That finding, which comes from a new study published in Monday's advance online edition of Cancer, is based on two breast cancer studies that together included nearly 2,500 women aged 55-79 in Washington state. The group included 1,140 women who had had breast cancer.
Amanda Phipps, of the Fred Hutchinson Cancer Research Center in Seattle and her colleagues found that "triple-negative" breast cancer was half as common among women who said they breastfed for six months or more compared with women who hadn't breastfed.
Estrogen-sensitive or luminal breast cancers were also 20 per cent less common among women who breastfed for at least six months than among mothers who didn't breastfeed, the researchers said.
The aggressive cancer type is called "triple negative" breast cancer because the tumor doesn't respond to a critical regulatory protein known as "HER2" or two key hormones, estrogen and progestin, which also are fundamental to many hormone-based treatments for breast cancer. That makes the cancer treatment difficult as hormonal treatments cannot be used on these tumors.
Researchers also found that starting to menstruate before age 13 was also linked to a higher risk of HER2 breast cancer and entering menopause late, after age 55, and taking estrogen-plus-progesterone hormone therapy were linked to risk of estrogen-sensitive breast cancer.
The reason could be that since women don't menstruate while breastfeeding, they are exposed to fewer hormones thus preventing certain types of cancers. Another possibility is that the structure of breast cells change during breastfeeding, making them less susceptible to mutate.
Scientists however cautioned that the study doesn't prove that breastfeeding prevented breast cancer or that not breastfeeding raises breast cancer risk but notes that observational research has linked breastfeeding to lower rates of breast cancer.
