Studies of breast cancer survivorship are improving lives by increasing understanding of how lifestyle and other factors influence disease outcomes.

It is well established that breastfeeding is great for babies, and now Kaiser Permanente research is showing that it may also have benefits for moms. A new study by research scientist Marilyn L. Kwan, PhD, and colleagues in the Kaiser Permanente Northern California Division of Research, found that women diagnosed with breast cancer who had ever breastfed a child had about a 30 percent overall decreased risk of dying from the disease or having it recur. We talked with Kwan about her intriguing new study, which was published April 28 in the Journal of the National Cancer Institute.

Why did you conduct this study?

While breast cancer is the most commonly diagnosed cancer in women, it is also the most treatable. Scientists here at the Division of Research have several long-term studies under way that are helping to increase our understanding of how lifestyle factors can influence long-term outcomes among the estimated 2.9 million women in the United States who have been diagnosed with breast cancer.

How was this study conducted?

We studied 1,636 women – most were members of Kaiser Permanente Northern California – who participated in the Life After Cancer Epidemiology (LACE) and Pathways studies of breast cancer survivorship. The women completed a questionnaire that included breastfeeding history, and we obtained additional medical data from chart reviews and Kaiser Permanente’s comprehensive electronic health record. We also examined the biology of the women’s tumors with a test that uses 50 genes to determine the subtype of each woman’s tumor — genes that have been associated with the aggressiveness of the cancer and risk of recurrence

What were the most important findings?

We followed the women for 9 years after their initial breast cancer diagnoses, and those who had previously breastfed a child for any length of time were 28 percent less likely to die from the disease and had a 30 percent overall decreased risk of the disease recurring. The risks of mortality and recurrence were even lower among women who breastfed for six months or longer.

Assessing the biologic subtype of the tumors revealed that the benefits of breastfeeding were strongest among women diagnosed with the luminal A subtype, which includes the most commonly diagnosed breast cancers. These types of cancer are less likely to spread, are treatable with hormonal therapies, and generally have a better prognosis.

Why do women who breastfeed seem to have better breast cancer outcomes?

Researchers theorize that breastfeeding may increase the maturation of ductal cells in the breast, making them less susceptible to carcinogens, or it may facilitate the excretion of carcinogens and lead to slower growing tumors. Likewise, breastfeeding may set up a molecular environment in the breast that makes the tumor more responsive to anti-estrogen therapy.

How does this study add to our knowledge about the benefits of breastfeeding?

This is the first study we’re aware of that examined the role of breastfeeding in cancer recurrence and by tumor subtype. Numerous previous studies have shown that breastfeeding reduces the risk of developing breast cancer in the first place, although the science is inconclusive regarding how much that risk is reduced — perhaps between 5 and 10 percent.

The decision to breastfeed is a personal one; this study adds to a growing body of research that will help inform women and their doctors about how breastfeeding can possibly affect their future health.

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