Aug. 16, 2011 -- African-American women may reduce their risk of getting an aggressive breast cancer known as ER-negative by breastfeeding, new research suggests.
Julie R. Palmer, ScD, of Boston University, found that having multiple children raised the risk of ER-negative breast cancer in African-American women, but that breastfeeding reduced it.
"Women with two or more kids had 1.5 times the risk of getting ER-negative breast cancer compared with women who had no kids," says Palmer, a professor of epidemiology. "We found that increased risk was really only there for women who had not breastfed."
Palmer's study is published in Cancer Epidemiology, Biomarkers & Prevention.
ER-negative (estrogen receptor-negative) breast cancer is much less common than ER-positive (estrogen receptor-positive). ER-positive requires estrogen to grow.
"ER-negative is more difficult to treat," Palmer says. It has a worse outlook.
Palmer and her colleagues wanted to see how childbirth and breastfeeding might affect the risk of this cancer.
They evaluated women who participated in the Black Women's Health Study. In this ongoing study, 59,000 African-American women were followed from 1995 until 2009.
When they first entered the study, the women answered detailed questions about childbirth and breastfeeding.
They completed another questionnaire every two years. They told researchers whether they had any diagnosis of cancer.
For this study, Palmer identified 457 women with ER-positive cancer and 318 with ER-negative.
''For the most common type of cancer, ER-positive, we found what everyone has found for breast cancer overall," she says. "The more babies you have, the lower the risk of developing [this] cancer."
Breastfeeding didn't affect that association.
"But for the other subtype, we found the opposite relationship," she says. "The more babies you have, the higher the risk of [ER-negative] breast cancer you have."
However, the risk declined in those who had breastfed.
If a woman does not breastfeed, the breast goes through a process called involution, Palmer says. This is marked by increasing inflammation in the breast tissue. This inflammation may be associated with breast cancer.
Women who breastfeed typically reduce the number of breastfeeding sessions, stopping gradually, she says. As result, experts believe the breast returns to its pre-pregnancy state in a more orderly manner, and with less inflammation.