Why breastfeeding protects mums from breast cancer

New research has shown why this part of becoming a mum can help reduce their risk of breast cancer - and it could lead to vaccines in the future for others.

Pregnancy and breastfeeding cause special immune T cells to gather in the breast and act like guards ready to attack cells that might turn into cancer.

It is these biological guardians that Australian researchers have shown for the first time give breastfeeding mums an extra defence against breast cancer, the most common cancer in women.

A new study published Tuesday in the journal Nature explains a long-term protective effect of child-bearing against breast cancer, and importantly why it has a lasting impact on a woman’s immune system.

The researchers said it was completing a full cycle of pregnancy, breastfeeding, and breast recovery that caused these special T cells to accumulate in the breast.

Study lead, oncologist and clinical scientist Professor Sherene Loi from the Peter MacCallum Cancer Centre. Image: Supplied

The study was led by medical oncologist and scientist Professor Sherene Loi from the Peter MacCallum Cancer Centre.

She said the findings could shape the future of breast cancer treatment, and ultimately lead to a vaccine.

“We looked at studies in over 1000 women diagnosed with breast cancer after their last child and found those who breastfed did better from triple negative breast cancer than those who had not,” Professor Loi said.

She said while there was no guarantee women who breastfed would not get breast cancer, this study provided an explanation of this long-lived protection and suggested a new pathway to prevent or treat breast cancer using immunotherapy.

Professor Laura Mackay from the Doherty Institute was also part of the research team. Image: Supplied

The research team included Drs Balaji Virassamy, Peter Savas and Michael Harris at Peter Mac with Professor Laura Mackay at the Peter Doherty Institute.

“We found women who breastfed had tumours with higher numbers of these protective T cells and, in some groups, they lived longer after diagnosis of breast cancer,” Professor Loi said.

While it has long been known that having children reduces a woman’s breast cancer risk, the reasons had not been clear and pregnancy-related hormonal changes were thought to be a major factor.

This research highlights the involvement of the immune system.

Speaking before travelling to Berlin to deliver the paper at an international cancer congress, Professor Loi said breast cancer rates were increasing, in particular triple negative breast cancer in younger women.

The research shows for the first time the biological reason breastfeeding mums have an extra defence against breast cancer.

While lifestyle and genetic issues were a factor, she said it was largely believed reproductive issues such as women having children later in life and a decrease in breastfeeding contributed.

Professor Loi said it was still unclear how long a woman should breastfeed to get the best protection, but that six months appeared optimal, “although some breastfeeding was better than none”.

“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 triple-negative breast cancer,” she said.

Professor Loi said key from the research was that pregnancy and breastfeeding left behind long-lived protective immune cells in the breast and body that can help reduce the risk and improve defence against breast cancer and potentially other cancers and disease.

Originally published as Why breastfeeding protects mums from breast cancer