Overweight and obese women are at increased risk of breast cancer and of being diagnosed with advanced disease, according to results of a large study published Wednesday in the Journal of the National Cancer Institute.

And it’s not because overweight and obese women get fewer mammograms than normal-weight women. “Even after taking into account how often women were screened for breast cancer, overweight women had a 10 percent increased risk of advanced disease and obese women a 56 percent to 82 percent increased risk of advanced disease compared to normal-weight women,” Dr. Karla Kerlikowske opined.

It’s also not because mammograms are less accurate in heavy women. The researchers were able to show that breast tumors are easy to detect in overweight and obese women, so the increase in advanced disease among overweight and obese women is not because more tumors are missed on mammography in these women. From 1996 to 2005, they collected mammography data on 287,115 postmenopausal women not using hormone therapy who underwent a total of 614,562 mammograms. A total of 4,446 of these women were diagnosed with breast cancer within 12 months of a mammography exam.

The researchers found that the overall breast cancer rates rose in tandem with weight, as did rates of advanced tumors.

As mentioned, differences in mammography use and accuracy did not explain the higher rates of breast cancer among overweight and obese postmenopausal women not taking hormone therapy. The reason may be that being overweight increases circulating estrogen, which in turn promotes tumor growth. Fortunately, 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.

SOURCE: Journal of the National Cancer Institute, November 26, 2008.