Running is undoubtedly great for your health, but some research suggests that when it comes to pounding the pavement, too much of a good thing might actually have the opposite effect.

A number of earlier studies have suggested that people who run more than 20 miles a week or at an average pace of 7.5 mph or faster are more likely to have shorter lifespans than those who run slower over shorter distances. In other words, when "increasing mileage and pace, the benefits of running seem to disappear," cardiologist Martin Matsumara told The Huffington Post over the phone this week. "[These studies suggest that] running fast and far may be toxic to the heart in some way."

But some running enthusiasts are skeptical. In 2012, for example, a writer for Runner's World took issue with a study linking endurance running with reduced longevity by pointing out that the researchers had not considered other health factors -- such as body mass index, smoking habits and hypertension -- when making their conclusions.

A new study conducted by Dr. Matsumara, however, is now challenging this counter-argument.

Co-director of the Cardiovascular Research Institute at Pennsylvania's Lehigh Valley Health Network, Matsumara said he wanted to find out if running farther and faster really causes people to not live as long, or if other factors are at play. "I wondered, is there something these high-mileage runners are doing that's causing them to live shorter lives?" he said.

After looking at the backgrounds and habits of 3,800 runners, Matsumura said he didn't find any evidence that high-mileage runners had particularly unique habits or troubling medical histories. In fact, other than how long or how far the runners ran, he said his team found no difference between those who ran longer and faster, and those who didn't.

"We looked at medical background, smoking habits, heart risk factors, use of medications like NSAIDs [nonsteroidal anti-inflammatory medications], and we didn't find any risk factors that would explain the loss of benefit," Matsumara told the HuffPost.

In fact, when it came to the use of NSAIDs -- which have been linked with heart problems -- runners who ran less than 20 miles a week tended to take more of the painkillers, he said.

Ultimately, Matsumara said researchers still need to figure out the optimal dose of running for health and longevity.

"The underlying cause of the [link] between training mileage and longevity remains unclear and should be the topic of further study," he wrote in a bulletin prepared for his recent presentation of the new study.

In the meantime, Matsumara says that people should absolutely not stop running. "Runners in general enjoy longer and better health," he said.

He added, however, that adopting a more moderate running regimen might be the key when it comes to reaping all the possible health benefits. He recommends running no more than 2 or 3 hours a week; but as a 2013 Boston Globe report on this topic points out, researchers are divided on what "moderate" really means.

"Until further studies can help physicians personalize their advice on exercise, researchers agree that the safest bet is to listen to your body and cut back if you experience pain or excessive fatigue between workouts," the Globe wrote. "Many people check off marathons and triathlons from their bucket lists and then ease back into shorter workouts."

The Centers for Disease Control and Prevention recommends that adults engage in at least 75 minutes of vigorous activity (like running or jogging) or 150 minutes of moderate activity (like brisk walking) every week.