The effect of physical activity on breast cancer risk: A cohort study of 30,548 women

Background: In epidemiological studies abundant physical activity has been related to decreased breast cancer risk, though the results have been incon sistent. The purpose of this paper was to study the association of physical activity at leisure and commuting to work and incidence of breast cancer. Methods: The study cohort consisted of 30,548 female participants of the Fi nnish adult health behaviour survey, based on annual random samples of Finn s aged 15-64, collected in 1978-1993. By the end of 1995, 332 breast cancer cases had been diagnosed in the cohort. Relative risks of breast cancer we re adjusted for age at survey, body mass index (BMI), education, length of follow-up, parity and age at first birth using Poisson regression models. R esults: Compared to women exercising less than once a week, the adjusted re lative risk of breast cancer for women exercising once a week was 0.80 (95% confidence interval (CI): 0.58-1.10), for women exercising 2-3 times per w eek 0.92 (95% CI: 0.78-1.22) and for women exercising daily 1.01 (95% CI: 0 .72-1.42). Women who reported commuting, walking or bicycling to work 30 mi n or more daily had slightly lower adjusted risk of breast cancer (RR: 0.87 , 95% CI: 0.62-1.24) than women working at home, being unemployed or drivin g a car to working place. Conclusion: Although a small protective effect of regular physical activity for breast cancer incidence was found in physica l activity when commuting to work, the role of the physical activity in bre ast cancer prevention is still an open question.