Extreme exercise such as marathons may permanently damage the heart and trigger rhythm abnormalities, warn researchers.
They say the safe ‘upper limit’ for heart health is a maximum of an hour a day – after which there is little benefit to the individual.
A review of research evidence by US physicians says intensive training schedules and extreme endurance competitions can cause long-term harm to people’s hearts.
Activities such as marathons, iron man distance triathlons, and very long distance bicycle races may cause structural changes to the heart and large arteries, leading to lasting injury.
Lead author Dr James O’Keefe, of Saint Luke’s Hospital of Kansas City, said exercise was generally beneficial for health but could tip into becoming harmful when taken to excessive lengths.
He 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.’
A review published in the medical journal Mayo Clinic Proceedings (must credit) looked at studies detailing the mechanisms, pathophysiology, and clinical manifestations of cardiovascular injury from excessive endurance exercise.
Dr O’Keefe and colleagues said research suggests that extreme endurance training can cause transient structural cardiovascular changes and elevations of cardiac biomarkers, all of which return to normal within one week.
But for some individuals, over months and years of repetitive injury, this process can lead to the development of patchy scarring of certain areas of the heart, and abnormal heart rhythms.
In one study, approximately 12 per cent of apparently healthy marathon runners showed evidence for patchy myocardial scarring, and the coronary heart disease event rate during a two-year follow up was significantly higher in marathon runners than in runners not doing marathons.
The review said it had been known that elite-level athletes commonly develop abnormal electrocardiogram readings.