Moderate physical activity may reduce postmenopausal breast cancer risk, according to the results of a prospective study in the October 25 issue of the Archives of Internal Medicine.
“Physical activity has many health benefits,” write A. Heather Eliassen, ScD, from Brigham and Women’s Hospital and Harvard Medical School in Boston, Massachusetts, and colleagues. “Although greater activity has been related to lower postmenopausal breast cancer risk, important details remain unclear, including type, intensity, and timing of activity and whether the association varies by subgroups.”
Among 95,396 postmenopausal women enrolled in the Nurses’ Health Study, the investigators determined specific and total activity self-reported every 2 to 4 years since 1986 and examined the associations of activity (measured as hours of metabolic equivalent task values (MET-hours) with breast cancer risk. Hazard ratios (HRs) and 95% confidence intervals (CIs) were determined from Cox proportional hazards models.
There were 4782 cases of invasive breast cancer documented during 20 years of follow-up from 1986 to 2006. Breast cancer risk was lower in women reporting higher amounts of recent total physical activity (” 27 MET-hours per week; approximately 1 hour per day of brisk walking) vs those reporting less than 3 MET-hours per week (< 1 hour per week of walking; HR, 0.85; 95% CI, 0.78 – 0.93; P < .001 for trend for simple update, and HR, 0.88; 95% CI, 0.79 – 0.98; P = .003 for trend for cumulative average).
Compared with women who were least active at menopause and did not increase activity during follow-up (< 9 MET-hours per week; approximately 30 minutes of average-paced walking on most days of the week), those who increased their activity (< 9 MET-hours per week at menopause and ” 9 MET-hours per week during follow-up) had a reduced risk (HR, 0.90; 95% CI, 0.82 – 0.98). Among specific activities modeled simultaneously, brisk walking predicted lower risk (per 20 MET-hours per week [5 hours per week]: HR, 0.91; 95% CI, 0.84 – 0.98; P = .01).
“The association with total activity did not differ significantly between estrogen and progesterone receptor”positive and -negative tumors (P = .65 for heterogeneity),” the study authors write. “Our findings suggest that moderate physical activity, including brisk walking, may reduce postmenopausal breast cancer risk and that increases in activity after menopause may be beneficial.”
Limitations of this study include reliance on self-report for physical activity; possible underestimation of the true association between physical activity and breast cancer risk; and homogeneity of the study population, limiting generalizability.
“The equivalent of 5 h/wk of brisk walking was sufficient to reduce the risk of breast cancer, an amount consistent with the US government’s guidelines for adults to achieve additional health benefits beyond minimal activity,” the study authors conclude. “The lack of significant difference by ER/PR [estrogen/progesterone receptor] status, BMI [body mass index], and PMH [postmenopausal hormone] use and the attenuation but not elimination of the association with adjustment for weight change suggest that activity may be acting, at least in part, on a nonhormonal pathway.”
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