There was no reduced risk in women who did not have a family history of the disease. Nor was there any difference depending on how long the mother breast-fed or the intensity of breast-feeding (whether the baby was breast-fed exclusively or not).
The reduced risk did not seem to have any link with hormones, given that the risk did not differ with the amount of time a woman went without a period while breast-feeding.
The researchers postulated other hypotheses to explain the link.
"It may be something about the first couple of days postpartum if the woman doesn't breast-feed," Stuebe said. "The breast tissue has to shut down, and there's a lot of inflammation and discomfort. Perhaps on a molecular level there's going some kind of damage."
This theory is supported by the fact that women who used medication to stop lactating also had a lower risk.
"This data would suggest it's more of the effect of milk being taken out of the breast tissue after pregnancy that's beneficial," Stuebe said. "We know that just being pregnant reduces the risk of breast cancer compared with not having been pregnant. Getting milk out afterwards appears to be part of the phenomenon."
The real value of the study, added Dr. Richard Bleicher, co-director of the breast surgery fellowship at Fox Chase Cancer Center in Philadelphia, may be less in the clinical implications but in the fact that it helps point researchers towards avenues for understanding the mechanisms of breast cancer.
The U.S. government's Women's Health Information site has more on breast-feeding.
SOURCES: Alison M. Stuebe, M.D., assistant professor, obstetrics and gynecology
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