New study links breastfeeding to reduced risk of breast cancer

The new findings offer another reason to support breastfeeding among those who can and choose to do so, especially Black women.  

Breastfeeding has long been connected to various benefits like quicker postpartum recovery, weight loss, reduced risk of developing disease, and now, a reduced risk of breast cancer.

According to a new study from the Peter MacCallum Cancer Centre in Melbourne, published recently in Nature, breastfeeding has been linked to a stronger immune system and lower risk of developing breast cancer later in life. 

“We found that women who have breastfed have more specialized immune cells, called CD8⁺ T cells, that ‘live’ in the breast tissue for decades after childbirth,” said Professor Sherene Loi, the oncologist who led the research in a press release.  

“These cells act like local guards, ready to attack abnormal cells that might turn into cancer,” Loi continued. “This protection may have evolved to defend mothers during the vulnerable post-pregnancy period, but today it also lowers breast cancer risk, especially the aggressive type called triple-negative breast cancer.”

Loi explained that completing a full cycle of pregnancy, breastfeeding, and breast recovery led to these protective cells accumulating in the breast — a finding later confirmed in lab models. 

“When breast cancer cells were introduced, the models with this reproductive history were far better at slowing or stopping tumor growth — but only if T cells were present,” she said.

For instance, the study analyzed more than 1,000 women who had developed triple-negative breast cancer, one of the most difficult forms of the disease to treat. Researchers found that those who had breastfed not only showed better outcomes but also had more of these protective T cells in their tumors than those who hadn’t.

While it’s long been known that having children can lower a woman’s risk of breast cancer, scientists have never fully understood why. Hormonal changes during pregnancy were thought to play the biggest role, but the new study suggests the immune system may be the key driver. Researchers say understanding this mechanism could one day lead to new ways to prevent or even treat certain forms of breast cancer.

Beyond the science, the findings may offer another reason to support breastfeeding among those who can and choose to do so. In the U.S., Black women are still disproportionately less likely to breastfeed compared to other groups. Data from the Centers for Disease Control and Prevention show that while 84% of mothers nationwide start breastfeeding, only around 74% of Black mothers do. At six months, that number drops to 19% for Black mothers, compared to 27% for white women.

Experts cite a range of factors behind the gap, including limited hospital lactation support and cultural stigmas, economic pressures, and a lack of paid maternity leave. For some, returning to work just weeks after giving birth makes sustaining breastfeeding nearly impossible.

Still, there’s been progress. The rise of community-led programs such as Loyal Lactation, more culturally competent lactation training for hospital staff, and policy changes like extended paid leave are helping to close the gap.