The Cooper Institute
 

Founded in 1970 by the "Father of Aerobics"
Kenneth H. Cooper MD, MPH

 
 

Questionable Conclusion Regarding Risk of Jogging in Copenhagen City Heart Study

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Thursday, Jun 11, 2015

Data from the Cooper Center Longitudinal Study1, as well as from other large databases worldwide have shown conclusively that a low level of cardiorespiratory fitness (CRF) significantly increases the risk of all-cause, cardiovascular disease, and cancer mortality. As you may know, CRF is defined as the maximal ability to utilize oxygen at the cell level, and is sometimes referred to as ‘cardiovascular fitness’ or ‘aerobic power’. CRF can be measured in a clinical setting via a maximal treadmill exercise test, or in a field setting via the 1.5 mile run or 1 mile walk tests. You may have taken one or more of these tests. Moving from a low to a moderate level of CRF provides a substantial decrease in mortality risk, while moving from a moderate to high level of CRF provides a further but smaller decrease in risk.

A recent study published in the Journal of the American College of Cardiology2 attracted the attention of worldwide media, who used headlines such as “Fast Running is as Deadly as Sitting on Couch.” Having been a competitive runner many years ago, I must admit that I raised my eyebrows when I saw these kinds of headlines, so I quickly checked out the details to see what all the fuss was about. The study followed 1098 joggers and 413 sedentary individuals in Copenhagen, Denmark; all subjects were apparently healthy when the study began. Using jogger responses to a questionnaire, the researchers divided them into light (n=576), moderate (n=262), and strenuous (n=40) jogger groups. A total of 145 all-cause deaths occurred during a ~13 year follow-up period, with 128 deaths in the sedentary group and 17 deaths in the jogging groups. Light and moderate joggers had significantly lower risks of dying when compared to the sedentary group. Surprisingly, strenuous joggers were reported to have the same risk of dying as the sedentary group. Further inspection reveals a potential flaw in this conclusion. Because there were only 2 deaths in the group of 40 strenuous joggers, there was not enough statistical power to warrant any conclusion regarding this group. Let me give you an analogy. Say that 2 blindfolded men ran across a busy highway and were not struck by a car. Would anyone conclude based on those two events, that it is perfectly safe for everyone to run blindfolded across a busy highway? Of course not! Similarly, a total of two deaths in a tiny sample of 40 strenuous joggers suggest an inadequate sample size to anyone who is familiar with research design and statistics. As a comparison, a 2013 Cooper Center Longitudinal Study paper3 published in Circulation:Heart Failure  showed that moderate to high levels of CRF significantly reduced the risk of dying from heart failure. The sample size in that study was 44,674 men who were tracked for an average of 20 years, with 153 heart failure deaths occurring during the follow-up period. Now that’s a Texas-sized sample size where a meaningful conclusion can be stated!

It is very surprising that the limitations of the Copenhagen jogging study were not more strongly emphasized in the manuscript. One must have an adequate sample size with an adequate number of events i.e., deaths before any type of meaningful conclusion can be made.

While no one doubts that you can get too much of a good thing with exercise,  this study did little to identify how much is too much.

To learn more about cardiorespiratory fitness and the role of interval training, consider the Cooper Institute’s 1-day Interval Training workshop. You don’t need to be a fitness professional to take this course, the general public is welcome.

 

References

1Blair, S.N., Kohl, H.W., Paffenbarger, R.S., Clark, D.G., Cooper, K.H., Gibbons, L.W. (1989). Physical fitness and all-cause mortality. JAMA. 262(17):2395-2401. 2Schnohr, P., O’Keefe, J.E., Marott, J.L., Lange, P., Jensen, G.B. (2015). Dose of jogging and long-term mortality: the Copenhagen city heart study. JACC. 65(5):411-419. 3Farrell, S.W, Finley, C.E, Radford, N.B, Haskell, W.L. (2013). Cardiorespiratory fitness, body mass index, and heart failure mortality in men: Cooper Center Longitudinal Study.Circ Heart Failure. 6(5):898-905.