Physical Activity and Breast Cancer Risk McTiernan A, Kooperberg C, White E, et al: Recreational physical activity and the risk of breast cancer in postmenopausal women: The women's health initiative cohort study. JAMA 2003;290(10):1331-1336.

Lee I: Physical activity in women: how much is good enough? JAMA 2003;290 (10)1377-79(editorial).

Although previous studies have shown an inverse association between physical activity and breast cancer risk, little is known about the intensity and duration of activity needed. The study population was a large (n=74,171) ethnically and racially diverse group of women aged 50 to 79 recruited from 40 sites throughout the United States. During a mean follow-up of 4.7 years, 1,780 new cases of breast cancer were identified. Women who engaged in strenuous physical activity at least 3 times a week at age 35 years had a 14% decreased risk of breast cancer (RR 0.86; 95% CI 0.78-0.95) compared with less active women. Women who undertook the equivalent of 1.25 to 2.5 hours per week of brisk walking had an 18% reduced risk of breast cancer (RR 0.082; 95% CI 0.68-0.97) compared with inactive women. Strenuous activity reduced risk the most for women in the lightest weight group, although risk reduction also occurred for moderately overweight women.

Subgroup analysis for ages, family history, use of hormone therapy, and parity showed the benefits of physical activity on risk of breast cancer were present across all categories.