By Debbie Saslow, PhD
There are a limited number of things that women can do to lower their risk of breast cancer, including getting regular physical activity, limiting alcohol, and maintaining a healthy weight. Breastfeeding has often been included in the protective behaviors against breast cancer, but the research has been inconsistent.
Looking at the research on breastfeeding and breast cancer risk, it is clear that this has been a difficult area to study. If breastfeeding does lower risk, the level of protection is small and depends on women breastfeeding for a long time. In countries such as the U.S., most women who breastfeed their babies stop after several months, or they breastfeed less frequently as they start to supplement with formula and baby food. Women who have many children and breastfeed each baby for a long time seem to be at somewhat lower risk of breast cancer than women who have smaller families and breastfeed for a shorter time. Studies that have found that breastfeeding does lower breast cancer risk have also found that protection builds up over time (that is, duration of breastfeeding) and number of children that are breastfed.
The major study (Collaborative Group on Hormonal Factors in Breast Cancer; Lancet, 2002 Jul 20; 360 (9328): 187-95) that supports breastfeeding as protective against breast cancer was published in 2002. The researchers analyzed 47 studies in 30 countries; these studies had information about 50,000 women with invasive breast cancers and 97,000 women without breast cancer. The study authors found that the rate of breast cancer diagnoses was slightly lower among women who had breastfed and among women who had breastfed for longer periods of time.
For every 12 months of breastfeeding (either with only 1 child, or as the total period of time for several children), the risk of breast cancer decreased by 4.3%, compared to women who did not breastfeed. Risk decreased by 3.4% for each child breastfed, compared to women who did not breastfeed. This lower risk did not differ by women's age, race, numbers of births, age at birth of first child, family history, or country of residence.
A more recent analysis from 2008 looked at 31 studies that were published after the 2002 study was done. Some of these 31 studies found a small decrease in breast cancer risk among women who breastfed, but others did not. The authors compared risk among women who had breastfed and those who never breastfed, and also risk among women who breastfed for different amounts of time. In both cases the results were inconsistent, with about half the studies showing slightly reduced risk and half showing no difference in risk.
In December 2012, the National Cancer Institute sponsored a workshop on the breast changes that occur as part of pregnancy and breastfeeding, and any associated risk of breast cancer. The goal of the workshop was to summarize what is known in this area and to make recommendations for future research.
Workshop participants concluded that overall, research suggests that breastfeeding has only a slight effect on breast cancer risk and that effect is only among women who have breastfed for a long time. They also concluded that breastfeeding seems to be more protective against the most aggressive types of breast cancer, including tumors in women with mutations in the BRCA1 gene, basal-like cancers, hormone-receptor negative, and possibly triple negative tumors.
The group also acknowledged the many difficulties of doing research in this area. One of their recommendations for future research was to try to study the individual effects of pregnancy and breastfeeding; it is difficult to separate these two factors when studying breast cancer risk.
A second recommendation was to collect more detailed information when asking women about breastfeeding, including how long they breastfed each child, how long they breastfed without supplementing with other liquids or foods, reasons for stopping breastfeeding, and reasons for not breastfeeding. Two large studies in the U.S., the Nurses' Health Study II and the Black Women's Health Study, are now collecting more detailed information from study participants.
Although it's not clear just how much breastfeeding may impact cancer risk, it has many other health advantages for mothers and especially for babies. The American Academy of Pediatrics recommends that women breastfeed each of their children for the first 6 months, and continue breastfeeding for 1 year or longer as other foods are introduced.
Dr. Saslow is director of breast and gynecological cancers for the American Cancer Society.