If running a marathon is on your lifetime to-do list, here’s another reason to stick like glue to your training schedule: A new study presented this week at the Canadian Cardiovascular Congress 2010 shows that participating in these races can temporarily hurt the heart. Plus, the least-fit runners experience the most damage.

Study leader Eric Larose, M.D., of Laval University said that he and his colleagues hoped to shed some light on the great disconnect between exercise and cardiovascular health: While regular exercise appears to decrease the risk of heart disease and death, those same risks increase during strenuous exercise, such as running a marathon.

To see what happens in a marathon runner’s heart, the researchers recruited 20 runners who were training for a marathon. These runners had various levels of previous marathon experience—some were running their first marathon; one was running his thirtieth.

Larose and his colleagues put these runners through a battery of tests at various stages of the race: about 2 months before the marathon began, a few hours after the subjects crossed the finish line, and 3 months afterward. Their main finding was that the heart does undergo some injury during the marathon. Specifically, runners experience some inflammation to sections of the left ventricle, or a section of the heart that aids in pumping blood. Those who were considered “fittest” showed less injury than those who were less fit. However, by the 3-month mark, signs of injury had disappeared in all the runners.

These findings could explain why some runners die during marathons, something Larose witnessed himself while running a race himself years ago. They also deliver an important message to runners about training, Larose notes.

“It’s extremely important to rigorously follow your training program,” he says. “You can’t cheat the marathon, because it could come back to really hurt you in the end.”