Breast Cancer Risk Minimized by Breastfeeding

Yale School of Medicine
Monday, 18 June 2001

Women who breastfeed their children, particularly if the first child is breastfed for more than 13 months, have a reduced risk of breast cancer, according to a study by a Yale researcher and published in the British Journal of Cancer.

The study involving Conecticut women showed a stronger association between reduced risk of breast cancer and women who breastfed their children. The protection also was more pronounced in pre-menopausal years rather than following menopause.

The study by Tongzhang Zheng, associate professor in the Department of Epidemiology and Public Health at Yale School of Medicine, was conducted between 1994 and 1998 and included 608 breast cancer cases and 609 matched controls.

Zheng recently published a study of women in China showing lifetime reduced breast cancer risk among those women who breastfed for two years or more. He said a protective effect associated with longer duration of breastfeeding has been consistently reported in countries where the prevalence of prolonged breastfeeding is high.

"For example, four studies from China, where more than half the women lactate at least three years, suggest that long-term lactation is protective among both pre- and postmenopausal women," he said. "It will be very interesting to see if the risk will change for the new generations in China who tend to marry later and who have been subjected to the one child policies implemented in the past decade, particularly in the large cities."

Zheng said there are several theories about the apparent connection between reduced risk of breast cancer and prolonged lactation. Among these are reduced exposure to certain reproductive hormones suppressed during prolonged breastfeeding; a protective effect from physical changes in the breast that accompany milk production; a reduction in the concentrations of toxic organochlorines in the breast with increasing cumulative duration of lactation and an expression, during lactation, of transforming growth factor-Beta, which is hormonally regulated negative growth factor in human breast cancer cells.

The study was supported by a grant from the National Cancer Institute/National Institute of Environmental Health Science. The co-authors were Yawei Zhang of Yale; Bing Zhang of McGill University in Montreal, Canada; Peter Boyle of the European Institute of Oncology in Milan, Italy, and Shelia Zahm of the National Cancer Institute.

For more information, or to contact Yale School of Medicine, see their website at: info.med.yale.edu/ysm/

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