Monday November 3, 2008 (foodconsumer.org) -- Physical activity or exercise may reduce risk of breast cancer in certain groups of women, according to a review study published in the August, 2008 issue of British Journal of Sports Medicine.

The study led by Friedenreich CM and Cust AE of Alberta Cancer Board found that 76 percent of 62 studies showed physical activity was associated with a reduction of 25 to 30 percent in breast cancer risk/

The researchers found stronger risk reduction in breast cancer risk among postmenopausal women, women with normal BMI, non-white racial groups, those with hormone receptor negative tumours, women without a family history of breast cancer and parous women who engaged in recreational activity, lifetime or later life activity and vigorous physical activity.

The review was based on 34 case-control and 28 cohort studies published before September 2007.

Another recent study led by Michael F Leitzmann and colleagues at the National Cancer Institute found that postmenopausal women with body mass index lower than 25 kg/m2 engaged in vigorous physical activity were 23 percent less likely to develop breast cancer.    But no such association was found in overweight and obese women.

For the study, the researchers followed 32,000 women enrolled in the Breast Cancer Detection Demonstration Project Follow-up Study to examine if there was an association between risk of breast cancer and physical exercise.

Physical activity was surveyed by self-administered questionnaire and postmenopausal breast cancer cases were identified through self-reports, death certificate and state cancer registries.    During the 11-year follow-up, 1506 new incident cases of postmenopausal breast cancer were identified.

The researchers found that after adjusting a variety of risk factors, there was a weak inverse association between total physical activity and postmenopausal breast cancer.

The study was published in the Oct. 2008 issue of Breast Cancer Research.

Most studies are observational, meaning that it not necessarily that physical activity indeed causes the reduction in breast cancer risk.   However, the possibility is real because there is evidence that physical activity boots the body's immune response and helps fight inflammation, which are believed to be important in the fight against cancer.

Addition, exercise can also alter levels of certain hormones including sex hormones and insulin-like growth factors, which can promote the growth and spread of tumors.

Those who want to know the biological mechanisms whereby physical activity may influence the risk of breast cancer may read the review by Friedenreich and Cust.