TOO much exercise can scar the heart and increase the risk of sudden death.

Research shows extreme endurance sports such as marathons, triathlons and long-distance bike races can cause structural changes to the heart and arteries.

Usually recovery occurs within a week. But for some, repetitive injury over months and years of training and competition can lead to fibrosis, or scarring, in the heart, say scientists. This can lead to an increased likelihood of potentially fatal abnormal heart rhythms.

Dr James O'Keefe, from Saint Luke's Hospital in Kansas City, US, who led a review of the evidence, said: "Physical exercise, though not a drug, possesses many traits of a powerful pharmacologic agent. A routine of daily physical activity can be highly effective for prevention and treatment of many diseases, including coronary heart disease, hypertension, heart failure, and obesity.

"However, as with any pharmacologic agent, a safe upper-dose limit potentially exists, beyond which the adverse effects of physical exercise, such as musculoskeletal trauma and cardiovascular stress, may outweigh its benefits."

The research is published in the June issue of Mayo Clinic Proceedings.

Endurance sports such as ultramarathon running or professional cycling have been associated with as much as a five-fold increased risk of atrial fibrillation, one kind of abnormal heart rhythm.

Excessive sustained exercise may also be linked to coronary artery calcification, and dysfunctional and stiffened large arteries.

One study showed that around 12% of apparently healthy marathon runners had signs of heart scarring. Their chances of suffering a heart problem was also higher than average.