Breastfeeding is associated with a 30% overall decreased risk of breast cancer recurrence among women who have previously had one subtype of the disease, according to a new study published in the Journal of the National Cancer Institute.

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Breastfeeding may make the breasts less susceptible to carcinogens, leading to slower growing tumors.

The team behind the new study, from the Kaiser Permanente Division of Research in Oakland, CA, claims that this is the first study to examine the role of breastfeeding history in cancer recurrence.

The new study is part of ongoing research in breast cancer from the Kaiser Permanente team. Last year, the team discovered that patients with specific HER2+ breast cancer tumors have a low risk of cancer recurrence within 5 years of diagnosis. In 2013, they found that consuming high-fat dairy products following diagnosis of breast cancer increases likelihood of dying from the disease.

In the new study, the team found that breastfeeding appeared to improve outcomes of women diagnosed with the luminal A subtype of breast cancer.

Women with these cancers had a 30% decreased risk of breast cancer recurrence if they had a history of breastfeeding. Breastfeeding mothers were also found to be 28% less likely to die from luminal A breast cancer. The protection was found to be strongest for women who had a history of breastfeeding for 6 months or more.

However, no significant improved outcomes for women with other subtypes of breast cancer were associated with breastfeeding.

Why does breastfeeding have a protective effect for women with luminal A cancers?

Luminal A tumors are the most commonly diagnosed of breast cancers. These tumors generally have better outcomes than other tumor subtypes as they are less likely to metastasize and can be treated with hormonal therapy, such as tamoxifen or aromatase inhibitors.

Marilyn L. Kwan, PhD, research scientist with the Kaiser Permanente Division of Research and lead author of the study, explains a potential mechanism behind the protective effects of breastfeeding for women with these cancers:

"Women who breastfeed are more likely to get the luminal A subtype of breast cancer, which is less aggressive, and breastfeeding may set up a molecular environment that makes the tumor more responsive to anti-estrogen therapy."

"Breastfeeding may increase the maturation of ductal cells in the breast," hypothesizes senior author Bette J. Caan, "making them less susceptible to carcinogens or facilitate the excretion of carcinogens, and lead to slower growing tumors."

In 2014, Medical News Today reported on another study published in the Journal of the National Cancer Institute, which observed that African-American women who had four or more births and who never breastfed had a 68% higher chance of developing breast cancer compared with women who had only one birth but who breastfed their child.

A 2013 study published in the Journal of Clinical Nursing also found that women who breastfed for over 6 months were less likely to develop early breast cancer than women who did not breastfeed.

In that study, by researchers at the University of Granada in Spain, women who did not breastfeed were found to get breast cancer 10 years earlier on average than breastfeeding mothers.

However, that study also found that smoking appeared to counter the protective effects of breastfeeding. Female smokers were diagnosed with breast cancer at a younger age and had no significant benefit from breastfeeding.

The University of Granada team suggested that hormonal changes taking place during pregnancy and lactation could be responsible for the protective benefits of breastfeeding.