"This is good news for women with a family history of breast cancer," says Alison Stuebe, assistant professor of obstetrics and gynecology at the University of North Carolina at Chapel Hill School of Medicine and lead author of the study, which is published in the Aug. 10 issue of Archives of Internal Medicine.

"Our results suggest a woman can lower her risk of cancer simply by breastfeeding her children," Stuebe says.

Among women with a mother or sister with breast cancer, the researchers found that those who had breastfed were less than half as likely to develop premenopausal breast cancer as those who had not breastfed. The authors did not find a difference in risk among women without a family history of breast cancer.

For women with a family history, the reduction in risk with breastfeeding was similar to taking an anti-estrogen drug such as Tamoxifen for five years. But unlike Tamoxifen, Stuebe says, "Breastfeeding is good for mothers and for babies."

Stuebe and colleagues reviewed data from the Nurses' Health Study II, a long-term study of more than 100,000 women from 14 states. Stuebe's study followed more than 60,000 who reported at least one pregnancy in 1997, when breastfeeding was assessed in detail, and followed them through 2005 to determine how many developed invasive breast cancer.

How long a woman breastfed seemed to be less important than whether or not she had breastfed, Stuebe says. The reduction in risk was similar whether women breastfed for a lifetime total of three months or for more than three years. Also, there was no significant difference in risk for women who breastfed exclusively versus those who breastfed while supplementing with other foods.

Why breastfeeding reduces risk of breast cancer is unknown. The authors suspect that when women do not breastfeed, inflammation and engorgement shortly after birth causes changes in breast tissue that may increase risk for breast cancer. Breastfeeding followed by weaning may prevent this inflammation.

When the researchers compared data about women who breastfed and those who did not, there was a 25 percent total reduction in incidence of premenopausal breast cancer. But, Stuebe says, that statistic was accounted for by women without a family history of the disease.

"We did not find an association between breastfeeding and premenopausal breast cancer among women without a family history of breast cancer," Stuebe says. "This could be because there's something about genetically caused breast cancer that's affected by breastfeeding, or it could be because rates of breast cancer were so low in women without a family history that we couldn't see an association in this data set."

Stuebe says the research underscores the public health impact of policies that help mothers successfully breastfeed. In a recent CDC study, more than half of women said they stopped breastfeeding earlier than they wanted to. "Mothers and babies need supportive hospital policies, paid maternity leave, and workplace accommodations so that they can meet their breastfeeding goals," Stuebe says. "Public health begins with breastfeeding."

Note: If no author is given, the source is cited instead.

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• Breastfeeding
• Women's Health
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• Colostrum
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