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Women who are obese and not physically active may be more likely than other women to develop triple-negative breast cancer (breast cancer that is estrogen receptor-negative, progesterone receptor-negative, and HER2-negative). These results were published in Cancer Epidemiology, Biomarkers, & Prevention.

Each year roughly 200,000 U.S. women are diagnosed with breast cancer. Many of these breast cancers will be hormone receptor-positive, meaning that they are stimulated to grow by the circulating female hormones estrogen and/or progesterone. Treatment of hormone receptor-positive breast cancer often involves hormonal therapies that suppress or block the action of estrogen.

Breast cancers that are estrogen receptor-negative, progesterone receptor-negative, and HER2-negative are called triple-negative breast cancers. Triple-negative breast cancers tend to be more aggressive than other breast cancers and have fewer treatment options.

Obesity—often defined as a body mass index (BMI) of 30 or higher—increases the risk of serious health problems such as coronary heart disease, high blood pressure, stroke, type 2 diabetes, and certain types of cancer, including postmenopausal breast cancer. Increased estrogen levels among obese postmenopausal women have been thought to contribute to the increased risk of breast cancer, but obesity may also contribute to breast cancer that is not hormone responsive (such as triple-negative breast cancer).

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To further evaluate the relationship between obesity and risk of breast cancer, researchers evaluated information about 155,723 postmenopausal women enrolled in the Women’s Health Initiative studies. The researchers focused on two broad types of breast cancer: 1) triple-negative breast cancer (diagnosed in 307 of the study participants); and 2) estrogen receptor-positive breast cancer (diagnosed in 2,610 of the study participants).

Although triple-negative breast cancer and estrogen receptor-positive breast cancer have important biological differences, the results of this study suggest that obesity and inactivity may increase the risk of both.

Reference: Phipps AI, Chlebowski RT, Prentice R et al. Body size, physical activity, and risk of triple-negative and estrogen receptor-positive breast cancer. Cancer Epidemiology, Biomarkers, & Prevention. Early online publication March 1, 2011.

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