Women with the highest levels of cadmium in their urine have more than a two-fold higher risk of breast cancer than women with the lowest levels, according to a new study. However, further studies are needed to determine if these elevated levels are a cause or effect of breast cancer.

Although cadmium, a heavy metal, has been classified as a probable cancer-causing substance by the US Environmental Protection Agency, until now no human studies have investigated its link with breast cancer, Dr. Jane A. McElroy told Reuters Health.

The findings from “animal studies have supported an association, and cadmium has been found in breast tissue,” noted the researcher, from the University of Wisconsin Comprehensive Cancer Center in Madison.

McElroy and her team compared urinary levels of cadmium in 246 breast cancer patients and in 254 age-matched controls. The subjects were contacted by telephone to determine the presence of known breast cancer risk factors.

In the study, reported in the Journal of the National Cancer Institute, women with cadmium levels above a certain cut-off were 2.29-times more likely to have breast cancer than those with lower levels. This held true after accounting for established risk factors.

Exactly how cadmium might cause breast cancer is unclear, but there is evidence that it mimics the effects of estrogen. “It actually competes with estrogen for the alpha receptor site,” McElroy said.

McElroy believes that if the current findings are replicated in a larger study and cadmium’s role is confirmed, it could lead to tighter restrictions on how the heavy metal is disposed of in the environment.

SOURCE: Journal of the National Cancer Institute, June 21, 2006.