Science has proved that mothers and babies benefit from breast-feeding in all sorts of ways, and now researchers at the Fred Hutchinson Cancer Research Center have shown that this practice can reduce the risk of a particularly aggressive form of breast cancer.
"It's a form more common among younger women and African-American women," said Amanda Phipps, a scientist in the public health division of the Seattle cancer center and lead author of a report published Monday in the journal Cancer. "We wanted to find out what puts women at risk for this."
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's why we can't use hormonal treatments on these tumors," Phipps said.
Triple-negative breast cancer is an aggressive cancer estimated to be involved in anywhere from 15 percent to 30 percent of all breast cancers, she said. Little is known about why some women are at higher risk of developing this form of cancer.
Phipps and her colleagues at Fred Hutchinson examined two groups of women from 55 to 79 years old with and without a diagnosis of breast cancer. They compared 1,476 women without the cancer with 1,140 women with several different forms of breast cancer including the "triple negative," the most common "luminal" form and another subtype associated only with the HER2 protein.
The researchers then examined these women with respect to their reproductive health histories including breast-feeding practices, onset of menstruation and menopause. All of these, Phipps said, provide indicators of the hormone levels over time for these women.
"We've known for a long time that breast-feeding reduces your risk of breast cancer, and that it was probably related to hormones," she said. But there has been little specific research, she said, on which hormones might affect the risk of these more aggressive forms of breast cancer.
By comparing the molecular nature of these breast cancer types with each woman's reproductive history, Phipps and her colleagues found that breast-feeding for at least six months corresponded with a lower risk of both the common luminal form as well as the triple-negative form of breast cancer. They also found that early onset of menstruation (before age 13) was associated with a higher risk of the HER2 breast cancer.
Late onset of menopause and the use of hormone treatments were found to increase the risk of luminal breast cancer. They found no risk differences associated with the number of children or the mother's age at first birth.
While it is common knowledge that female hormone levels change with the onset of menstruation or menopause, and so may be related to hormonally influenced cancer risks, it is not fully understood why breast-feeding would be correlated with this cancer-hormone link.
"One possible explanation is that while women are breast-feeding, they aren't menstruating and so their hormones aren't cycling," Phipps said.
So the more women breast-feed, she said, the less chance their hormones may have to trigger a cancer. Another theory, she said, is that breast-feeding alters breast cell structures in ways that make them less prone to develop into cancer cells.
The findings support the overall view that breast-feeding can reduce a woman's risk of cancer, Phipps said. More importantly for scientists, she said, it shows that different reproductive health behaviors have different effects on various forms of breast cancer.
"It helps explain why some women are at higher risk and also why certain therapies are not effective against these more aggressive forms of breast cancer," she said.