The Protective Role of Breastfeeding in Reducing Breast Cancer Risk

Often referred to as “liquid gold,” breast milk contains all the nutrients, antibodies, and stem cells that infants need for growth and development. Breastfed infants have a lower risk of asthma, obesity, sudden infant death syndrome (SIDS), ear infection, and other conditions.

The health benefits of breastfeeding extend to the mother as well. Increased levels of oxytocin result in less postpartum bleeding and a faster return of the uterus to its pre-pregnancy state. Breastfeeding can have longer-term impacts as well. Beyond the postpartum period, breastfeeding can lower a mother’s risk of breast cancer.

Building protective benefits over time

“The longer you breastfeed, the greater the reduction in breast cancer risk,” says Jasmine McDonald, PhD, associate professor of epidemiology in the Mailman School of Public Health and co-associate director for education and training at the Herbert Irving Comprehensive Cancer Center (HICCC). “If you breastfeed for a year or more in total, you will receive a significantly reduced risk of developing breast cancer in the future.”

The effect is cumulative, explains McDonald, meaning that what matters is the total time spent breastfeeding all your children. For example, a mother who breastfeeds one child for one year has an approximate 4.3% lower chance of being diagnosed with breast cancer than if she never breastfed. However, another woman who breastfeeds two children for 6 months each will also receive the same benefit. Lastly, a mother who breastfeeds each of her two children for a year reduces her risk by approximately 8.6%.

Breastfeeding decreases the number of menstrual cycles that a woman will have throughout her lifetime. Less menstrual cycles mean less exposure to hormones that are produced by her ovaries – namely, estrogen and progesterone. Studies have demonstrated a link between greater exposure to ovarian hormones and an increased risk of developing breast cancer.

However, more research is needed to understand how different variables like formula supplementation impact the benefits of breastfeeding.

“We don't have enough information to know if there is a difference in risk between those who are supplementing and pumping, versus those who are pumping and chestfeeding, versus those that are solely chestfeeding. We just don't know,” she says. “But overall, breastfeeding has a long-term protective effect against the development of breast cancer.”

Both the American Academy of Pediatrics (AAP) and World Health Organization (WHO) recommend breastfeeding exclusively for the first six months, with continued breastfeeding along with appropriate complementary foods for up to 2 years or longer.

Overcoming breastfeeding disparities to improve cancer outcomes

Part of McDonald’s research includes examining cancer disparities, and she believes that lower rates of breastfeeding in Black women may play a role in the high mortality rate from breast cancer seen in that population. Black women have a 40% higher mortality rate than White women and are disproportionally affected by more aggressive subtypes, such as estrogen receptor-negative breast cancer.

“Breastfeeding is associated with a reduced risk for all breast cancer subtypes, but the most benefits are seen with estrogen receptor-negative breast cancer,” says McDonald. “Given that Black women have a higher rate compared to white women of being diagnosed with estrogen receptor-negative breast cancer, and being diagnosed at younger ages, it's thought that breastfeeding disparities could be a contributor."

McDonald emphasizes that policies, programs, and initiatives are needed to overcome barriers to breastfeeding, particularly for Black, immigrant, and low socioeconomic status families. Paid leave, along with dedicated nursing time and space at work, can improve breastfeeding rates — and in turn, protect more women against cancer.