Marathon running is bad for you, and it's best to keep exercise to a maximum of 50 minutes a day say doctors

High intensity exercise continued over hours and repeated regularly over years and decades “stretches” the heart

Athletes run past Buckingham Palace in London during the athletics event men's marathon (Getty Images)

Marathon running is bad for you if you keep on doing it, doctors say.

One or a few is fine but after that it is best to restrict vigorous exercise to a maximum of 50 minutes a day they say.

High intensity exercise continued over hours and repeated regularly over years and decades “stretches” the heart, disrupting muscle fibres and causing micro-tears that do permanent damage.

There are signs of damage after a single marathon but these rapidly return to normal after a week. If the damage is repeated, however, it can eventually lead to scarring and stiffness.

Cardiologists from the US say in an editorial in the journal Heart that the idea that more and more high intensity exercise is good for you is a myth.

Vigorous exercise is good for health but only if it is limited to a maximum daily dose of between 30 and 50 minutes.

“If you really want to do a marathon or full distance triathlon, it may be best to do just one or a few and then proceed to safer and healthier exercise patterns,” they warn.

“A routine of moderate physical activity will add life to your years as well as years to your life. In contrast, running too far, too fast, and for too many years may speed one’s progress to towards the finishing line of life.”