There have been some hints, however, that people can exercise too much, especially if the exercise is intense. In past studies, researchers scanning the hearts of longtime endurance athletes, such as marathon runners, have found scarring in those athletes’ heart muscles and also hefty deposits of coronary plaques, which can break free and block arteries, causing a heart attack. But most of those earlier studies were small and provided a single snapshot of the athletes’ hearts; they did not follow people for years to see whether their heavy exercise and any subsequent buildup of plaques was linked to heightened risks for heart attacks and shorter lives.

So, for the new study, scientists at the Cooper Institute in Dallas and other institutions decided to delve into just that issue.

They also happened to have a useful trove of data readily at hand, in the health records of tens of thousands of people who had undergone exams at the affiliated Cooper Clinic. Many of these exams included scans of people’s hearts, along with detailed questionnaires about their exercise habits.

The researchers focused on the records of 21,758 men, most of them in their 50s. (They did not include women but plan to in a follow-up study.) They categorized the men into groups, based on how much they exercised. Those in the sweating-the-most group worked out vigorously for at least five hours a week and often more. The researchers used a mathematical measure (known as metabolic equivalent of task) to characterize the men’s workouts. But in practical terms, these extreme exercisers were doing the equivalent of running about six miles a day.

A second group completed somewhat less exercise, and a third group finished less than half as much exercise each week as the most avid exercisers.