Running marathons and other gruelling races is bad for the heart and could lead to serious cardiac problems in later life.

Those who enjoy a lean look from taking part in marathons, triathlons and iron man challenges are more likely to need pacemakers in old age, a new study says.

Tests on mice, funded by the British Heart Foundation, show microscopic changes take place in the body due to exercise training.

This can disrupt the electrical pulse of the heart, causing the super-fit to suffer abnormal heart rhythms.

The number of people taking part in marathons is going up every year.

Elderly athletes with a lifelong history of endurance training and competing are prone to heart rhythm disturbances, known as arrhythmias. 

This is due to molecular changes in the heart's pacemaker from the exercise training, according to scientists from the University of Manchester.

Normal adults have a resting heat beat of between 60 and 100 per minute.

But the hearts of athletes beat as slow as 30 times a minute - or even lower at night when there can be long pauses between beats.

Cyclists Sir Chris Hoy and Miguel Indurain have resting heart rates of 30 and 28 beats per minute, the scientists say.

Author Dr Alicia D'Souza said: 'The heart rate is set by the heart's pacemaker, but this is controlled by the nervous system.

'The "vagal" nerves lower the heart rate and therefore it was assumed the low heart rate of athletes is the result of over activity of the vagal nerves. 

'But our research shows this is not the case. Actually, the heart's pacemaker changes in response to training and in particular there is a decrease in an important pacemaker protein, known as HCN4, and this is responsible for the low heart rate.'

The researchers say these molecular changes in the cardiac structure which govern heart rhythm may help us to understand the more frequent occurrence of heart rhythm disturbances or even loss of consciousness in athletes. 

Professor Mark Boyett, lead researcher on the study, added: 'This is important because although normally a low resting heart rate of an athlete does not cause problems, elderly athletes with a lifelong training history are more likely to need an artificial electronic pacemaker fitted.'

More than 500 marathons take place in Europe and America each year with around one million taking part. The number is forecast to rise by five per cent each year. 

But Professor Boyett stressed that although endurance exercise training can have harmful effects on the heart, 'it is more than outweighed by the beneficial effects'.

Professor Jeremy Pearson, Associate Medical Director at the British Heart Foundation, said: 'This study shows the heart's electrical wiring changes in mice that exercise for long periods, and these changes in heart rhythm are sustained afterwards. 

'If the findings are reproduced in humans they could have implications for heart health in older athletes. 

'But much more research is needed before we could draw that conclusion.'

(The research is published in the journal Nature Communications)

Written by Mark Branagan / The news retrieved from

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