Too much exercise can scar the heart and increase the risk of sudden death, experts claim.
Research shows that extreme endurance sports such as marathons, triathlons and long-distance bicycle races can cause structural changes to the heart and large arteries.
Usually recovery occurs within a week. But for some individuals, repetitive injury over months and years of training and competition can lead to patches of 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 Kansas City, in the 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, say the scientists. Excessive sustained exercise may also be linked to coronary artery calcification, and dysfunctional and stiffened large arteries.
A famous victim of excess exercise may have been legendary US ultramarathon runner Micah True who died suddenly while on a routine 12-mile training run on March 27, it is claimed.
True, nicknamed Caballo Blanco (Spanish for "white horse"), would run as much as 100 miles in one day. After his death at 58, his heart was found to be enlarged and scarred. He died from a lethal heart rhythm irregularity.