Women of Mexican descent who had more children and breastfed were more likely to develop an aggressive breast cancer, according to results from the Ella Binational Breast Cancer Study. This finding contradicts the traditional epidemiologic view that factors thought to be protective, such as giving birth at a younger age, breastfeeding, and having multiple children, decrease a woman’s risk of breast cancer.  

The Ella Study enrolled 1,041 Mexican and Mexican American female patients age 18 years and older with breast cancer. The women were enrolled at two sites in the United States and three sites in Mexico. During the 4-year study, scientists examined the association between reproductive factors and three tumor subtypes: luminal A, HER2, and triple negative. The luminal tumor is most common and starts in the inner cell lining of the mammary ducts. The HER2 tumor is positive for human epidermal growth factor receptor 2, a protein that plays a role in aggressive breast cancer. Triple-negative breast cancer is difficult to treat and has the worst prognosis of the three subtypes.

Women in the Ella study gave birth to their first child at age 23 years on average, had two to three children, and breastfed for 12 months or longer. This reproductive pattern would be classified as low risk based on epidemiologic studies in non-Hispanic white women. Yet all of the women in the study developed breast cancer and were more than twice as likely to have triple-negative breast cancer.

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“We found that breastfeeding in women of Mexican descent is associated with triple negative breast cancer,” said lead author María Elena Martínez, MPH, PhD, who is codirector of the Reducing Cancer Disparities research program at the University of California at San Diego Moores Cancer Center . “This was quite surprising. No other study has seen this correlation before. Most studies show health benefits of breastfeeding. Our results are both puzzling and disconcerting because we do not want to give the wrong message about breastfeeding.”

Martínez said further research is needed in populations with unique risk-factor patterns. Conclusions drawn from the Ella study need to be tested in populations with similar reproductive profiles to determine if the results are due to common biologic factors or specific genetic or environmental factors.  The study was published in Cancer Epidemiology, Biomarkers & Prevention (2013; 10.1158/1055-9965.EPI-13-0560).