New Haven, Conn. - Breastfeeding for two or more years reduces a woman's risk of developing breast cancer by 50 percent, according to a study conducted by a Yale researcher among women in China.
The researcher, Tongzhang Zheng, associate professor of epidemiology and public health at Yale School of Medicine, said he conducted the study in China because, unlike Western nations, long term breastfeeding is part of the Chinese culture.
"In Chinese society, it is socially acceptable to breastfeed for a long time," said Zheng. "And it is considered good for the child."
His research, published recently in the American Journal of Epidemiology, followed three studies conducted in the early 1980s in Shanghai, Beijing and Tianjin, China. The earlier studies found a more than 50 percent reduction in breast cancer among women who reported a lifetime breastfeeding of more than 109 months compared to women who never breastfed.
A limitation of that first study was that it only reported lifetime duration of lactation and breast cancer risk for all women. Recent studies from Western populations suggest that the effect of breastfeeding may be limited to pre-menopausal women and may be affected by other factors such as the number of children breastfed and the age at first breastfeeding.
In 1997-1999, Zheng's group conducted a study in four large hospitals and several smaller hospitals in the Shandong Province of China to investigate the relationship between lactation and breast cancer risk.
This study, unlike the earlier studies, included detailed information on menstrual and reproductive factors, number of children breastfed, mean duration for each child breastfed, lifetime duration of breastfeeding, and age at first breastfeeding. A total of 404 female breast cancer cases were identified from the participating hospitals and all of the patients involved participated in the study.
"I should emphasize that this is a crucial time to do a study in China because of the country's one child policy. If we do not take the opportunity to do this study now, the opportunity will be lost," Zheng said.
What he found was a 50 percent reduction in breast cancer risk among those women who breastfed for more than 24 months per child, compared to women who breastfed their children for less than 12 months.
Zheng said the breastfeeding for more than 24 months also has a long term effect. It was found not only to reduce the risk of breast cancer diagnosed among pre-menopausal women, it was also found to reduce the risk of breast cancer among post-menopausal women as well. However, the age of first breastfeeding and the total number of children breastfed did not seem to have a significant impact on the breast cancer risk.
Studies in Western countries showing that breastfeeding does not play a significant role in reducing breast cancer risk might be explained in that few women in Western nations breastfeed for more than four months to one year, Zheng said.
"Based on the data from the National Cancer Institute, the average duration of lactation prior to supplementation was about 14 weeks in the United States," he said.
"We probably will never be able to resolve this issue in cultures where they do not have long-term breastfeeding history," Zheng said. "But our findings in China are clear. The longer duration of lactation -- whether it is based on breastfeeding of a first child, or breastfeeding over a lifetime -- leads to a significantly reduced risk of breast cancer."
Co-authors of the study were Yi Liu, M.D.; Li Duan, M.D.; Bing Zhang, MPH; Yan Wang, M.D.; Yongxiang Chen, M.D.; and Patricia Owens, research associate in the Department of Epidemiology and Public Health.