New US research has found that despite previous concerns, extreme levels of exercise do not appear to pose any danger to heart health.
Led by researchers at the University of Texas Southwestern Medical Center, the new study looked at 21,758 male athletes age 40 to 80 who were generally in good health and without cardiovascular disease.
The men were followed between 1998 and 2013 and asked to report on their physical activity levels. Most of the athletes were runners, but some were cyclists, swimmers, or rowers, or trained in more than one of these sports.
Participants also underwent coronary calcium scanning, an imaging test which shows how much calcium -- and therefore cholesterol deposits -- has accumulated in the blood vessels that supply the heart. The test helps doctors identify which patients without cardiac symptoms are at a low, intermediate, or high risk for heart attack and decide whether there is a need for medication and lifestyle modifications to reduce this risk.
Women were not included in the study as they have a lower mortality rate than men.
The findings, published in JAMA Cardiology, showed that high levels of coronary artery calcification were more common among the highly active men.
However, the researchers also found that higher coronary calcium scores did not increase the high-intensity athletes' risk for cardiovascular or all-cause mortality when compared with men who were less active.
High-volume, high-intensity exercise was defined in this study as at least five to six hours per week at a pace of 10 minutes per mile. The average amount of high-intensity exercise in the group was eight hours per week.
"The question has never been whether exercise is good for you, but whether extreme exercise is bad for you. For the past decade or so, there's been increasing concern that high-volume, high-intensity exercise could injure the heart. We found that high volumes of exercise are safe, even when coronary calcium levels are high," said lead author Dr. Benjamin Levine.
"The most important take-home message for the exercising public is that high volumes of exercise are safe. The benefits of exercise far outweigh the minor risk of having a little more coronary calcium."
Despite the positive findings Dr. Levine advises that the protective effect of exercise cannot undo poor lifestyle habits.
"You cannot overcome a lifetime of bad behaviors -- smoking, high cholesterol, hypertension -- just from doing high levels of physical activity, so don't use that as a magical cure," said Dr. Levine, who also recommends caution when starting a new training program.
"If you want to train for a marathon, you have to have a long-range plan to build up slowly before you achieve those volumes and intensity of exercise."