Recreational physical activity and risk of breast cancer

Physical activity has been proposed as a modifiable risk factor for breast cancer because of its effects on circulating sex hormones. Overall, results from previous studies support the hypothesis that regular physical activity may reduce the risk of invasive breast cancer; however, the association between physical activity and in situ breast cancer is not well-understood. We examined the association between physical activity and breast cancer risk in two different study populations. The first study was the American Cancer Society's Cancer Prevention Study II Nutrition Cohort, a prospective cohort followed from 1992 through 1997. Information on physical activity was obtained in 1992 via a self-administered questionnaire for 72,608 postmenopausal female participants who were cancer-free. During the five-year follow-up, l,520 incident breast cancer cases were identified among these women. Women who were most physically active (>42.0 MET-hours/week) at baseline had 29% lower incidence rates than active women with the least activity (>0--7.0 MET-hrs/wk) (95% CI, 0.49--1.02). The difference in risk was largest for localized breast cancer, and for women who did not use HRT at enrollment. The second study was a population-based case-control study of 567 in situ breast cancer patients and 616 properly screened control subjects from Los Angeles County between March 1, 1995 and May 31, 1998. The risk of in situ breast carcinoma (BCIS) was approximately 35 percent lower among women with ever lifetime exercise activity compared to ever inactive women. We observed no linear trend in BCIS risk with increasing levels of exercise activity, measured either by average hours per week or by MET-hours per week (p-trend = 0.24 and p-trend = 0.27, respectively). When examining risk of BCIS associated with exercise activity during specific time periods, we observed a modest inverse relationship with exercise activity within the ten years after menarche (p-trend = 0.18) and a significant inverse trend with exercise activity during the ten years prior to a woman's reference date (p-trend = 0.02) and BCIS risk. These findings suggest that exercise activity, particularly in recent years, may be important in predicting risk of both invasive and in situ breast carcinoma.