Postmenopausal women who maintain a regular, moderate to vigorous exercise program reduce their risk for breast cancer, even if they did not exercise in the past, according to a study published online October 1 in BMC Cancer. “Among the few modifiable risk factors for breast cancer, a high versus low level of physical activity has been consistently associated  with 20-40% reduced risk of postmenopausal breast cancer,” write Tricia M. Peters, MPhil, of the National Cancer Institute in Bethesda, Maryland, and colleagues of the National Institute of Health-American Association of Retired Persons Diet and Health Study. “In order to better characterize the physical activity-breast cancer relation, further investigation of specific parameters of physical activity such as the intensity and the time of life of physical activity that may be most effective for breast cancer prevention among postmenopausal women is needed.” The study, begun in 1995, consisted of 118,899 women, ages 50 to 71 years, who answered questions about their exercise habits during 4 periods of their lives: ages 15 to 18 years, 19 to 29 years, 35 to 39 years, and in the past 10 years. Participants also indicated the number of hours exercised per week, from less than 1 to more than 7, and whether their activities were light (eg, bowling and fishing) or moderate to vigorous (eg, jogging and swimming) for each of the periods. During 6.6 years of subsequent tracking, 4287 breast cancers, mostly estrogen receptor (ER)–positive (84%; n = 1352), were diagnosed among the participating women. Researchers found that women who maintained a high level of activity for more than 7 hours a week during the 10 years before the study reduced their risk for breast cancer by 16% vs more sedentary women in age-adjusted and multivariate (each relative risk [RR], 0.84; 95% confidence interval [CI], 0.76 – 0.93) models. Adjustment for light exercise during the recent decade did not significantly affect the risk (RR, 0.85; 95% CI, 0.76 – 0.95). Further adjustment for body mass index (BMI) had limited impact on the correlation between brisk activity and the risk for breast cancer (RR, 0.87; 95% CI, 0.78 – 0.96). The authors noted that their results regarding physical activity after menopause are consistent with previous findings. “Our observation that recent physical activity showed a stronger inverse association with breast cancer risk than historical activity is supported by two systematic review and three prospective studies among postmenopausal women,” they point out, citing a review published in Epidemiology (2007;18:137-157) and a cohort study in the Journal of the American Medical Association (2003;290:1331-1336), among others. Combined light and moderate to vigorous exercise across the lifespan somewhat affected risk (P = .49 -.53). A greater positive relationship between risk and all activity, regardless of intensity level, existed for ages 19 to 29 years and ages 35 to 39 years (P = .79 for both light and moderate to vigorous activity) vs light and moderate to vigorous exercise during the 10 years before the study or for the 15- to 18-year-old period (P = .35 for light activity; P = .28 for moderate to vigorous activity). Reasons for the link between activity and reduction of breast cancer risk may include the ability of exercise to reduce levels of endogenous sex hormones, modulate insulin and insulin-like growth factors, increase immunity, and reduce ongoing inflammation, according to the researchers. One limitation of the study was a relatively low response rate by the 3.5 million members of the American Association of Retired Persons who initially received the questionnaires. The authors also note that subjects’ ability to remember physical activity that occurred 10 years ago vs in the more distant past may have colored the results. They point out that future studies of lifelong physical activity are needed to verify their findings and provide more details about how exercise intensity and timing affect breast cancer risk. “Although controlled trials or intervention studies are the ideal study designs for disentangling the ‘dose’ of physical activity that may influence breast cancer risk, the cost and duration of such studies for researching the association of physical activity intensity and timing with primary breast cancer limits their feasibility,” the authors write. “However, interventions and experimental research will be imperative to investigate the mechanism by which physical activity intensity and timing influence breast cancer risk.” The Intramural Research Program of the National Institutes of Health, National Cancer Institute supported this study. The study authors have disclosed no relevant financial relationships. BMC Cancer. Published online October 2, 2009. Abstract Clinical Context Breast cancer is the most common cancer affecting US women. Exercise has been shown to be protective, with a 20% to 40% reduced risk for postmenopausal breast cancer associated with high vs low physical activity levels. However, the relationship between timing of exercise and intensity with the risk for breast cancer is uncertain. This is a prospective cohort study of postmenopausal women to examine the association between self-reported recent and historical levels of physical activity and intensity of activity and the risk for breast cancer. Study Highlights

• The National Institute of Health-American Association of Retired Persons Diet and Health Study was initiated in 1995 and 1996 with a cohort of men and women aged 50 to 71 years from 6 US states and 2 metropolitan areas.
• Excluded were men, women with prevalent cancer at baseline, implausible measurements of height or weight, no physical activity or menopausal data, or no follow-up data. 118,899 women were followed up prospectively for 6.6 years.
• Breast cancer cases were identified through linkage with 11 state cancer registries, with a sensitivity of more than 90% for case finding.
• Histology of breast cancer was obtained from the cancer registries, and tumor ER and progesterone receptor status was available for 7 of 11 states.
• Breast cancer was classified as ductal, lobular, or mixed ductal-lobular.
• Physical activity was by self-report with duration per week in 6 categories from never to more than 7 hours per week, and the first 2 were collapsed into 1 for a total of 5 categories.
• Light-intensity activities included bowling, golfing, table tennis, gardening, fishing, and light housework.
• Moderate- to vigorous-intensity activity included hiking, jogging, handball, aerobics, and heavy housework.
• Exercise was defined as light or moderate/vigorous, for 4 age periods (ages 15 – 18 years, 19 – 29 years, 35 – 39 years, and the previous 10 years).
• The physical activity instrument was developed with reference to the Physical Activity Scale for the Elderly.
• Mean age at baseline was 62.9 years, 91.2% were Caucasian, and 31.5% were college educated.
• Light activities were more prevalent than moderate to vigorous activity, and moderate to vigorous activity was more likely during younger age periods.
• At 6.6 years of follow-up, 4287 incident breast cancers were diagnosed, including 736 in situ tumors.
• The majority (84%) were ER-positive tumors.
• Women reporting moderate to vigorous physical activity in the past 10 years were more likely to drink alcohol and less likely to smoke vs inactive women.
• Women with breast cancer were more likely to have a positive family history of breast cancer, earlier age at menarche, later age at first birth, lower age at menopause, and higher BMI and alcohol intake.
• There was no association between exercise (light or moderate to vigorous) at ages 15 to 18 years, 19 to 29 years, or 35 to 39 years and postmenopausal cancer risk.
• For exercise within the past 10 years, women reporting moderate to vigorous, but not light, activity for 7 hours per week or more had a 16% reduced risk for breast cancer (RR, 0.84) independent of BMI and of light activity.
• The association was significant for ER-positive (RR, 0.77), but not ER-negative, tumors; was of similar magnitude for invasive and in situ tumors; and did not vary by tumor histologic features.
• The risk estimates were not affected by BMI at age 18 years, history of mammography, previous benign breast disease, or age at menopause.
• The authors concluded that only moderate to vigorous physical activity of at least 7 hours weekly within, but not before, 10 years was associated with reduced risk for ER-positive breast cancer in postmenopausal women.

Clinical Implications

• The risk for breast cancer in postmenopausal women is associated with moderate to vigorous physical activity within, but not before, 10 years.
• The risk for ER-positive breast cancer in postmenopausal women is 16% lower in those reporting 7 hours or more per week of moderate to vigorous physical activity vs no activity.