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Phys Ed Gretchen Reynolds on the science of fitness.

An ingenious new study of marathon runners and their non-running spouses should reassure anyone headed for a spring marathon that prolonged training doesn’t damage the heart, a concern that has been raised in previous research. At the same time, becoming fit as a marathoner doesn’t seem to protect the heart to the extent you might expect, although it may have unexpected benefits for your spouse.

While we all know that exercise is healthy, some research has begun to raise questions about whether it’s possible to overdo a good thing. A few studies have found that long-time endurance athletes can have a heightened risk for abnormal heartbeats, and even for scarring of the heart muscle. Likewise, experiments with lab animals have found possible links between prolonged, extremely strenuous running and undesirable changes in the structure and function of the heart.

But the actual incidence of runners having a heart attack during a marathon race is vanishingly small, a finding that seems to suggest that marathon training can’t be excessively hard on hearts or there would be greater, obvious consequences.

Such inconsistencies in the data about prolonged endurance exercise and heart health prompted researchers to wonder if perhaps past studies had been too imprecise. It’s difficult to isolate the risks associated with strenuous exercise from other lifestyle factors, said Beth Taylor, an assistant professor in the health sciences department at the University of Hartford who led the new study, which was published last month in BMJ Open. Runners whose hearts seemed to have been affected by their exercise habits might also have smoked, gorged on junk food or otherwise imperiled their hearts, separately from how much they worked out.

So Dr. Taylor and her colleagues decided to better control for such factors by studying marathon runners along with their domestic partners, who presumably would be sharing their lifestyles if not their physical exertions. If cardiac health differed among these couples, the scientists felt, they could reasonably conclude that training had played a role, since so many lifestyle factors would be the same.

With that idea in mind, Dr. Taylor and her colleagues contacted a slew of runners who had qualified and signed up for the 2012 Boston Marathon, inquired if they had non-running spouses or partners, and asked if both would be willing to have their hearts scanned and cardiovascular disease risk assessed.

Forty-two of the runners said yes, along with their spouses or partners. Half of the runners were women. Their ages ranged from 33 to 59, although most were in their mid- to late 40s. Their partners were around the same age but considerably less active, averaging fewer than two sessions of moderate exercise per week. Many did not formally exercise at all, although most reported frequently walking, gardening or undertaking other types of moderate activity.

The day before the 2012 race, the racers and their partners visited a makeshift lab next door to the race expo, where they filled out questionnaires about their exercise and health histories. Scientists then drew blood to determine the volunteers’ cholesterol and triglyceride profiles and measured their height, weight, pulse rate, blood pressure and other vital signs. Finally, each volunteer underwent a noninvasive heart scan to reveal the buildup of arterial plaques, an indication of heart disease.

Not surprisingly, the marathon runners were significantly thinner than their partners, although few of the partners were overweight. The runners also generally had lower blood pressure, heart rates, bad cholesterol and other indicators of cardiac health.

But running did not insulate the racers altogether from heart disease, the scientists found. Some of the racers, particularly the oldest ones, carried large deposits of plaques in their arteries, a worrying sign. These older racers also tended to have the highest tallies on a numerical assessment of heart attack risk called the Framingham risk score, which considers medical and lifestyle factors that, along with genetics, can contribute to the development of atherosclerotic plaques.

In essence, the scans showed that marathon training did not cancel out the depredations of age, longstanding bad health habits or a family history of cardiac problems, Dr. Taylor said.

On the other hand, the scientists found no relationship between the number of hours the runners trained or how fast they ran and the levels of plaque in their arteries, indicating that marathon training had not directly damaged any of these racers’ hearts.

Over all, Dr. Taylor said, the study’s data suggests that if you’re training for a marathon or otherwise doing frequent and prolonged endurance exercise, you’re probably not hurting your heart and are likely strengthening it. But you should be aware of your past health habits and family history and monitor any symptoms, such as shortness of breath, that could be a sign of potential heart troubles.

Perhaps the more surprising takeaway of the study, Dr. Taylor said, is that marathon training’s cardiac benefits may be transferable. “The spouses of the runners were quite healthy, too,” she pointed out. More so than many people, they walked and moved around frequently, and had generally robust cardiac risk profiles. Dr. Taylor’s conclusion: if you want improved heart health but can’t be a runner, marry one.