Relationship of Leisure-Time Physical Activity and Mortality
ABSTRACT & COMMENTARY
Synopsis: Among twin pairs, physical activity statistically significantly reduced the risk of death in a prospective study.
Source: Kujala UM, et al. JAMA 1998;279:440-444.
Many studies have shown that physical activity decreases the risk of death in both men and women. However, some investigators have suggested that genetic selection or childhood experiences might select individuals who are more likely to be physically active and, thus, confound the conclusion that exercise reduces the risk of death. This paper reports the results of a prospective cohort study of twins from Finland. Twins were chosen in an effort to minimize genetic and childhood experience biases to better study the true benefit of exercise.
In the study, 19,126 potential participants were identified. Those with chronic diseases were excluded. The final cohort included 15,902 subjects (7925 males and 7977 females). Questionnaires were sent to all members of the study group in 1975, and information concerning many variables, including physical activity and causes of premature death (smoking, alcohol use, obesity, etc.) were asked. The response rate was high. Zygosity was determined by the usual epidemiologic methods.
Physical activity was categorized several different ways, but, for most end points, the cohort was divided into sedentary, occasional exercisers, and conditioned exercisers. Appropriate statistical and data management techniques were used to study the subjects. The study was completed in 1994. Only 15% of the cohort was sedentary, with the remaining individuals being almost equally divided between occasional exercisers and conditioned exercisers. When mortality during follow-up was examined for the entire cohort, the sedentary group fared worst, and the vigorous exercisers had the lowest risk of death.
When the analysis was restricted to same sex twin pairs, the risk of death was found to decline with increasing physical activity in both genders. Unfortunately, it was impossible to perform an analysis on monozygotic twin pairs because, in most cases, physical activity of both members of the pair was identical. Kujala and colleagues conclude that physical activity is a better predictor of mortality than genetics or childhood experiences.
COMMENT BY KENNETH NOLLER, MD
This is an important study for many reasons and can be read on a number of levels. Overall, the study supports the conclusion that exercise is good. Furthermore, the study suggests that exercise can improve survival even in individuals who are closely matched genetically and have similar childhood backgrounds. This conclusion holds for women as well as men. Many previous studies of exercise have failed to include women.
The study can also be read as an example of the way to conduct a correct, prospective, cohort study. For anyone who is interested in research, I would suggest that the Methods section of this article be read in some detail. Studies such as this are extremely difficult (and expensive) to perform. The many adjustments and calculations that must be done are enormous and time-consuming. In addition, as with all prospective cohort studies, the investigator has to be patient. In this case, nearly 20 years elapsed between the start and end of the study.
Finally, the article can be read as a reason for each of us to emphasize even more strongly the importance of exercise for all of our patients. In general, counseling and education are far more important at the time of a routine annual visit than is any part of the physical examination. (Dr. Noller is Professor and Chairman, Department of OB/GYN, University of Massachusetts Medical Center, Worcester.)
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