Breastfeeding may lower risk of cancer

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Women who breastfeed appear to have lower risk of developing pre-menopausal breast cancer than those who don’t, according to a new study released today in in the Archives of Internal Medicine. The stats are especially compelling for women with a family history of the disease. Among that group, women in the study who’d breastfed had just 41% the disease risk of women who did not breastfeed. 

The results come from researchers at Harvard University and the University of North Carolina, Chapel Hill. They used data from 60,075 women who’d had at least one baby, and who were monitored between 1997 and 2005 as part of the Nurses’ Health Study II. The researchers wanted to see what kind of an effect breastfeeding had on cancer risk, once they’d controlled for other known risk factors, including body weight, age at first period, oral-contraceptive use and alcohol consumption.

The breastfeeding effect was strong, at least among those women with a family history of breast cancer. But it’s still not clear why or how, exactly, giving milk might prevent cancers from forming. Breastfeeding is of course correlated with oodles of other lifestyle factors, so it’s always possible that one of those is responsible for the observed effect. (Remember, too, that breast-cancer risk increases with age until women are well into their 70s, so pre-menopausal breast cancer should not be your only concern.) And yet the researchers behind this new study do have one good reason to think that the link they’ve found could be causal. It seems that women who used drugs to suppress lactation got the very same benefit. The researchers hypothesize, therefore, that something in the breast really does respond badly when milk goes unused. In particular, they write, “if a woman does not breastfeed, she experiences abrupt engorgement [of the breast right after childbirth], and mammary tissue may become progressively inflamed.” They believe this inflammation could play some role in cancer formation.

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