resting heart rate of just 30 beats a minute. But in old age, this can then become dangerous.
Professor Boyett, whose study was published in the journal Nature Communications, now plans to carry out the same tests on people. Professor Jeremy Pearson, of the British Heart Foundation, which funded the research, said: "If the findings are reproduced in humans they could have implications for heart health in older athletes."
The warning comes as growing numbers try out gruelling races. More than one million people run marathons in Europe and the US annually and the figure is predicted to rise by 5 per cent a year.
- Daily Mail