Women who breast-feed their babies and later develop breast cancer are less likely to have the cancer return or to die from it than women who do not breast-feed, new research shows.
"We found in this study of over 1,600 women with breast cancer that those who previously breast-fed had a 30 percent overall decreased risk of their breast cancer recurring," said study leader Marilyn Kwan, a research scientist at the Kaiser Permanente division of research in Oakland, Calif. "We also found those who previously breast-fed had a 28 percent reduced risk of dying from their breast cancer."
The study was published online April 28 in the Journal of the National Cancer Institute.
Previous research has found that breast-feeding is linked with a lower risk of developing breast cancer in the first place, the researchers said.
"If a woman breast-feeds, she reduces her risk of developing breast cancer by about 5 percent to 10 percent, although other factors come into play," Kwan said, such as the number of children she has had. "We think this is one of the first [studies] to examine the role of breast-feeding and breast cancer outcomes -- prognosis and survival," she added.
Her team found that breast-feeding's protective effect in lowering the chances of recurrence or death from breast cancer was strongest against the most commonly diagnosed breast cancers.
The study builds on previous evidence about the link between breast-feeding and breast cancer, said Dr. Joanne Mortimer, director of Women's Cancer Programs and co-director of the Breast Cancer Program at City of Hope Cancer Center, in Duarte, Calif.
The message that's reinforced in the new research, Mortimer explained, is that ''women who breast-feed get less aggressive breast cancer." While that has been known, the new study adds detailed findings about tumor types, she said.
Kwan's team looked at two groups of women, some diagnosed from 1997 to 2000 and others diagnosed from 2006 to 2013. Most were patients at Kaiser Permanente, a large health maintenance organization.
The researchers found an association between breast-feeding and a more favorable outcome for breast cancer patients, but the study didn't prove cause-and-effect. The investigators found the link to be statistically significant with breast cancers known as luminal A subtypes, including tumors known as estrogen receptor-positive (ER-positive), the most common of all breast cancers. However, they did not find the link to be statistically strong for other subtypes of breast cancer.
While Kwan found an overall 30 percent reduced risk of breast cancer recurrence among those who breast-fed, when they took the length of breast-feeding into account, any duration decreased risk, but the link was not as strong for those who did so for less than six months.
The American Academy of Pediatrics (AAP) recommends exclusive breast-feeding for about six months, followed by continued breast-feeding as foods are introduced, for up to a year or longer. Breast-feeding helps protect babies from diseases such as diabetes, from infection and from becoming overweight. Mothers who breast-feed have a lower risk of developing breast and ovarian cancers, according to the AAP.
Kwan and others speculate that breast-feeding may be protective for a number of reasons. For instance, it reduces a woman's lifetime number of menstrual cycles, and that lowers the accumulated exposure to hormones that can help some cancers grow. Breast-feeding also increases differentiation, or maturation, of the ductal cells in the breast, perhaps making them more resistant to cancer, Kwan explained.
Mortimer said that possible mechanism makes sense, based on a concept that the breast isn't fully developed until a woman has lactated.