A large international study shows that breastfeeding is associated with a lower risk of developing hormone-receptor–negative (HRN) breast cancer, an aggressive form of the disease. This breastfeeding meta-analysis, published in Annals of Oncology (2015; doi:10.1093/annonc/mdv379), shows the risk was reduced by up to 20% in women who breastfed.

HRN breast cancers are more likely to be aggressive and life threatening. This subtype is more commonly diagnosed in women younger than 50 years. Women of African American or Sub-Saharan African descent are more likely to develop HRN breast cancers, as are women with the BRCA1 gene mutation. Other factors may put these women at even higher risk for developing HRN breast cancer, including obesity and multiple early pregnancies. Furthermore, women with these multiple risk factors are least likely to breastfeed.

In the United States, HRN breast cancers represent about 20% of all breast cancers. This subtype of breast cancer has no receptors for the hormones estrogen or progesterone; approximately two-thirds of these HRN cancers also have no receptors for human epidermal growth factor receptor 2 (HER2). Breast cancers with no receptors for estrogen, progesterone, or HER2 are called triple-negative (TN).

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HRN and TN breast cancers are more often deadly because they tend to be diagnosed at later stages, respond to fewer treatment options, and are less likely to be cured by current therapies. In the absence of estrogen, progesterone, and HER2 receptors, medicines that target these receptors, such as tamoxifen, aromatase inhibitors, trastuzumab, and pertuzumab, are ineffective.

“Further evidence to support the long-term protection of breastfeeding against the most aggressive subtypes of breast cancer is very encouraging and actionable,” said Marisa Weiss, MD, president and founder, Breastcancer.org, and director of breast health outreach, Lankenau Medical Center in Wynnewood, Pennsylvania. “Breastfeeding is a relatively accessible, low-cost, short-term strategy that yields long-lasting natural protection.”

This work highlights the need for more public health strategies that directly inform women and girls about the maternal (and fetal) benefits of breastfeeding before and during a woman’s childbearing years. Also important, this message should be reinforced by these women’s health care professionals.

Removing the barriers to breastfeeding at home, in the community, and in the workplace is critical.

“All approaches will be necessary in order to protect the most women against the devastation of breast cancer over their lifetimes,” said Farhad Islami, MD, PhD, director of Interventions, Surveillance and Health Services Research, American Cancer Society.

“Pregnant women and young mothers are highly receptive and motivated to make healthy choices. We need to encourage women who are able to breastfeed to do so for their breast health, in addition to the health of their children,” said Paolo Boffetta, MD, associate director for population sciences at the Tisch Cancer Institute at the Icahn School of Medicine at Mount Sinai in New York, New York. “Further prospective research will be necessary to further understand the full impact of breastfeeding duration and its effect on other subtypes.”

The research is a collaboration between Breastcancer.org; Icahn School of Medicine at Mount Sinai; Washington University in St. Louis, Missouri; and the American Cancer Society.