By Bahar Gholipour, Staff Writer
Published: 09/03/2013 04:03 PM EDT on LiveScience
Taking hormones to treat the symptoms of menopause is thought to increase women's risk of breast cancer, but this risk doesn't rise equally in all women, a new study finds.
The increase in risk varies depending on a woman's race, body mass index (BMI) and breast density, and some women may benefit from hormone therapy while facing little increase in cancer risk, the study found.
The researchers looked at nearly 1.65 million postmenopausal women ages 45 and older, and found that leaner women, as well as women with denser breasts, were more likely to see the detrimental effects of hormone replacement therapy (HRT) on their breast-cancer risk.
Among underweight and normal-weight women (defined as having a BMI lower than 25) in the study, those who used HRT had a 35 percent increased risk of breast cancer compared with those who did not use HRT. For obese women (those with a BMI of 30 or higher), the risk of breast cancer did not appear to be affected by hormone use. [6 Foods That May Affect Breast Cancer Risk]
Among women with extremely dense breasts, those who took HRT had a 40 percent increased risk of breast cancer compared with those who didn't take hormones, according to the study. High breast density means the breast is made up of more connective tissue, relative to the amount of fat tissue.
The effects of HRT also appeared to depend on race -- among white and Hispanic women in the study, those who used HRT had more than 20 percent increased risk of breast cancer compared to their counterparts who didn't use HRT, but HRT didn't seem to increase the risk of breast cancer in black women.
"Black women, obese women and women with breast tissue composed largely of fat may benefit from HRT use with minimal excess breast-cancer risk," the researchers at the University of Chicago wrote in the study, published today (Sept. 3) in the Journal of the National Cancer Institute.
Previous studies had shown that using hormones to alleviate menopause symptoms is linked with an increase in breast-cancer risk. The findings led to fewer hormone prescriptions for postmenopausal women, and a concurrent decrease in breast-cancer incidence shortly thereafter, which has been attributed to the reduced use of HRT, the researchers said.
In the new study, in line with the previous findings, there was an overall link between HRT use and a higher rate of breast cancer. On average, 578 in 10,000 women who used HRT developed breast cancer, compared with 546 in 10,000 women who didn't use HRT.
The study has some limitations, the researchers said. For instance, the study didn't look at the type of hormone therapy the women used, or for how long.
"The true effect of long-term HRT use on breast-cancer risk might be higher than observed, especially in subgroups of women identified to be sensitive to HRT," the researchers said.
Future studies with more detailed information about other risk factors, including duration of HRT use, are needed to determine the increase in breast-cancer risk that may be due to HRT, the researchers said.