How mammary tissue returns to its pre-pregnancy state might explain the apparent protective effect of breastfeeding, the researchers say. (CBC)

Women with a family history of cancer may lower their risk of pre-menopausal breast cancer by breastfeeding their children, a new study suggests.

Among those with a mother or sister with breast cancer, researchers in the U.S. found a 59 per cent reduction in the incidence of pre-menopausal breast cancer for women who breastfed, researchers reported in Monday's online issue of the Archives of Internal Medicine.

To come to that conclusion, Dr. Alison Stuebe, a professor of obstetrics and gynecology at the University of North Carolina at Chapel Hill School of Medicine, and her colleagues reviewed health data from 60,075 pre-menopausal women, mainly nurses, who reported at least one pregnancy in 1997.

Participants were followed through 2005 to see how many developed invasive breast cancer.

"This association was restricted to women with a first-degree family history of breast cancer," the study's authors concluded.

"The observed 59 per cent reduction in risk compares favourably with hormonal treatments such as tamoxifen for women at high risk for breast cancer. Moreover, breastfeeding is associated with multiple other health benefits for both mother and child. These data suggest that women with a family history of breast cancer should be strongly encouraged to breastfeed."

The findings suggested it did not matter how long women breastfed — the results were the same whether a woman with a family history of the disease breastfed for three months or three years. Overall, 87 per cent of the participants had breastfed for at least some time after the birth of their children.

The protective effect did not apply among women without a family history.

Family history mattered

"We did not find an association between breastfeeding and pre-menopausal breast cancer among women without a family history of breast cancer," Stuebe said in a release.

"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."

Whether women breastfed exclusively or supplemented with other foods, or whether they experienced a loss of menstruation as a result did not seem to make a difference, the researchers said.

Among those who never breastfeed, 72.2 per cent used medications to suppress lactation and also appeared to have a lower risk of breast cancer.

But use of the medications are linked to an increased risk of blood clots in the mother, and their use has declined in the U.S. since the late 1970s.

Prevents inflammation?

That finding points to a possible link between a malfunction in how mammary tissue returns to its pre-pregnancy state, the researchers said.

They suspect when women do not breastfeed, inflammation and engorgement shortly after birth causes changes in breast tissue that may increase the risk for breast cancer. Breastfeeding followed by weaning may prevent this inflammation, the study's authors speculated.

They noted it is difficult to separate out the effects of breastfeeding in such observational studies. They called for more research into the interaction between breastfeeding history, family history and genes associated with breast cancer risk, to confirm the observations and explore how they may occur.