Physical inactivity or lack of exercise may dramatically increase risk of breast cancer, according to a new report published in the Dec 2008 issue of Cancer Causes and Control.

The report by Coyle Y.M at the University of Texas Southwestern Medical Center in Dallas, TX says exercise lowers estrogen levels that if high would cause a higher risk of breast cancer.

Coyle suggests that estrogen induces breast cancer by increasing breast epithelial cell proliferation, the metabolism of estrogen to genotoxic metabolites and the silencing of tumor suppressor genes (TSGS) that have been implicated in breast carcinogenesis.

According to the author, animal studies suggest that exercise slows breast tumor growth by promoting changes in cellular proliferation and apoptosis.

Human studies albeit limited also suggest that exercise produces favorable changes in estrogen metabolism which in turn reduces the risk of breast epithelial cell proliferation.

Coyle says that exercise reduces promoter hypermethylation of TSGS in breast carcinogenesis by lowering estrogen levels.

Further research is needed to clarify the mechanisms that relate to exercise as a negative modulator of breast cancer risk, the author concludes.

Many studies have suggested that regular physical activity or exercise reduces risk of cancer.

One recent study involving 5,968 women led James McClain of the National Cancer Institute and colleagues confirmed previous studies that have shown people who did physical exercise regularly were at lower risk of developing cancer.

The study reported at a meeting of the American Association for Cancer Research also found among women who were in the upper half with regard to the amount of physical activity each week, those who slept less than seven hours per night were 47 percent more likely to develop cancer than those who slept longer.

One study led by Michael F Leitzmann and colleagues at the National Cancer Institute and published in the Oct. 2008 issue of Breast Cancer Research found that postmenopausal women with body mass index lower than 25 kg/m2 who engaged in vigorous exercise were 23 percent less likely to develop breast cancer.      But no such association was found in overweight and obese women.

The researchers followed up 32,000 women who 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.

Another study led by Freedman DM and colleagues from National Cancer Institute and published in Oct 21, 2008 issue of Cancer Causes and Control found exercise such as walking and hiking for 10 or more hours per week rendered the greatest protection against breast cancer in women, a 43 percent reduction in the risk.