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Original Article
Volume 336:1269-1275 May 1, 1997 Number 18
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Physical Activity and the Risk of Breast Cancer
Inger Thune, M.D., Tormod Brenn, M.Sc., Eiliv Lund, M.D., Ph.D., and Maria Gaard, M.D.

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ABSTRACT

Background Because physical activity may affect hormonal concentrations and energy balance, we decided to investigate whether everyday exercise is related to the risk of breast cancer.

Methods During 1974 to 1978 and 1977 to 1983, a total of 25,624 women, 20 to 54 years of age at entry, enrolled in health surveys and answered questionnaires about leisure-time and work activity.

Results During a median follow-up of 13.7 years, we identified 351 cases of invasive breast cancer among the 25,624 women in the cohort. Greater leisure-time activity was associated with a reduced risk of breast cancer, after adjustments for age, body-mass index (the weight in kilograms divided by the square of the height in meters), height, parity, and county of residence (relative risk, 0.63; 95 percent confidence interval, 0.42 to 0.95), among women who exercised regularly, as compared with sedentary women (P for trend = 0.04). In regularly exercising women, the reduction in risk was greater in premenopausal women than in postmenopausal women, and greater in younger women (<45 years at study entry) than in older women (>45 years) (relative risk, 0.38; 95 percent confidence interval, 0.19 to 0.79). In stratified analyses the risk of breast cancer was lowest in lean women (body-mass index, <22.8) who exercised at least four hours per week (relative risk, 0.28; 95 percent confidence interval, 0.11 to 0.70). The risk was also reduced with higher levels of activity at work, and again there was a more pronounced effect among premenopausal than postmenopausal women.

Conclusions Physical activity during leisure time and at work is associated with a reduced risk of breast cancer.


Source Information

From the Institute of Community Medicine, University of Tromsø, Tromsø (I.T., T.B., E.L.), and the Cancer Registry of Norway, Oslo (M.G.) — both in Norway.

Address reprint requests to Dr. Thune at the Institute of Community Medicine, University of Tromsø, N-9037 Tromsø, Norway.

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