Breastfeeding Associated with Reduced Risk of Breast Cancer Among Women with Family History
Archives of Internal Medicine, 24 August 2009
Women with a family history of breast cancer appear to have a lower risk of developing the disease before menopause if they have ever breastfed a child, according to a report published in the Archives of Internal Medicine.
Alison M. Stuebe, M.D., M.Sc., then of Brigham and Women's Hospital and Harvard Medical School, Boston, and now of the University of North Carolina at Chapel Hill, and colleagues used information from 60,075 women who had given birth and who participated in the Nurses' Health Study between 1997 and 2005. Each woman completed a detailed questionnaire on demographic characteristics, body measurements and lifestyle factors, with follow-up questionnaires every two years. Breastfeeding history was assessed in detail on the 1997 questionnaire, and on each subsequent follow-up the women were asked to report whether they had been diagnosed with breast cancer.
Through the end of the study—June 2005—a total of 608 cases of premenopausal breast cancer were diagnosed, at an average age of 46.2 years. Women who had a first-degree relative with breast cancer had a lower risk of developing the disease if they had ever breastfed than if they had never breastfed. The association did not appear to change based on duration of breastfeeding, whether breastfeeding was exclusive or whether the woman experienced amenorrhea (absence of menstruation) as a result. There was no association between breastfeeding and breast cancer among women without a family history.
Women who did not breastfeed but used medication to suppress breast milk production also appeared to have a lower risk of breast cancer than women who neither breastfed nor used lactation suppression. This association could be related to disordered involution, or a malfunction in the process by which mammary tissue returns to its pre-pregnant state caused by engorgement and inflammation, the authors note.
"Future studies of interactions among breastfeeding history, family history and genotypes associated with breast cancer risk will be needed to confirm these associations and explore underlying mechanisms," they write. In the meantime, the observed risk reduction compared favorably to that experienced by high-risk women taking hormonal treatments such as tamoxifen, they continue. "Moreover, breastfeeding is associated with multiple other health benefits for both mother and child. These data suggest that women with a family history of breast cancer should be strongly encouraged to breastfeed."
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In both the high- and low-dose groups about 30 percent of participants experienced a clinical benefit — their tumors either shrank or stopped growing. Interestingly, the researchers demonstrated that they could predict fairly accurately which patients would have this positive response. They conducted standard positron emission tomography (PET) scans before estrogen treatment and 24 hours later. If metastatic tumors flared, or glowed more brightly, in the PET scans after estrogen was started, they were much more likely to be affected by estrogen therapy. In 80 percent of women with PET flare reactions, tumors responded to estrogen therapy, and in 87 percent of women without PET flares, tumors did not respond to estrogen.
The participants filled out questionnaires to indicate whether they had adverse reactions to estrogen during the study. Adverse reactions could include headaches, bloating, breast tenderness, fluid retention, nausea and vomiting. Patients receiving the high estrogen dose had more severe side effects.
"The older women in the study were, the fewer estrogen-related symptoms they had," says Ellis also professor of medicine in the Division of Oncology. "But overall, we demonstrated clearly that the low dose was better tolerated than the high dose and was just as effective for controlling metastatic disease."
In the 30 percent of participants who responded to estrogen, tumors often began to grow again after a period of months or years. But in a third of these recurring cases, the researchers showed that the women's tumors had become resensitized to anti-estrogen therapy. The tumors shrank or stopped growing when the patients went back on their original aromatase inhibitor treatment.