The results mark the first time the relationship has been shown in humans, said Michael Thun, vice president of epidemiology and surveillance research at the American Cancer Society, in a written statement.
However, "it is unclear whether the association between cadmium concentrations in the urine of breast cancer patients relates to the cause or treatment of the disease or to some other factor," he said.
Cadmium is a naturally occurring metal that's found in soil, water, certain foods, tobacco and batteries.
Previous studies have demonstrated links between cadmium exposure and lung cancer, prostate cancer and kidney disease.
In addition, cell culture studies have shown that cadmium acts like estrogen once it enters the body, said Mary Beth Martin, an associate professor of oncology at the Lombardi Cancer Center at Georgetown University Medical Center.
In an earlier study in rats, Martin and colleagues found that low doses of cadmium stimulated signs of early breast cancer such as changing the density of breast tissue. That study also suggested that cadmium could cause adverse effects on a woman's fetus.
For the current study, researchers analyzed urine samples from almost 500 Wisconsin women, aged 20 to 69, between September 2004 and February 2005. Each participant also answered questions about their medical background, reproductive history, smoking history and food intake. Foods such as liver, kidney and tuna contain high levels of cadmium.
Even after adjusting for factors such as smoking, which doubles the normal intake of cadmium in the body, women with the highest levels of cadmium - more than 0.58 micrograms per gram - in their urine were twice as likely to develop breast cancer as women with the lowest levels - less than 0.26 micrograms, said Jane McElroy, co-author of the study and an associate scientist at the University of Wisconsin Comprehensive Cancer Center.
A person's normal intake of cadmium is one to three micrograms per day.
The results are published in the Journal of the National Cancer Institute.
"Everybody has cadmium in their bodies," McElroy said. "If this indeed ends up being a direct relationship, we need to figure out ways to limit it in the environment and its exposure to humans."
"This could certainly have some impact as far as cancer screening," said Marilyn Frenn, an associate professor of nursing at Marquette University. "If we are going to advise people at risk to get further testing, we need to know what would be the best things to test for."
The Environmental Protection Agency classifies cadmium as a probable human carcinogen due to "limited evidence from occupational epidemiologic studies of cadmium (that) is consistent across investigators and study populations."
"Because cadmium is a natural element in the Earth's crust, it can't be eliminated," said Tara Bergeson, an environmental toxicologist at the Wisconsin Department of Natural Resources.
"However, residents can take actions to reduce their exposure to cadmium," she said.
For example, keep nickel-cadmium batteries out of reach of young children, avoid cigarette smoke and eat a balanced diet to reduce the amount of cadmium in the body, she said.
Also, people who work with cadmium should take precautions to avoid carrying cadmium-containing dust home from work on clothing, skin, hair or tools, Bergeson said. |
