Exercise and other physical activity is associated with lower risk of breast cancer. This raises the question as to whether a lifestyle that includes long periods of sitting increases the risk of breast cancer (even in regular exercisers). Previous studies have reported that sitting at work compared to more active occupations is associated with increased risk.
However, this does not answer the question as to whether sitting is in and of itself harmful. Now a new prospective study has reported that postmenopausal women with sedentary occupations who engage in regular exercise are not at increased risk simply as a result of sitting for long periods at work.

Association of sitting with breast cancer risk

Physical activity is thought to reduce risk of breast cancer through its influence on circulating sex hormones, insulin sensitivity, inflammation, and weight control. Previous studies have not focused on sitting, per se, but have included sitting as a variable in examining the association between physical activity and risk of breast cancer.
A large U.S. study of postmenopausal women reported that routine activity during the day at work or at home that included heavy lifting or carrying was associated with a 38% lower risk of breast cancer compared to mostly sitting.
Another large study, this time of Chinese working women aged 40 to 70 years, collected data on exercise during adolescence and adulthood, household activity, occupational sitting time, work-related physical activity, and walking and cycling for transportation.
Breast cancer risk was found to be 19% lower among women in the lowest fourth of work-related sitting time and 27% lower in the highest fourth of average work-related energy expenditure. Having both an active job and exercise participation did not in and of itself confer any additional reduction in risk, according to analysis of joint effects.

Latest research finds sitting can be offset by regular exercise

A prospective study recently presented at the annual American Association for Cancer Research (AACR) meeting was designed to investigate the influence of both physical activity and long periods of sitting on risk of breast cancer in postmenopausal women. The study included 73,608 initially cancer-free postmenopausal women in the American Cancer Society Cancer Prevention Study II (CPS-II) Nutrition Cohort, which began enrollment in 1992. A total of 4,622 of the women were diagnosed with breast cancer during the 1992 to 2007 study period.
Women who engaged in the most physical activity (over 42 metabolic equivalent (MET) hours per week) were found to have a 22% lower risk of breast cancer than women with the lowest levels of activity (less than 7 MET hours/week). Eight MET hours/week amounts to approximately 160 minutes of fast walking per week. In the study, walking an hour per day reduced breast cancer risk by 15%. Time spent sitting was not found to be associated with breast cancer risk (comparing at least six hours of sitting per day with less than three hours).
When the data was analyzed according to tumor stage, the association between physical activity and risk of breast cancer was found only for tumors localized in the breast. Lack of physical activity was not found to be associated with in situ disease (DCIS or LCIS), regional disease (breast cancer in lymph nodes or chest wall), or distant metastasis, a finding that needs to be explained. In addition, the association did not differ by ductal or lobular histology or hormone receptor (ER/PR) status. Generally speaking, the association between physical activity and risk held regardless of body mass index (BMI) or use of hormone replacement therapy (HRT) (also known as menopausal hormone therapy). However, the lower risk of breast cancer associated with the highest level of physical activity was observed only in women not using HRT.

Bottom line

Physical activity is associated with lower risk of postmenopausal breast cancer and the benefits of physical activity do not appear to be limited to any specific subgroup of women. Sitting for long periods is not in and of itself associated with breast cancer risk; women with sedentary occupations who engage in regular exercise are not at increased risk as a result of long periods of sitting at work.