Why do pregnancy, breastfeeding reduce risk of breast cancer? Latest science finds clues

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It's been known for decades that breastfeeding and childbirth reduce the risk of breast cancer. Now medical researchers are gaining clues on why and hoping these insights could help create a pill to mimic the protective effects of nursing.

In October, researchers based in Australia found women who breastfed had more specialized immune T-cells in their breast tissue. Prof. Sherene Loi, a medical oncologist and the study's lead author, likened the cells to "local guards, ready to attack abnormal cells that might turn into cancer."

Loi hopes her findings could help prevent breast cancer in all women, regardless of whether or not they had children.

The study, published in the journal Nature, builds on past work that observed how pregnancy and breastfeeding are protective against the development of breast cancer with modern immunology to suggest a possible explanation why.

It’s an important question, according to Dr. Steven Narod, a professor of public health at the University of Toronto and director of the Breast Cancer Research Unit at Women's College Hospital, who was not involved in the study.

Narod notes the Australian study doesn’t offer evidence of what the specific immune T-cells are doing in the breast. But if we start to understand how breastfeeding protects against cancer, that could lead to new treatments.

"We like the idea of breastfeeding as a way of preventing breast cancer, but we'd rather find a pill," Narod said. 

"If we really understood what it was, perhaps a hormone or something equivalent, maybe that could be used as a preventive treatment."

Some of the work on this question focuses specifically on young women — including those carrying mutations in tumour-suppressing genes known as BRCA genes.

When Narod and co-investigator Dr. May Lynn Quan in Calgary analyzed data from women in their 30s with breast cancer from across Canada, they found the disease often follows a poorer course than in post-menopausal women. 

People with BRCA1 and BRCA2 genes that aren't working properly due to mutations face increased risks of cancers including breast, ovarian and prostate.

Dr. Stephanie Wong, a surgical oncologist in Montreal with an interest in breast cancer prevention, studied breastfeeding after breast cancer in young BRCA carriers. Wong said she thinks it is important to understand the cellular and molecular building blocks underpinning breast cancer in certain women and its prevention in others. 

"There is rarely one single risk factor that explains why one in eight women develop breast cancer," Wong said in an email. "We usually think about a constellation of factors ― hormonal exposures, lifestyle factors, environment, family history, and genetics ― as playing a role in the development of the majority of breast cancers."