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Overweight Women at Increased Risk of Advanced Breast Cancer

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A nationwide study of over 280,000 women showed that postmenopausal women who are overweight or obese have advanced breast cancer at significantly higher rates than women of normal weight or less than normal weight.

The study, published in the November 26, 2008 issue of the Journal of the National Cancer Institute, was led by Karla Kerlikowske, MD, a physician at San Francisco VA Medical Center and a professor of medicine, epidemiology, and biostatistics at the University of California, San Francisco.

From 1996 to 2005, researchers collected ongoing mammography data on 287,115 postmenopausal women who were not using postmenopausal hormone therapy. They found that overall breast cancer rates went up in direct relationship to weight, as did rates of advanced cancer – specifically, rates of large invasive tumors, advanced-stage tumors, and high nuclear grade tumors. Nuclear grade is a measure of tumor cell growth and rate of cell division; the higher the grade, the more aggressive the tumor.

“The reason may be that being overweight increases circulating estrogen, which in turn promotes tumor growth,” says Kerlikowske.

She says this conclusion is supported by the fact that in the study, increased weight affected only estrogen-receptor positive tumors, and not ER negative tumors.

Kerlikowske led a previous study indicating that estrogen-progestin postmenopausal hormone therapy increases both overall risk of breast cancer and risk of more advanced breast cancer in postmenopausal women.

Fortunately, says Kerlikowske, “this is a modifiable risk factor. There are very few breast cancer risk factors that can be modified. Not taking postmenopausal hormones is one. Maintaining a healthy weight is another.”

One way to do that, she suggests, is regular exercise. “There are no controlled studies, but there’s very good observational data that exercise can decrease your risk of breast cancer,” she says. “So you can lower your risk by maintaining ideal body weight and by exercising on a regular basis, and get cardiovascular benefits as well.”