The largest study yet to examine whether certain "environmental estrogens"--synthetic chemicals that can act like hormones in the body--might be contributing to breast cancer has found no evidence for such a link. The research, to be published tomorrow in the New England Journal of Medicine, deals a heavy blow to a popular theory for explaining the recent rise in breast cancer.
Breast cancer rates are highest in developed countries, leading some researchers to suggest that industrial pollution could be an important factor. Among the prime suspects are two types of environmental estrogens found in everyone's fat tissue--polychlorinated biphenyls (PCBs) and DDE, a metabolic product of the pesticide DDT. Four years ago, a U.S. study of 58 women with breast cancer seemed to confirm the hypothesis: It suggested that women with high blood levels of DDE had a four-fold greater risk for breast cancer than women with normal levels. But a larger study a year later found no link between breast cancer and DDE or PCBs.
Now, a team led by epidemiologist David Hunter of the Harvard School of Public Health has analyzed 240 breast cancer cases that occurred in a group of nearly 33,000 women after they gave blood samples in 1989 and 1990 as part of the Harvard Nurses' Health Study. Comparing DDE and PCB levels in blood from the 240 women to those of a control group of women without cancer, the researchers found women with higher levels were at no greater risk for breast cancer. While "no epidemiology study" can answer the question once and for all, Hunter says, "the weight of the evidence is now strongly against the hypothesis" of a DDE or PCBs-breast cancer link.
The Harvard findings and "other recent studies should reassure the public that weakly estrogenic organochlorine compounds such as PCBs, DDT, and DDE are not a cause of breast cancer," writes Stephen Safe, a toxicologist at Texas A&M University, College Station, in an editorial in the same issue of NEJM. But not all researchers are ready to exonerate the chemicals. Epidemiologist Devra Davis of the World Resources Institute in Washington, D.C., notes that older women tend to have higher DDE levels than the nurses, whose median age was 59. As a result, she says, the chemicals' effects might have been masked by stronger risk factors--such as smoking and radiation the nurses were exposed to in their work.