ST. PAUL  — Exercise usually is regarded as good medicine for your heart, but maybe there’s such a thing as an overdose, according to a new study that involved researchers and marathoners from Minnesota.

The study, recently published in the journal Missouri Medicine with a cover title “Marathons in the Long Run Not Heart Healthy,” compared heart scans done on 23 sedentary men and 50 male marathon runners recruited from longtime participants of the Twin Cities Marathon.

The results show the runners “paradoxically” had greater volume of coronary plaque. That condition narrows the vessels leading to the heart, which can result in a heart attack.

Robert Schwartz, a Minneapolis Heart Institute Foundation researcher, and his fellow researchers wrote that “an emerging body of scientific data” is starting to suggest that “chronic, high-intensity exercise” may be stressing the heart and accelerating problems like plaque build up and cardiac chamber stiffness.

“It is plausible, not proof by any stretch, that metabolic changes when running, could be moderately toxic to arteries,” Schwartz said

The runners in the study, men who had done at least 25 Twin Cities Marathons in a row, had a mean age of 59 and had lower heart rates and were thinner than the sedentary men, who had a mean age of 55.

The runners also had higher levels of good cholesterol, were less likely to have hypertension and none had diabetes, compared with four of the 23 men in the sedentary control group. But more than half the runners had a history of smoking, compared with less than 40 percent of the sedentary men.

The study said “long-term marathon running in men may not engender protection against coronary artery plaque development” despite providing other benefits in blood pressure, lipid levels and glucose metabolism.

The results do not prove a cause-and-effect relationship between marathon running and accelerated coronary plaque, according to the study.

And doctors aren’t certain whether that higher volume of coronary plaque in otherwise fit runners is as damaging as it would be among couch potatoes, Schwartz said.

The study cited another heart study in Copenhagen that showed that levels of physical activity produced a “U-shaped” curve in mortality.

Moderate levels of exercise resulted in the lowest mortality rates. But mortality rates rose at the ends of spectrum in physical activity, among those who were sedentary or who were high-volume, high-intensity runners.

“In other words, excessive running may have abolished the remarkable improvements in longevity conferred by lower doses of running,” the Minnesota plaque study authors wrote.

Sudden deaths among runners during marathons are rare — about one death in 100,000 participants, according to the plaque study.

“However, the bigger concern may be the fact that excessive exercise ultimately deprives the individual from reaping the significant longevity benefits conferred by moderate exercise,” the study said.

The study noted that the number of marathon participants has risen dramatically in the past 40 years, approaching half a million in the United States in 2012.

“We suspect some runners might choose shorter, less exhausting challenges if they were aware of the potential adverse cardiac effects of chronic extreme endurance efforts,” the study said.

Or maybe not.

“Are far as not running, are you crazy? We’re addicted,” said Timothy Zoerb, 57, of Eden Prairie, who participated in the marathon heart study.

Zoerb, who has run 37 marathons, said a heart scan showed that his coronary arteries were fine. But he said he watches what he eats, and not all runners do.

“I think there’s some kind of bravado: ‘I can do whatever I want because I’m a runner,’ “ Zoerb said.

“You certainly don’t want to tell people that exercise is bad,” said Paul Arbisi, 58, of Edina, who also participated in the study.

He said his coronary plaque levels were within acceptable levels, and he doesn’t intend to break his streak of 32 Twin Cities Marathons. The clinical psychologist said there are emotional and psychological benefits to marathon running that go beyond physical health.

Roger Carlson, 70, of Stillwater said that after running 7,813 days straight, he stopped after taking part in the study and learning he had major blockage in a coronary artery.

But he had a stent put into his artery and resumed running 12 days later. He said his current running streak is more than 1,760 days, including running with a catheter after prostate surgery.

He said running makes him feel fit and full of life, despite what the heart scan says.

“I’m willing to bet that anyone’s whose run more than two marathons is going to say, ‘I don’t care; I’m going to run,’” Carlson said. “I never had any symptoms. I think generally my heart is healthy. It’s just full of plaque.”

But one person who has given up marathoning is Peter McCullough, a cardiologist at Baylor University Medical Center at Dallas.

McCullough, 51, has run 54 marathons, but he’s given up the long-distance event because of concerns it may be hurting his heart.

“I’ve personally been woken up about it,” said McCullough, who co-authored an editorial overview in the Missouri Medicine issue about marathon running.

McCullough said the plaque buildup seen in the study of Minnesota marathoners “really shouldn’t be happening in runners, and it really shouldn’t be happening more than nonrunners.”

McCullough said he suspects a certain percentage of the population — perhaps as many as one-fourth of marathon runners — might have a genetic variation that makes marathon running unhealthy for their hearts.

“The exercise community has had a mantra that more exercise is better,” he said. “We’re finding out that more is not better.”