There's good news for those who adhere to the "slow and steady wins the race" adage: A light jog a few times a week may help you live longer, a new study from Denmark suggests. In contrast, running too hard may have drawbacks, the study found.

Researchers analyzed information from about 1,000 healthy joggers ages 20 to 86, and about 400 people who were healthy, but did not jog, and were mostly sedentary.

The analysis showed that light joggers were about 78 percent less likely to die over the 12-year study than those who were sedentary. "Light joggers" were defined as those who ran at a speed of about 5 mph (8 km/h) a few times a week, for less than 2.5 hours per week total. [7 Common Exercise Errors and How to Fix Them]

In contrast, those who jogged strenuously were just as likely to die during the study period as those who were sedentary, according to the research published Feb. 2 in the Journal of the American College of Cardiology. Strenuous joggers were defined as those who ran at a speed of more than 7 mph (11 km/h), for more than four hours per week.

The finding "suggests there may be an upper limit for exercise dosing that is optimal for health benefits," study co-author Dr. Peter Schnohr, of the Copenhagen City Heart Study and Frederiksberg Hospital in Denmark, said in a statement. "If your goal is to decrease risk of death and improve life expectancy, jogging a few times a week at a moderate pace is a good strategy. Anything more is not just unnecessary, it may be harmful."

Dr. Karol Watson, co-director of preventive cardiology at the University of California, Los Angeles, agreed, and said that many previous studies have produced similar findings: A moderate amount of jogging is linked with the best outcomes in terms of a longer life span, but when people run too far for too long, the health benefits start to drop off.

"[Humans] weren't meant to do mountain biking or marathon running every day ... and you don't have to" to live longer, said Watson, who was not involved in the study.

Being a marathon runner is still likely going to be good for heart health overall, but those runners should be aware that there is a slight increase in mortality over a given period for extreme runners compared to moderate runners, Watson said.

Other experts stress that more research is needed to determine whether there really is an upper limit on how much exercise is good for you.

"The goal is not to unnecessarily frighten people who wish to participate in more-strenuous exercise," Duck-chul Lee, of Iowa State University's Department of Kinesiology, and colleagues, wrote in an editorial accompanying the study in the journal. Although most research suggests that, beyond a certain point, more physical activity is not necessarily better, "we still need more data to truly determine 'is more actually worse?'" they said.

The authors of the editorial also noted that in the new study, the "strenuous" jogging group included only 40 people, while the other groups included hundreds. If the study had included more people who jogged strenuously, the researchers may have found a link between strenuous jogging and a decreased risk of dying during the study, the editorial authors said. Also, the study relied on participants' own reports of how much they run, which may not have been entirely accurate.

The study's authors offered a possible explanation for the negative effects linked to strenuous exercise in the results. It could be that long-term, strenuous endurance exercise has harmful effects on the heart, the researchers said. Some studies of marathon runners have found that these athletes have a higher rate of heart scarring than people who don't run marathons.

The study also adds to a growing body of evidence that has shown that even small amounts of exercise can have health benefits. In the study, people who jogged less than one hour a week were less likely to die than those who didn't jog at all.

The best outcomes in the study were associated with running between 1 and 2.4 hours per week, with no more than three days of running per week, at an average or slow pace. "Many adults will perceive this to be a goal that is practical, achievable and sustainable," the researchers said.        

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