Jonathan Knowles/Getty Images

Recently, researchers in Britain set out to study the heart health of a group of dauntingly fit older athletes. Uninterested in sluggards, the scientists recruited only men who had been part of a British national or Olympic team in distance running or rowing, as well as members of the extremely selective 100 Marathon club, which admits runners who, as you might have guessed, have completed at least a hundred marathons.

All of the men had trained and competed throughout their adult lives and continued to work out strenuously. Twelve were age 50 or older, with the oldest age 67; another 17 were relative striplings, ages 26 to 40. The scientists also gathered a group of 20 healthy men over 50, none of them endurance athletes, for comparison. The different groups underwent a new type of magnetic resonance imaging of their hearts that identifies very early signs of fibrosis, or scarring, within the heart muscle. Fibrosis, if it becomes severe, can lead to stiffening or thickening of portions of the heart, which can contribute to irregular heart function and, eventually, heart failure.

The results, published online a few weeks ago in The Journal of Applied Physiology, were rather disquieting. None of the younger athletes or the older nonathletes had fibrosis in their hearts. But half of the older lifelong athletes showed some heart muscle scarring. The affected men were, in each case, those who’d trained the longest and hardest. Spending more years exercising strenuously or completing more marathon or ultramarathon races was, in this study, associated with a greater likelihood of heart damage.

The question of whether years of intense endurance training might, just possibly, be harmful to the heart is hardly new. It arises whenever a seemingly healthy distance runner, cyclist or other endurance athlete suffers a heart attack. It’s also sometimes invoked by those looking for an excuse not to exercise.

But, to date, science has been hard pressed to establish a clear cause-and-effect link between strenuous exercise and heart damage. A much-discussed 2008 German study of experienced, older marathon runners, for instance, found signs of fibrosis in their hearts more frequently than in a group of less active older men. But some of the racers had taken up regular exercise only late in life, after decades of smoking and other bad health habits. It was impossible to say whether their current heart damage predated their marathon training.

The new study of elite lifelong athletes avoids that pitfall. None of the athletes were new to exercise. Only one had ever smoked. But even so, the study can’t directly prove that the older athletes’ excruciatingly heavy training loads and decades of elite-level racing caused heart scarring, only that the two were associated with each another.

But another new study, this time in laboratory rats, provides the first solid evidence of a direct link between certain kinds of prolonged exercise and subtle heart damage. For the study, published in the journal Circulation, Canadian and Spanish scientists prodded young, healthy male rats to run at an intense pace, day after day, for three months, which is the equivalent of about 10 years in human terms. The training was deliberately designed to mimic many years of serious marathon training in people, said Dr. Stanley Nattel, a cardiologist who is director of the electrophysiology research program at the Montreal Heart Institute Research Center and a senior author of the study.

The rats had begun their regimens with perfectly normal hearts. At the end of the training period, heart scans showed that most of the rodents had developed diffuse scarring and some structural changes, similar to the changes seen in the human endurance athletes. A control group of unexercised rats had developed no such remodeling of their hearts. The researchers also could manually induce arrhythmias, or disruptions of the heart’s natural electrical rhythm, much more readily in the running rats than in the unexercised animals. Interestingly, when the animals stopped running, their hearts returned to normal within eight weeks. Most of the fibrosis and other apparent damage disappeared.

What does all of this mean for those of us who dutifully run or otherwise make ourselves sweat several times a week? Probably not much, realistically, said Dr. Paul Thompson, the chief of cardiology at Hartford Hospital in Connecticut and an expert on sports cardiology. He was one of the peer reviewers for the British athlete study.

“How many people are going to join the 100 Marathon club” or undertake a comparable amount of training? he asked. “Not many. Too much exercise has not been a big problem in America. Most people just run to stay in shape, and for them, the evidence is quite strong that endurance exercise is good” for the heart, he said.

Dr. Nattel agrees. “There is no doubt that exercise in general is very good for heart health,” he said. But the emerging science does suggest that there may be a threshold of distance, intensity or duration beyond which exercise can have undesirable effects.

Unfortunately, it remains impossible, at the moment, to predict just what that threshold is for any given person, and which athletes might be most vulnerable to heart problems as a result of excessive exercise, said Dr. Paul Volders, a cardiologist at the University of Maastricht in the Netherlands, who wrote an editorial accompanying the recent rat study.

“Let’s say we ask 100 people, all same age, all same gender, to start a marathon training program at the age of 20 years,” Dr. Volders wrote in an e-mail. If the runners continued their training uninterrupted for 30 years and scientists then scanned their hearts, “it is very likely (one may say: for sure) that there will be major differences in the tissue of the chambers of the heart between these people,” he wrote. For some, the changes will be beneficial; for others, probably not.

Similarly, because most of the research has been done in men and male animals, it is unclear whether the hearts of long-term female athletes are affected in the same fashion. But Dr. Nattel said it seems likely that the latest finding would also apply to women.

So for now, the best response to the emerging science of excessive exercise is to just keep exercising, but with a low-level buzz of caution. If your heart occasionally races, which could indicate arrhythmia, or otherwise draws attention to itself, Dr. Nattel said, consult a doctor.

But if you exercise regularly and currently have no symptoms, “I think it’s safe to say that you should keep it up,” Dr. Thompson said.