The findings, published in the Heart journal, show that repeatedly pumping large volumes of blood around the body could affect heart rythm. It can overstretch the heart's chambers, thicken its walls and change the elctrical signalling.

If you want to run a marathon, the scientists advise that you do just "one or a few" and then move on to "safer and healthier exercise patterns".

The study shows that a marathon-damaged heart will heal quickly, but repeated endurance can lead to scarring.

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

They conclude that most people should do no more than 30 to 50 minutes of intense exercise a day and that no amount of light excercise is harmful.

The research was carried out by Dr James O’Keefe and Carl Lavie, from St Luke’s Mid America Heart Institute in Kansas City, and the Pennington Biomedical Research Centre in Baton Rouge, US.