Papers

Mortality in joggers: population based study of 4658 men

Peter Schnohr, chief cardiologist ^a, Jan Parner, statistician ^b, Peter Lange, chief physician ^c. 

^a Copenhagen City Heart Study, Bispebjerg University Hospital, DK-2400 Copenhagen NV, Denmark, ^b Department of Biostatistics, University of Copenhagen, DK-2200 Copenhagen N, Denmark, ^c Department of Respiratory Disease, Hvidovre University Hospital, DK-2650 Hvidovre, Denmark

Correspondence to: P Schnohr peterschnohr{at}dadlnet.dk

Since 1970 jogging has become an increasingly popular form of exercise, but the public's concern over its harmful effects has been raised following reports of deaths during jogging. During the past two or three decades the number of joggers and jogging races has increased further, prompting an interest in mortality of joggers.

	Participants, methods, and results

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The Copenhagen city heart study is a prospective population study of cardiovascular disease in 19 698 men and women aged 20 years and over randomly selected from the population of Copenhagen.¹ Overall, 4658 men (response rate 72%) aged 20-79 years with no prior myocardial infarction attended two examinationsone in 1976-8 and the other in 1981-3. Jogging status was ascertained by asking the participants whether they were joggers. The cohort was followed until 30 November 1998. Information about deaths was obtained from the Danish national population register, which is almost 100% complete.

We observed a major trend in jogging habits over the five year period between the first and second examination. At the first examination, 217 (4.7%) men reported active jogging. Of these, 96 (2.1%) were still actively jogging five years later. Between the two examinations, 106 (2.3%) started jogging, showing an almost constant number of joggers. Overall, 4335 men were non-joggers at both examinations. Crude estimates of death rates showed a higher mortality among non-joggers and a lower mortality among persistent joggers.

An analysis of the influence of jogging on time to death was conducted by using a Cox proportional hazards regression model, with age as the time axis.² In addition to jogging, the model included diabetes, smoking, household income, education, and alcohol consumption as potential confounding variables, and systolic blood pressure, concentrations of plasma total cholesterol and plasma high density lipoprotein cholesterol, and body mass index as intermediate variables. A significant effect of jogging was found only for the group that were joggers at both examinations, with an estimated relative risk of 0.37 (95% confidence interval, 0.19 to 0.71; P=0.003). Jogging was therefore included as a binary variable in the final model: persistent jogging or no jogging or jogging at only one of the two examinations. The relative risk of death in persistent joggers was significantly lower than that in non-joggers or those who jogged at only one of the two examinations (0.39, 0.19 to 0.73; P=0.005) (table). Similar results were found when including only potential confounding variables.

	Comment

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Regular jogging is not associated with increased mortality in men, as shown by the significantly lower mortality in joggers than non-joggers in our study. The lower mortality of joggers could be an effect of the physical training, but it could also be due to other lifestyle attributes or a combination of both. Numerous studies in the disciplines of epidemiology, work physiology, psychology, and biochemistry have all pointed towards a beneficial effect of physical activity on health, but the optimal intensity, frequency, and duration of physical activity has yet to be established.

Although our study was observational, the men were randomly selected from a general population. The estimated effect of jogging did not depend on the inclusion of intermediate variables, supporting the association of jogging with lower mortality.

Whether light, moderate, or vigorous exercise should be recommended to the public has changed through the years. Although light exercise has some value, moderate and vigorous exercise is now considered more favourable for health. ^{3 4} Our study supports this by showing that even a vigorous activity such as jogging is associated with a beneficial effect on mortality.

	Acknowledgments

   Contributors: PS initiated and conducted the study and contributed to the writing of the manuscript with PL. JP was responsible for the statistical analysis of the data and contributed to the writing. PS will act as guarantor for the paper.

	Footnotes

Funding: Danish Heart Foundation.

Competing interests: None declared.

	References

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1.	Appleyard M, Hansen AT, Schnohr P, Jensen G, Nyboe J. The Copenhagen City heart study. A book of tables with data from the first examination (1976-78) and a five-year follow-up (1981-83). Scand J Soc Med 1989; 170(suppl 41): 1-160.
2.	Andersen PK, Borgan Oe, Gill RD, Keiding N. Statistical models based on counting processes. New York: Springer Verlag, 1993.
3.	Fentem PH. Benefits of exercise in health and disease. BMJ 1994; 308: 1291-1295[Free Full Text].
4.	Pate RR, Pratt M, Blair ST, Haskell WL, Macera CA, Bourchard C, et al. Physical activity and public health. JAMA 1995; 273: 402-407[Abstract].

(Accepted 11 May 2000)