There is a growing pile of studies claiming that excessive running is bad for health. The latest one, published in this month's Journal of the American College of Cardiology, should be taken with just as big a grain of salt as the others.
Researchers in Denmark set out to discover what too much running might do to the heart.
"Long-term excessive exercise may be associated with coronary artery calcification, diastolic dysfunction and large artery wall stiffening," lead author Dr. Peter Schnohr of Copenhagen's Frederiksberg Hospital wrote in the study.
Based on data from the Copenhagen Heart Study, the researchers concluded that people who ran faster than 11 kilometres per hour (what they termed "strenuous jogging") for more than four hours a week had no difference in their mortality rates than sedentary people.
In other words, running harder and more frequently won't make you live any longer than a couch potato, the study concludes.
However, jogging at a slower pace for less time each week, researchers found, vastly decreased a person's mortality risk.
"The dose of running that was most favourable for reducing mortality was jogging 1 to 1.4 hours per week, with no more than three running days per week, at a slow or average pace [of 8 km per hour]," the authors write.
Such runners were found to be 78 per cent less likely to die than sedentary people.
"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," Dr. Schnohr writes. "Anything more is not just unnecessary, it may be harmful."
Indeed, it may be – emphasis on "may."
As with all such previous studies, it's worth delving in to the data before we decide there's no point going for a run today.
Danish researchers followed 1,098 healthy joggers and 3,950 people who do not jog for nearly a decade. At the end of that period, 28 of the runners had died while 128 of the people who did not run passed away.
A number of problems in the design of the study and its conclusion were raised in an accompanying editorial written by Duck-chul Lee, a kinesiologist at Iowa State University, who did not participate in the study.
These problems are as follows: The study only looked at white people between the ages of 20 and 93; it did not take into account other forms of exercise, which seems like a pretty huge oversight; it also relied on participants to self-report, which could lead to bias or error; and, finally, having only 127 strenuous joggers in the study may not have provided a large enough sample size to accurately determine mortality risk.
"Further studies are needed to better evaluate this controversial issue," the editorial says. "Ideally these studies will be well-controlled interventions, because we certainly agree that the goal is not to unnecessarily frighten people who wish to participate in more strenuous exercise."
There's surely a point at which too much running becomes bad for your health by damaging the heart, but no study has yet to find it conclusively.