Vigorous running – faster than 7 mph, more than 2.4 hours a week, more than three times a week – could be almost as harmful as sitting around doing nothing, according to a new cardiology study that’s likely to stoke the debate over how much exercise is too much.

The paper, published Monday in the Journal of the American College of Cardiology, suggests there may be a point at which exertion becomes dangerous, outweighing the benefits of exercise. While the study didn’t specify how strenuous running could hurt people, researchers noted the stress that intense exercise can put on the cardiovascular system.

Previous research has shown that taxing workouts may put excessive strain on the heart, though plenty of studies have also underscored the many health benefits of exercise. The findings are forcing hard-core runners to reflect on whether it’s possible to overdo a habit they’ve long considered healthy.

At least once a year, Tina Petrusic’s doctor tries to get her to cut back on her running – suggesting half marathons over full marathons or maybe trying another sport, mainly because of joint and internal organ damage from the constant pounding on the pavement. She has no plans to slow down.

MODERATION IN THE LONG RUN

“If I want to feel healthy, I want to feel happier, I go and run,” said Petrusic, 40, an asset manager for PWA Real Estate in Pittsburgh.

Researchers in the paper published Monday looked at 5,048 people in the Copenhagen City Heart Study, a long-term examination of thousands of people that has been the basis for many reports on cardiovascular health. The researchers sifted 12 years’ worth of data on 1,098 healthy joggers and 413 healthy but sedentary non-joggers. The study looked at jogging hours and frequency as well as the person’s perception of how fast they ran.

The data excluded participants who were already diagnosed with heart disease, stroke or diabetes to control for self-selection bias, and scientists repeated the analysis excluding all deaths within the first two years of follow-up and found similar results.

The researchers found those who jogged one to 2.4 hours a week at a slow or average pace with no more than three running days in a week had the best survival rates. Anything longer than that left runners with death rates closer to those who didn’t run at all, the authors said.

“2.4 hours of exercise is ridiculous if you’re trying to qualify for the Boston Marathon,” said study co-author James O’Keefe. “But if you’re trying to live to be 90 and still have a good heart and still have good health, that’s probably an ideal amount of exercise.”

David Bach, a cardiologist and avid runner who has completed more than 35 marathons and several ultramarathons, cautioned that the findings may not paint a complete picture.

“I’m far from convinced based on data from this paper and other papers that there’s an actual increase in cardiovascular risk with higher levels of physical activity,” said Bach, a professor of medicine at the University of Michigan in Ann Arbor. “If this paper showed what it pretends to show – that running four hours a week is potentially detrimental – that is putting a very low lid on what is acceptable.”

STILL IMPORTANT TO GET OFF COUCH

Still, previous studies have hinted that moderate jogging may be the healthiest option. Running appears to lower the risk for mortality when an athlete doesn’t exceed more than 20 miles a week, log more than 5 to 7 miles per hour, or run more than two to five times a week, researchers at the Ochsner Health System of New Orleans and the University of South Carolina found in a 2012 paper.

It’s important for people not to selectively use the research published Monday as a reason not to exercise, Bach said. More than one-third of U.S. adults are obese, and the number of diabetics surged to 19.6 million in 2011 from 5.5 million in 1980.

“I don’t think we’re facing some sort of medical crisis in this country because people exercise,” he said.

In the paper published Monday, the researchers found that compared with sedentary non-joggers, runners who moved at a slow pace were associated with a 49 percent lower risk of dying. Jogging at an average pace had a 62 percent decreased risk, said study co-author Jacob Marott, a statistician with the Copenhagen City Heart Study. Those who ran fast only had a 6 percent lower risk of dying. A slow and average pace was considered about 5 mph, while a fast pace was more than 7 mph, he said.

For those who ran one hour to 2.4 hours a week, the risk of dying was 71 percent lower than those who didn’t exercise.

“With most things in life there is a sweet spot, a middle zone that is ideal,” said O’Keefe, a sports cardiologist at Saint Luke’s Mid America Heart Institute in Kansas City, Missouri. “Moderation is best because it’s a long slog through life. If you put too many miles on the engine, so to speak, in your young and middle ages, it’s a lot of extra wear and tear.”