In a small study, women who had been diagnosed with breast cancer were 30 percent less likely to see a return of the disease if they had a history of breast-feeding. While previous research has found a small protective effect of breast-feeding and breast cancer risk, researchers from Kaiser Permanente's Division of Research believe this is the first study to explore an association between nursing and a recurrence of cancer.
In the study, researchers asked 1,636 women who had breast cancer, mostly Kaiser patients, to fill out a questionnaire that included their history of breast-feeding. "We followed them for over nine years, and we identified the women who had a recurrence and also those who died of breast cancer," said lead author Marilyn Kwan, Ph.D., with Kaiser. "And we (found) this dramatic reduction in risk of recurrence."
In addition to the reduced risk of recurrence, the researchers found a 28 percent reduced risk of dying from the disease among women who breast-fed.
They study also looked at specific types of tumors. They found that women who had breast-fed were more likely to develop a more common -- and more easily treatable -- kind of tumor, known as luminal A subtype, which includes estrogen-receptor positive tumors.
"These tumors are less likely to spread, to metastasize, and they're treatable with hormonal therapy such as tamoxifen," Kwan said. "Because of this, they have better prognosis. Women who have luminal A tumors are more likely to survive."
The study was published in the Journal of the National Cancer Institute and was funded by the National Institutes of Health.
Kwan said that breast-feeding "can increase the maturation of the cells in the breast." She theorized that it may set up a "molecular environment that makes a tumor more responsive to therapy, so women, in the end, are less likely to have a recurrence or die from their disease."
Experts not affiliated with the study called it interesting, but noted that it was conducted among a fairly small sample of women. Of the 1,600 women in the study, 383 had a recurrence of cancer during the nine-year study period, and 290 women died of the disease. More than 200,000 women in the U.S. are projected to be diagnosed with breast cancer in 2015.
"It's a hypothesis that supports doing more research," said Dr. Otis Brawley, chief medical officer with the American Cancer Society. "We can't take this study and tell women, 'Breast-feed, and you are more likely to get the easier-to-treat breast cancer.' "
Intriguing Questions for African-American Women
Brawley also theorized that this study could have implications for African-American women who have higher rates of diagnosis of basal-type tumors, also called "triple negative," which are more aggressive. Women with these types of tumors face a poorer prognosis.
Thirty percent of African-American women are diagnosed with these basal-type tumors, but only 20 percent of Caucasian women are. African-American women have a higher mortality rate from breast cancer overall. And, Brawley noted, African-American women have lower rates of breast-feeding.
"Maybe breast-feeding, or lack thereof, is part of the reason for that difference," Brawley said of the breast cancer disparity. "I can only say maybe."
Karuna Jaggar, executive director of Breast Cancer Action, a San Francisco-based advocacy group, said that overall the study adds to our understanding of breast cancer. "(This research) underscores that breast cancer is a life course disease," she said. "Exposures and activities that happen earlier in life impact breast cancer risk and mortality," both positively and negatively.