This large study found that breastfeeding one baby -- even for as little as 3 months -- reduced the risk of breast cancer before menopause by 59% in women with higher-than-average risk compared to high-risk women who never breastfed.
Women in the study were considered high-risk if they had at least one first-degree relative (a mother or a sister) diagnosed with breast cancer.
Other research has shown clearly that women who have given birth to one or more babies have a lower risk of breast cancer than women who have never been pregnant. Other studies also have shown that breast cancer risk is lower among women who have breastfed one or more babies, but this link between breastfeeding and lower risk hasn't been found consistently in all studies.
In this study, the link between breastfeeding and lower pre-menopausal breast cancer risk was found only in women who were considered to have a higher-than-average risk. Breastfeeding didn't seem to lower pre-menopausal breast cancer risk in women who didn't have a first-degree relative diagnosed with breast cancer. The researchers aren't sure why this is.
Some premenopausal women with higher-than-average breast cancer risk can lower their risk by taking tamoxifen, usually for 5 years. Tamoxifen is a hormonal therapy medicine and works by blocking the effects of estrogen on breast cancer cells. This study found that breastfeeding -- even for a short period of time -- lowered pre-menopausal breast cancer risk about as much as taking tamoxifen for 5 years. Still, the researchers said that pre-menopausal women at high risk should talk to their doctors about the benefits of BOTH breastfeeding and tamoxifen. Breastfeeding may lower pre-menopausal breast cancer risk even more in high-risk women who choose to take tamoxifen.
Choosing to breastfeed is a personal decision. For some women, it may be more practical to bottlefeed. Still, breastfeeding for even a short time offers important health benefits to both mother and baby. For women with a family history of breast cancer, one of these benefits may be lower breast cancer risk.
If you have a family history of breast cancer and have higher-than-average risk, the information in the Lowering Risk for People at High Risk section can help you better understand risk and the choices you can make to keep your risk as low as it can be.
Breastfeeding a baby for even a few months may protect high-risk women from premenopausal breast cancer, researchers found.
Among women with a first-degree relative with breast cancer, those who nursed their children were at a significant 59% lower risk of premenopausal breast cancer than those who had a baby but never breastfed , Alison M. Stuebe, MD, MSc, of the University of North Carolina at Chapel Hill, and colleagues reported in the Aug. 10/24 issue of the Archives of Internal Medicine.
This association did not extend to women who did not have a family history of breast cancer but compared favorably to the impact of hormonal treatments (MedPage Today) -- like five years of tamoxifen (Nolvadex) -- for high-risk women, Stuebe said.
But since hormonal treatments have a proven benefit for high-risk women whereas "it is unclear if breastfeeding decreases breast cancer risk further in women on tamoxifen, both should be recommended," commented Virginia Kaklamani, MD, DSc, of Northwestern University in Chicago, who was not involved in the study.
The findings underscore the obligation of physicians to counsel about infant feeding, not just leave it as a checklist item for the nurse, Stuebe emphasized.
"For a long time how a mom fed her baby was thought of as a lifestyle choice and not something that was the obligation or responsibility of physicians to talk with them about," Stuebe said. "There's been a great deal of evidence in addition to this from both infant and maternal effects to suggest that really it's a health behavior, not a lifestyle choice."
Other reproductive factors, such as whether a woman has children, clearly influence breast cancer risk.
But because the link with breastfeeding has been inconsistent in the typically retrospective observational studies investigating it, Stuebe's group analyzed data from 60,075 premenopausal, parous women participating in the prospective cohort study of the Nurses' Health Study II from 1997 to 2005.
Overall, 87% of the women had breastfed for at least some time following the birth of their children. Among those who didn't breastfeed, medication to suppress lactation was used by 72.2%.
Women who nursed had a 25% lower risk of premenopausal breast cancer overall than those who had at least one baby but never breastfed (95% confidence interval 0.56 to 1.00).
Notably, the duration of lactation was not a factor in the apparent protective effect (P=0.95 for trend) -- the effect was significant for women who breastfed for as little as three months across all their pregnancies.
Nor were exclusivity of breastfeeding or duration of lactation-induced amenorrhea significant factors (P=0.74 and P=0.88 for trend, respectively).
However, the association did vary substantially by family history of breast cancer. Women who had a first-degree relative with breast cancer -- considered a high-risk group for premenopausal malignancy -- had a hazard ratio 0.41 for nursing (95% CI 0.22 to 0.75) whereas the link was not significant among women without such a family history.
Although women who had longer cumulative durations of breastfeeding -- previously assumed to yield lower risk -- tended to have had more children, restricting the analysis to women with only one birth strengthened the protective effect of nursing in those with a family history (HR 0.10, 95% CI 0.02 to 0.47, P=0.04 for interaction).
Another surprising finding, the researchers said, was that use of medications to suppress lactation tended to have a protective effect as well for women who had never breastfed compared with women who had done neither (adjusted HR 0.65, 95% CI 0.42 to 1.02). But it didn't add to the effect of having breastfed.
These once commonly used drugs went out of style for routine practice in the '70s and '80s for good reason, Stuebe cautioned. Estrogen or estrogen/testosterone therapy increased deep vein thrombosis risk, and bromocriptine (Parlodel) was linked to a small risk of stroke and heart attack in postpartum women.
Rather, this finding may implicate problems with involution, since genes and processes involved in shutting down the apparatus for breastfeeding are also related to breast cancer.
"Perhaps having a baby, becoming engorged, and never breastfeeding somehow increases breast cancer risk among women who are predisposed to breast cancer, whereas either preventing that engorgement process with medication or breastfeeding doesn't require this acute shutdown kind of process," Stuebe speculated.
Why the association would be different for women with a hereditary risk of breast cancer compared with other women is not clear, but may have to do with lower statistical power to detect the less common incidence of premenopausal tumors in women without a family history, she added.
But one collaborative meta-analysis actually found a reduction in breast cancer risk with breastfeeding only among women without a family history of the disease, noted Thomas A. Sellers, PhD, of the Moffitt Cancer Center in Tampa, Fla., who was not involved in the study.
"Before making recommendations on counseling for primary prevention, it is important to have greater consensus," he said.
Jennifer Harvey, MD, of the University of Virginia in Charlottesville, also took a cautious approach. "Each individual has a unique set of risk factors. They should all be taken into account. The fact that one breastfed would not preclude the use of tamoxifen or other strategies for risk reduction."
The authors noted several limitations of the study, including the fact that observational studies -- particularly studies of breastfeeding -- are subject to confounding and that generalizability was limited because their "cohort is comprised entirely of registered nurses, 90% of whom are of self-reported white ancestry."
This article was developed in collaboration with ABC News.
The Nurses' Health Study II is supported by a grant from the National Cancer Institute.
The researchers reported no conflicts of interest.
Kaklamani, Sellers, and Harvey provided no information on conflicts of interest.
Primary source: Archives of Internal Medicine Source reference: Stuebe AM, et al "Lactation and Incidence of Premenopausal Breast Cancer: A Longitudinal Study" Arch Intern Med 2009;169:1364-1371.
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