The study did not investigate why breast-feeding cut the risk of breast cancer, but researchers believe it may be related to reduced exposure to estrogen and female hormone cycles during lactation. The researchers interviewed the women to gather information about lactation, menstruation, and reproduction. The risk of breast cancer was also significantly lower for women who breast-fed more than 73 months over their lifetime. Of importance, the risk of breast cancer was reduced both before menopause and after menopause. By contrast, in China, breast-feeding children for the first two years is socially accepted.
Experts believe that estrogen exposure plays a role in breast cancer risk.
The more menstrual cycles a woman has throughout her lifetime, the higher her risk of breast cancer. Though the study did not specifically address the reason why breast-feeding may reduce breast cancer risk, Dr. Zheng suggests that the suppression of estrogen and female hormone cycles may be one explanation for the protective effect. However, further research is needed to understand how breast-feeding lowers this cancer risk. In the study, the number of children breast-fed and the age at the time of lactation did not appear to influence breast cancer risk. Many risk factors for breast cancer, including advancing age, family history, age of first menstruation, etc. are uncontrollable.