THE AUTHORS REPLY

Urho M. Kujala1, Jaakko Kaprio2 and Markku Koskenvuo3

1 Unit for Sports and Exercise Medicine, Institute of Clinical Medicine, University of Helsinki, Mannerheimintie 17, 00250 Helsinki, Finland
2 Department of Public Health, University of Helsinki, and Department of Alcohol Research, National Public Health Institute, Helsinki, Finland
3 Department of Public Health, University of Turku, Turku, Finland

We thank Professor Borgdorff for his comments (1). In our study (2), the risk of death was lower among persons who were persistently sedentary workers than among persistently nonsedentary workers (age- and sex-adjusted hazard ratio = 0.66). To answer the question addressed by Professor Borgdorff, we calculated the hazard ratios for death by category of participation in vigorous leisure physical activity among persistently sedentary workers and persistently nonsedentary workers (table 1). The beneficial effect of vigorous leisure physical activity tended to be stronger among persons who were sedentary at work. This is an expected finding, since totally sedentary people are at higher risk for metabolic syndrome diseases. However, the interaction between sedentary work and vigorous leisure activity was statistically nonsignificant for each vigorous activity category (table 1), and the overall interaction between sedentary work and vigorous leisure physical activity was statistically nonsignificant (p = 0.072) in the 12,885 individuals included in the analysis. Therefore, we did not proceed to pairwise analyses, where the number of discordant pairs was much lower and the statistical power was lower. Even though pairwise analyses are very informative (2), the rarity of discordant twin pairs makes the research challenging. Several large twin cohorts with comparable data would be needed to tackle such specific questions on subgroups in the population, using pairwise analyses with covariates.


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TABLE 1. Hazard ratios for all-cause mortality from 1982 to June 30, 2001, by vigorous leisure physical activity and work-related physical activity, among members of the Finnish Twin Cohort who were healthy and aged 24–60 years at the end of 1981
 

REFERENCES

  1. Borgdorff MW. Re: "Modifiable risk factors as predictors of all-cause mortality: the roles of genetics and childhood environment." (Letter). Am J Epidemiol 2003;158:392.[Free Full Text]
  2. Kujala UM, Kaprio J, Koskenvuo M. Modifiable risk factors as predictors of all-cause mortality: the roles of genetics and childhood environment. Am J Epidemiol 2002;156:985–93.[Abstract/Free Full Text]