Cardiovascular disease can occur in healthy endurance athletes, such as marathon runners, but may be difficult to distinguish from the effects of training on the heart, German researchers find.

Their study included 110 males, ages 50 to 72, who had all completed at least five marathons in the previous three years. The men had no symptoms or known history of cardiovascular disease or diabetes.

However, MRI scans showed that the marathon runners had heavier hearts, with an average left ventricular mass (LVM) of 141 grams compared to an average of about 77.5 grams in the general population. LVM is the weight of the muscle of the left ventricle, the heart's main pumping chamber.

"The change in the heart mass is the way the body reacts to the endurance training. However, in some runners, it may also be an early sign of cardiac disease," researcher Dr. Torleif A. Sandner of University Hospital, Munich University, explained in a prepared statement.

"It is difficult to differentiate an athlete's heart from one that has disease. Establishing criteria for what is normal in marathon runners of advanced age is one of the team's research goals," noted principal investigator Dr. Stefan Mohlenkamp.

He noted that more and more older adults are taking part in marathons but that pre-training screening of new endurance athletes doesn't look for problems specific to older participants.

"Conventional screening includes a blood pressure check, questions about heart disease in the family or chest pain, listening to the heart and lungs and possibly doing an echocardiogram. But these techniques can miss early, potentially life-threatening, cardiovascular disease," Mohlenkamp said in a prepared statement.

He said that doctors "need to determine how to safely declare an individual of advanced age fit for marathon running."