Walking for 1 hour or more daily was associated with a 14% lower risk for breast cancer, and high physical activity was associated with a 25% lower risk, compared with the risk for most sedentary women, according to findings from a large prospective study published online October 4 in Cancer Epidemiology, Biomarkers, and Prevention.
"Our results clearly support an association between physical activity and postmenopausal breast cancer, with more vigorous activity having a stronger effect," senior author Alpa Patel, PhD, from the Epidemiology Research Program, American Cancer Society, Atlanta, Georgia, said in a news release. "Our findings are particularly relevant as people struggle with conflicting information about how much activity they need to stay healthy."
Although epidemiologic studies have suggested an inverse association between physical activity and postmenopausal breast cancer, it was previously unknown whether moderate activities such as walking could be protective. Whether time spent sitting is linked to breast cancer risk and the effects of estrogen receptor (ER) status, body mass index (BMI), adult weight gain, or use of postmenopausal hormones (PMH) on such associations were also previously unknown.
Between 1992 and 2009, 4760 of 73,615 postmenopausal women in the prospective American Cancer Society Cancer Prevention Study II Nutrition Cohort were diagnosed with breast cancer.
At study enrollment, 9.2% of women reported no recreational physical activity. Among the remaining women, the average energy expenditure was equivalent to 3.5 hours per week of moderately paced walking, and most of the women participated primarily in moderate-intensity activities such as walking, cycling, aerobics, and dancing. Fewer women reported engaging in vigorous-intensity activities such as running, swimming, and tennis. Nearly half (47%) of women reported walking as their only recreational activity.
Compared with sedentary women, physically active women tended to be leaner, more likely to maintain or lose weight during adulthood, more likely to drink alcohol, less likely to be currently smoking, more likely to use PMH, and more likely to have undergone mammography in the past year.
The investigators used extended Cox regression to estimate relative risks (RRs) of breast cancer after multivariable adjustment for race, education, BMI, adult weight change, alcohol intake, smoking, age at menopause, number of live births/age at first live birth, breast cysts, hysterectomy, oophorectomy, family history of breast cancer, mammography, and PMH use.
Compared with the least-active group of women, who reported between 0 and less than 7 metabolic equivalent hours per week of physical activity, the most active women (>42 metabolic equivalent hours per week) had a 25% lower risk for breast cancer (95% confidence interval [CI] for RR, 0.63 - 0.89; P for trend = .01).
Among women who reported walking as their only recreational activity, those walking 7 or more hours per week had a 14% lower risk than those walking 3 or fewer hours per week (95% CI for RR, 0.75 - 0.98).
Sitting time was not associated with breast cancer risk. ER status, BMI, weight gain, and PMH use did not affect the observed inverse associations between physical activity and postmenopausal breast cancer.
"Without any other recreational physical activities, walking on average of at least one hour per day was associated with a modestly lower risk of breast cancer," Dr. Patel said. "More strenuous and longer activities lowered the risk even more."
The study's limitations include failure to consider total physical activity in those whose occupations involve manual activity and a lack of generalizability to populations that are not predominantly white, middle-aged or elderly, and well-educated.
Although current guidelines recommend at least 2.5 hours per week of moderate-intensity activity, or 75 minutes per week of vigorous-intensity aerobic activity, for adults to improve overall health, less than half of US adult women achieve these activity levels.
"[P]romoting walking as a healthy leisure-time activity could be an effective strategy for increasing physical activity among postmenopausal women," Dr. Patel concluded.
The authors have disclosed no relevant financial relationships.
Cancer Epidemiol Biomarkers Prev. 2013;22:1906-1112.