Dose of Jogging and Long-Term Mortality: The Copenhagen City Heart Study | Journal Scan

Feb 02, 2015
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Authors:
Schnohr P, O’Keefe JH, Marott JL, Lange P, Jensen GB.
Citation:
J Am Coll Cardiol 2015;65:411-419.
Summary By:
David S. Bach, MD, FACC

Study Questions:

Is there a “U”-shaped mortality relationship between exercise and mortality?

Methods:

As part of the Copenhagen City Health Study, 1,098 healthy joggers and 3,950 healthy nonjoggers have been prospectively followed (since 2001) for 12 years. Cox proportional hazards regression analysis was performed with age as underlying time-scale and delayed entry result.

Results:

Compared with sedentary nonjoggers, 1-2.4 hours of jogging per week was associated with the lowest mortality (multivariate hazard ratio [HR], 0.29; 95% confidence interval [CI], 0.11-0.80). Optimal frequency of jogging was from 2-3 times per week (HR, 0.32; 95% CI, 0.15-0.69), or ≤1 time per week (HR, 0.29; 95% CI, 0.12-0.72). Optimal pace was slow (HR, 0.51; 95% CI, 0.24-1.10) or average pace (HR, 0.38; 95% CI, 0.22-0.66). The joggers were divided into light, moderate, and strenuous joggers. The lowest HR mortality was found in light joggers (HR, 0.22; 95% CI, 0.10-0.47), followed by moderate joggers (HR, 0.66; 95% CI, 0.32-1.38), and strenuous joggers (HR, 1.97; 95% CI, 0.48-8.14).

Conclusions:

The authors concluded that their findings suggest a U-shaped association between all-cause mortality and jogging dose as calibrated by pace, quantity, and frequency of jogging. The authors stated that light and moderate joggers have a lower mortality than sedentary persons, whereas strenuous joggers have a mortality rate not statistically different from the sedentary group.

Perspective:

In this intriguing study based on self-reported survey data from an all-white population in Copenhagen, a very little bit of exercise seems to go a long way toward reducing cardiovascular risk, but just a little more exercise purportedly is detrimental. There are some issues with the study, making the conclusions more speculative than definitive:

  1. Contrary to the discussion, these data have nothing to do with any possible cardiac risk associated with endurance and ultra-endurance athletics. The heavy-duty exercisers in this study exercised either 4 times per week for a total of at least 2.5 hours, or no more than 3 times per week for a total of at least 4 hours –– nothing close to the training of endurance athletes.
  2. Although the introduction and discussion are all about cardiovascular mortality, the outcome assessed was all-cause mortality.
  3. There was little if any difference in body mass index across the spectrum of joggers, suggesting that there was not much difference in physical fitness between light and strenuous joggers.
  4. With the exception of strenuous joggers, there was a surprisingly high prevalence of smoking among joggers, raising questions about whether this population of joggers really is representative of habitual exercisers.
  5. There are no reported statistical comparisons between groups of joggers; the stated ‘significantly lower mortality rates’ with low and moderate compared to strenuous joggers does not appear to be supported by data. The wide CIs surrounding the reported HRs (Figure 1) suggest that there was no significant difference between groups of joggers.
  6. Any difference in mortality trends between more and less strenuous joggers was apparent only after adjusting for co-variables. The trends toward lower prevalence of smoking and diabetes among the strenuous joggers might be an explanation for why this group appeared to have a higher adjusted risk despite no increase in unadjusted risk.

Keywords: Exercise, Jogging, Physical Fitness, Risk Factors, Risk Reduction Behavior, Cardiovascular Diseases, Regression Analysis


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