A team of researchers and runners from the Heart and Stroke Foundation has found that the magnitude of abnormal heart segments was more widespread and significant in a group of less fit runners.
The team is using data from magnetic resonance imaging (MRI) to find out what is really going on in the marathoner's heart as the kilometers pile up.
"Marathon runners can be a lot less fit than they think," said Eric Larose.
Lack of real aerobic fitness may directly impact the ways the heart organizes itself to survive the stress of marathon running, he said.
During the marathon, less fit runners had signs the heart might be at greater risk of damage than that of runners who had better training or at least had better exercise capacity.
"Without proper training, marathon running can damage your heart. Fortunately the exercise-induced injury is reversible over time. But it could take up to three months to completely recover." said Larose.
They studied the effects using MRI measurements, which propel research beyond the traditional stethoscope as a means of estimating and measuring heart function.
"The heart isn't simply playing tricks - this may be an important adaptive survival mechanism, like the way the brain can switch function after a stroke.
"Unfortunately, as a result, the data produced by traditional means may be inconsistent and misleading.
"This means that, short of performing MRI in everyone, we are left with only one practical test that can accurately tell runners their level of cardiac fitness under stress, " said Larose.
The test, V02 max, directly measures body oxygen consumption and it is the best test to provide an accurate measure of a safe maximum heart rate (number of beats per minute) for runners.
In V02 testing, treadmills or stationary bicycles may be used to establish cardiac fitness.
Larose took healthy amateur runners and performed a full evaluation on them six to eight weeks before, and then immediately after, they ran a marathon. They underwent exercise tests, blood analysis, and magnetic resonance imaging.
"What we did notice in this study is a runner with less preparation before the marathon had lower V02 max, so they had lower exercise capacity. Compared to those runners with better training, they became more dehydrated and their hearts showed greater signs of injury.
"The less well trained runners also experienced greater loss of function associated with lower blood flow and greater irritation of heart segments," he said.
The findings were discussed at the Canadian Cardiovascular Congress 2010, co-hosted by the Heart and Stroke Foundation and the Canadian Cardiovascular Society.