Excessive endurance training can be too much of a good thing, research suggests

Regular exercise is highly effective for the prevention and treatment of many common chronic diseases, and improves cardiovascular health and longevity. However, recent research suggests that chronic training for, and competing in, extreme endurance exercise 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 myocardial injury. A study in the June issue of Mayo Clinic Proceedings reviews the literature and outlines in detail for the first time the mechanisms, pathophysiology, and clinical manifestations of cardiovascular injury from excessive endurance exercise.

"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," says lead author James H. O'Keefe, MD, of Saint Luke's Hospital of Kansas City, MO. "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."

Dr. O'Keefe and his colleagues present emerging data suggesting that extreme endurance training can cause transient structural cardiovascular changes and elevations of cardiac biomarkers, all of which return to normal within one week. For some individuals, over months and years of repetitive injury, this process can lead to the development of patchy myocardial fibrosis, particularly in the atria, interventricular septum, and right ventricle, and an increased susceptibility to atrial and ventricular arrhythmias. In one study, approximately 12% 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 controls.

Although it has been recognized that elite-level athletes commonly develop abnormal electrocardiograms and atrial and ventricular entropy, these adaptations traditionally have not been thought to predispose to serious arrhythmias or sudden cardiac death. However, it now appears that the cardiac remodeling induced by excessive exercise can lead to rhythm abnormalities. Endurance sports such as ultramarathon running or professional cycling have been associated with as much as a 5-fold increase in the prevalence of atrial fibrillation.

Chronic excessive sustained exercise may also be associated with coronary artery calcification, diastolic dysfunction, and large-artery wall stiffening.

Lifelong vigorous exercisers generally have low mortality and disability rates and excellent functional capacity, Dr. O'Keefe notes. He suggests that further investigation is needed to identify who is at risk for adverse cardiovascular remodeling, and to formulate physical fitness regimens for conferring optimal cardiovascular health and longevity.