Background: Epidemiologic evidence strongly suggests that cumulative exposure to ovarian hormones is a determinant of breast cancer risk. Because physical activity can modify menstrual cycle patterns and alter the production of ovarian hormones, it may reduce breast cancer risk; yet few epidemiologic studies have assessed this relationship. Purpose: The major ob-jective of this study was to determine whether young women (aged 40 and younger) who regularly participated in physical exercise activities during their reproductive years had a reduced risk of breast cancer. Methods: Using a case-control design, we conducted personal interviews of a total of 545 women (aged 40 and younger at diagnosis) who had been newly diagnosed with in situ or invasive breast cancer between July 1, 1983, and January 1, 1989, and a total of 545 control subjects. Case patients and control subjects were individually matched on date of birth (within 36 months), race (white), parity (nulliparous versus parous), and neighborhood of resi-dence. Lifetime histories of participa-tion in physical exercise activities on a regular basis were obtained during the personal interview. Results: After ad-justment for potential confounding fac-tors, we found that the average number of hours spent in physical exercise ac-tivities per week from menarche to 1 year prior to the case patient's diag-nosis was a significant predictor of reduced breast cancer risk (two-sided P for trend 3.8 hours/wk of exercise) women to inactive women, the ORs were 0.28 (95% CL = 0.16, 0.50) for parous and 0.73 (95% CL = 038, 1.41) for nulliparous women. Conclusions: Most previously identified risk factors for breast cancer are reproductive and menstrual events that cannot be readily altered. The protective effect of exer-cise on breast cancer risk in the women whom we studied suggests that physical activity offers one modifiable lifestyle characteristic that may substantially reduce a woman's lifetime risk of breast cancer. Implications: Whether the protective effects of exercise on breast cancer risk are due to alterations in ovarian function and whether they extend into women's menopausal years need to be established. Our results suggest that implementation of regular physical exercise programs as a critical component of a healthy lifestyle should be a high priority for adolescent and adult women. [J Natl Cancer Inst 86: 1403-1408, 1994]. © 1994 Oxford University Press.
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