Researchers from the Columbia University's Mailman School of Public Health found that breastfeeding reduces the risk for estrogen receptor-negative and progesterone receptor-negative breast cancer.
The study examined the association between reproductive risk factors - such as the number of children a woman delivers, breastfeeding and oral contraceptive use - and found an increased risk for estrogen receptor - and progesterone receptor - (ER/PR) negative breast cancer in women who do not breastfeed.
The results also indicated that having three or more children without breastfeeding was associated with an increased risk for ER/PR-negative breast cancer.
ER/PR-negative breast cancer often affects younger women and has a poor prognosis.
The researchers used data from three sites of the Breast Cancer Family Registry, which includes women with and without breast cancer from the US, Canada and Australia.
The study included 4,011 women with breast cancer and 2,997 population-based controls.
"Women who had children but did not breastfeed had about 1.5 times the risk for ER/PR-negative breast cancer," said Meghan Work, first author.
"If women breastfed their children, there was no increased risk for ER/PR-negative cancer. This is particularly important as breastfeeding is a modifiable factor that can be promoted and supported through health policy," Work said in a statement.
The investigators also found that oral contraceptive use was not associated with ER/PR-negative cancer risk, with the exception of those formulations available before 1975.
"These earlier formulations contained higher doses of estrogen and progestin than more recent versions," Work said.
The results are in line with previous findings that have demonstrated a breastfeeding benefit in triple-negative breast cancer, which includes estrogen and progesterone receptor negative cancers.
"The consistency of the association with breastfeeding and estrogen receptor negative tumours across a number of studies is particularly noteworthy as there have been few modifiable risk factors identified for this tumour subtype," said Mary Beth Terry, senior author of the paper.
The paper was presented at the 11th Annual AACR International Conference on Frontiers in Cancer Prevention Research.
