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MONDAY, Aug. 10, 2009 (Health.com) — Women with a family history of breast cancer may have a new weapon against the disease: breast-feeding. In a new study of more than 60,000 women, nursing a baby for at least three months cut the risk of breast cancer in half for those who had a family history of the disease.

The researchers say that breast-feeding could be the equivalent of taking the drug tamoxifen for five years, which is a well-known way to cut breast cancer risk in women with a family history of the disease.

“For women at high risk right now, the things we have to offer are tamoxifen, prophylactic mastectomy—that’s about it,” says Alison M. Stuebe, MD, of the University of North Carolina at Chapel Hill, who conducted the research while at Brigham and Women’s Hospital in Boston. “This study is really good news for women with a family history of breast cancer who are looking to reduce their risk.” The study was published Monday in Archives of Internal Medicine.

Some research has suggested that breast-feeding protects against cancer, but those are typically studies in which breast cancer patients and healthy women are asked to recall their behavior years or even decades in the past, Dr. Stuebe notes. These types of studies can be flawed due to “recall bias”—meaning that they rely on people’s sometimes faulty memory, she explains.

In the current study, Dr. Stuebe and her team avoided recall bias by following 60,075 women in the Nurses’ Health Study II over the course of nine years. In 1997, the women filled out a questionnaire about whether they had breast-fed each of their first four children, for how long, and whether they had used any lactation-suppressing medications. The researchers then followed them until 2005, during which time 608 were diagnosed with breast cancer.

Overall, the researchers found, women who had ever breast-fed had a 25% lower risk of premenopausal breast cancer. As long as they had nursed at least one of their babies for up to three months, the duration of breast-feeding didn’t seem to affect their risk. Whether they had exclusively breast-fed and whether they stopped menstruating during lactation also didn’t affect their risk.

Researchers found that breast-feeding didn’t affect risk for women who didn’t have breast cancer in their family. But for women with at least one close relative with breast cancer—a sister, mother, or daughter—nursing cut the risk of premenopausal cancer by 59% compared to those who didn’t breast-feed. By comparison, women who take tamoxifen or similar drugs for five years, which is recommended for those at particularly high risk of breast cancer, reduce their likelihood of developing breast cancer by 50%.

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