Study: Breastfeeding may reduce hormone receptor negative breast cancer risk

IN DEPTH REVIEW OF RESEARCH

Study background:

Previous studies suggested a relationship between breastfeeding and breast cancer: longer length of breastfeeding is linked with lower rates of breast cancer. Not all breast cancers are the same. Breast cancers can be categorized into different types based on the presence or absence of various markers, specifically estrogen receptor (ER), progesterone receptor (PR), and/or HER2, another cell marker.  If none of these markers are present, the cancer is considered to be “triple negative.” Many past studies on breastfeeding and breast cancer did not take into account the presence or absence of HER2.  In this paper, Dr. Marissa Weiss of Breastcancer.org and colleagues at the American Cancer Society, Washington University School of Medicine, and Icahn School of Medicine at Mount Sinai were interested in how this relationship may differ between different cancer types.

Researchers of this study wanted to know:

The association between breastfeeding and risk of specific types of breast cancer.

Population(s) looked at in the study:

Researcher looked 27 previous studies published through 2014. The studies used information from 36,881 human breast cancer cases, and provided information about the association between breastfeeding and type of breast cancer. 

Study findings: 

Breastfeeding was associated with:

1. a risk reduction of about 20% for triple-negative breast cancers.
2. a risk reduction of about 10% for HER2 positive cancers that were also ER and PR negative.

Breastfeeding did not greatly affect the risk of ER and PR positive breast cancers, regardless of HER2 status. 

Limitations: 

The association between breastfeeding and triple-negative breast cancer risk should be confirmed with further research, because only a small subset of the studies looked at triple-negative breast cancer.  The results from this study complement the findings of a previous study, which also found that breastfeeding reduced breast cancer risk in BRCA1 carriers (who are more likely to develop triple-negative breast cancers). However, other studies have not shown that breastfeeding reduces cancer risk. 

This report is a meta-analysis (a paper that uses data from previous studies). Meta-analyses have certain inherent weaknesses. One pitfall is that the analysis is only as strong at the studies included. Another is that the different study designs of the research included in the analysis may affect the overall analysis.

This study also did not take into account several factors, including the BRCA mutation status, income, race, ethnicity, or access to high quality medical care of the participants; these omissions may also have affected the meta-analysis results. Experts warn that given these limitations, more research is needed before we know with certainty whether or not breastfeeding reduces breast cancer risk, and if so, in what kind of breast cancer, and which women are most likely to benefit. Women who cannot breastfeed or who chose not to breastfeed should not be concerned, as other lifestyle choices have been shown to reduce breast cancer risk.  

Conclusions:

The association between breastfeeding and breast cancers that are hormone receptor-negative, both ER/PR-negative and HER2-positive, or triple-negative, is interesting. While this analysis did not find a strong association between breastfeeding and ER and/or PR-positive breast cancers, some studies included in this meta-analysis found a protective association between hormone receptor-positive breast cancer risk and breastfeeding when results were analyzed individually. More work needs to be done to look at this relationship, and to further study how breastfeeding may be conferring a reduced risk of hormone receptor-negative breast cancers. 

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