"The dose of running that was most favorable 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," the authors wrote. (A slow or average running pace was determined to be 5 mph, or a 12-minute mile.)
In an editorial that accompanied the paper, the authors noted that the study was limited by a number of factors.
The study participants included white men and women from Copenhagen between the ages of 20 and 93. However, it did not take into account other forms of physical exercise besides running, according to the editorial's lead author, Duck-Chul Lee, an Iowa State University kinesiologist, and his colleagues.
Lee and his colleagues, who did not participate in the Copenhagen study, wrote that since the study participants self-reported their running habits, they may be subject to error or bias. They wrote also that the 127 study participants who were identified as strenuous joggers may have been too small a group to accurately calculate mortality risk.
"Further studies are needed to better evaluate this controversial issue," wrote the authors of the editorial. "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."