Running a marathon can be bad for your heart, according to results of a study unveiled Monday at the Canadian Cardiovascular Congress in Montreal.Using MRI data to find out what’s happening to the runner’s heart as the marathon kilometres mount, Heart and Stroke Foundation researchers discovered the endurance race can damage the hearts of less-fit runners for up to three months.As part of the study, Laval University professor and cardiologist Dr. Eric Larose evaluated healthy amateur runners six to eight weeks before their run and immediately after.He assessed their aerobic endurance using the VO2 max test, which measures body oxygen consumption using treadmills or stationary bikes. Larose also subjected them to 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,” he said. “Compared to those runners with better training, they became more dehydrated and their hearts showed greater signs of injury.”Those with less training experienced lower blood flow and the segments of the heart — there’s 17 in the left ventricle — showed signs of greater irritation, he added.”Without proper training, marathon running can damage your heart. Fortunately the exercise-induced injury is reversible over time,” said Dr. Larose. “But it could take up to three months to completely recover.”With marathon running, particularly among boomers, growing in popularity, the study ought to be a warning to runners to train properly and stay hydrated, said Dr. Beth Abramson, a spokeswoman for the Hearth and Stroke Foundation.”Not everyone will need extensive testing before training to run a marathon, but speaking to your doctor about your cardiac risk is important,” she said.