In the July–August issue of IDEA Fitness Journal, we reported on a study that found participation in endurance activities like marathons was not harmful to health. A new study suggests the opposite may be true.

According to research from the Mayo Clinic Proceedings (2012; 87 [6], 587–95), “Chronic training for and competing in extreme endurance events such as marathons, ultramarathons, [Ironman®] distance triathlons, and very long distance bicycle races, can cause transient acute volume overload of the atria and right ventricle, with transient reductions in right ventricular ejection fraction and elevations of cardiac biomarkers. . . .”

Researchers analyzed data from several studies, both animal and human. Throughout those studies, the authors noticed potentially harmful side effects of training, including atrial fibrillation, higher than expected coronary artery calcium levels and calcified coronary plaque volume, abnormally large atria and myocardial fibrosis.

“In some individuals, long-term excessive endurance exercise training may cause adverse structural and electrical cardiac remodeling, including fibrosis and stiffening of the atria, right ventricle and large arteries,” the authors explained. “For now, on the basis of animal and human data, cardiovascular benefits of vigorous aerobic exercise training appear to accrue in a dose-dependent fashion up to about 1 hour daily, beyond which further exertion produces diminishing returns and may even cause adverse [cardiovascular] effects in some individuals.”