With many of us contemplating marathons or other prolonged endurance events in 2016, we, our spouses and other family members most likely have wondered whether such strenuous training could be harmful to our hearts. Could any of us, in making ambitious resolutions, exercise too much?
A newly published scientific review offers both reassurance and some caution. It found that while most athletes' hearts can withstand most exercise, there are exceptions. For some people in seemingly good health, heavy loads of exercise might be problematic. That's why all of us who work out should be as informed as possible about our family's cardiac history and our own potential genetic risks.
Exercise, of course, is in general extremely beneficial for heart health. Dozens of large-scale epidemiological studies have found that people who exercise in any amount, whether five minutes a day or two hours a day or more, are much less likely to develop or die from heart disease than people who are sedentary.
But these studies, while encouraging, contain a disquieting subtext. Their plotted data typically shows some type of bell curve to the heart benefits from exercise, meaning that the more people work out, the less they are at risk of cardiac problems — up to a point, and then the benefits plateau or decline.
To some scientists, that finding made little intuitive sense. If a little exercise is good for the heart, why shouldn't more be continually better?
Dr. Paul Thompson, the chief of cardiology at Hartford Hospital in Connecticut and for years a dedicated marathon runner, was particularly drawn to that issue. He and his colleagues at Hartford Hospital, along with scientists at the Radboud University Medical Center in Nijmegen, the Netherlands, recently decided to delve more comprehensively than had yet been done into the available data on the topic.
Many past studies have looked at individual aspects of prolonged exercise and heart health. But no past review had comprehensively examined all of the ways in which prolonged endurance exercise might affect the heart and whether, on aggregate, those studies indicated that we should — or should not — worry about the effects of heavy training.
So for the new study, being published next month in Physiological Reviews, Dr. Thompson and his collaborators gathered all of the studies they could find from the past 30 or more years related to exercise and heart health, and parsed their findings both singly and collectively.
Their findings should reassure most of us and our concerned loved ones.
"There is no evidence that there is a level of exercise that is dangerous or too much for a normal, healthy person," Dr. Thompson told me.
At the same time, those of us who work out should understand that frequent exercise training causes "profound changes in cardiac physiology and structure," as Dr. Thompson and his colleagues write in their review.
In the short term, these changes can mimic heart damage, they point out, with cardiac cells often becoming "leaky" after strenuous workouts or events, releasing proteins into the bloodstream that, in other circumstances, could indicate a heart attack. These proteins usually disappear within a few days, and the heart seems to recover fully, Dr. Thompson said.
But in the process, the heart adapts and changes. Its left and right ventricles enlarge. It begins to look quite different than a non-athlete's heart.
For most people, these changes are beneficial and also necessary for successful athletic performance. But, as Dr. Thompson and his colleagues write, their review of the studies indicates that there are outliers, seemingly healthy people for whom strenuous exercise holds unexpected dangers.
Perhaps most surprising, older marathon runners can be just as susceptible as their sedentary counterparts to atherosclerosis, or the buildup of dangerous plaques in the arteries, the studies show. Strenuous exercise does not prevent these plaques in people who are predisposed by heredity or lifestyle to the condition.
The act of exercising, however, may increase the risk that the plaques will rupture, precipitating a heart attack. Someone with atherosclerosis is more likely, studies show, to have a heart attack while running than while sitting quietly.
But many older athletes with atherosclerosis have no idea they suffer from the condition, which often causes few symptoms.
Similarly, according to the new review, people with certain inherited heart abnormalities, such as cardiomyopathy (an enlarged heart) or long QT syndrome, a disorder of the heart's electrical activity, may exacerbate their conditions with strenuous exercise. The cardiac remodeling that is beneficial for most exercisers, the reviewers write, is undesirable for them and could lead to premature death, possibly contributing to the statistical downturn in health benefits from extreme exercise seen in some studies.
The best response to this information is not, however, to panic and avoid strenuous exercise, Dr. Thompson said. The best response is "to know your family history of sudden death," he said. If a close family member has died unexpectedly of heart problems, talk to your doctor about whether you need to be tested for conditions such as atherosclerosis or cardiomyopathy.
The rest of us should pay attention to symptoms such as unusual fatigue, shortness of breath or chest pain during exercise, he said, but are much more likely to strengthen our hearts with exercise than harm them.