Nov. 26 -- TUESDAY, Nov. 25 (HealthDay News) -- Being overweight boosts the risk of getting advanced breast cancer for older women, according to a new study that looked at more than 287,000 women and took into account their mammogram habits.
The weight itself is to blame for the added risk, the researchers concluded.
"Women who are above their healthy weight have higher levels of circulating estrogens," noted study lead author Dr. Karla Kerlikowske, director of the Women Veterans' Comprehensive Health Center at the San Francisco Veterans Affairs Medical Center. "The estrogen is promoting tumor growth," she said.
In previous studies, Kerlikowske and her colleagues looked at postmenopausal women who took hormone replacement therapy (HRT) and found an increased breast cancer risk. In the current study, expected to be published in the Dec. 3 issue of the Journal of the National Cancer Institute, they looked at postmenopausal women not using HRT.
In years past, some research has suggested that the increased risk for breast cancer for obese women may be due to their not getting screened adequately, or because their tumors are perhaps more difficult to detect on mammography.
But those risk factors were ruled out in the current study. "We took into account how often they were screened and how well you could detect [the cancer]," Kerlikowske said. "There was still an increased risk."
In her study, Kerlikowske and her colleagues collected ongoing data from mammograms performed on more than 287,000 women past menopause. The women got routine mammograms. The researchers did not find that the tumors were harder to detect in women who were overweight or obese.
Nevertheless, "the risk of an advanced stage cancer for an obese women is 56 percent to 82 percent higher than for a normal-weight woman," Kerlikowske said. And the findings also revealed the more obese a woman was, the higher her risk for breast cancer. Women who were overweight but not obese had a 10 percent to 35 percent increased risk of breast cancer compared to normal-weight women, the researcher said.
According to Kerlikowske, adding strength to the idea that the increased estrogen in heavy women is fueling the tumor is the fact that the rate of tumors called "estrogen receptor-positive" (which are spurred on by estrogen) increased across the various weight groups, while ER-negative tumors did not.
Kerlikowske's team used the standard definitions of healthy weight, overweight, and obesity. For instance, a 5-foot, four-inch woman who weighs from 107 to 145 was considered at a healthy weight. The same woman weighing 146 pounds or more was considered overweight, and a weight of 175 pounds or more was considered obese.
About two-thirds of American adults are now either overweight or obese, according to recent government statistics.
Of the study, Kerlikowske said: "It's very representative. It's from mammography registries across the U.S."
The research provides valuable new information on basic biology and risk factors for breast cancer, said mammography researcher Dr. Joann Elmore, professor of medicine and adjunct professor of epidemiology at Harborview Medical Center in Seattle. "They basically found that there are biological effects of obesity, and this can influence breast cancer development or progression."
The study results, Elmore said, should inspire women who are above their healthy weight to shed some pounds. While many risk factors -- such as increasing age, being female, or having genetic mutations that raise breast cancer risk -- are not changeable, losing weight remains under a woman's control, she said.
Kerlikowske agreed. "Here is a risk factor to modify," she said.
There's more on breast cancer screening at the U.S. National Cancer Institute.
SOURCES: Karla Kerlikowske, M.D., professor, medicine, epidemiology and biostatistics, University of California, San Francisco; Joann Elmore, M.D., M.P.H., professor of medicine and adjunct professor of epidemiology, Harborview Medical Center, Seattle; Dec. 3, 2008, Journal of the National Cancer Institute