Consuming cadmium -- a toxic, heavy metal that has been classified as a probable human carcinogen -- may put women at greater risk of developing breast cancer, a new study shows.
It joins a small collection of evidence linking cadmium exposure to the disease, currently the second most common cancer among American women.
In the new study, published Thursday in the journal Cancer Research, researchers from the Karolinksa Institute in Sweden analyzed data from nearly 56,000 women who answered questionnaires about their food consumption. Relying on national estimates of the cadmium levels in various foods, the researchers determined that one-third of the women with the highest cadmium intake were 21 percent more likely to develop breast cancer than the one-third with the lowest intake.
Cadmium is present in many farm fertilizers, and also can enter food sources through the atmosphere. The highest concentrations, the study says, are found in shellfish and offal products -- foods like liver and tripe. It may also be present in cereals, potatoes, root crops and vegetables.
According to the authors, the new study is among the first to look specifically at dietary cadmium and breast cancer risk.
"Two previous studies from the U.S. have looked at the link between urinary cadmium concentrations -- a biomarker for long-term body burden of cadmium -- and breast cancer risk," said Bettina Julin, a researcher in the division of nutritional epidemiology and biostatistics at the Karolinska Institute and one of the study's authors. "This means they were unable to look at dietary cadmium alone ..."
Jane A. McElroy, an assistant professor with the University of Missouri's School of Medicine, authored one of those studies, published in the Journal of the National Cancer Institute in 2006. It found that women with the highest cadmium levels had twice the breast cancer risk than those with the lowest amount.
"Every study that has come out on this [topic] has shown a statistically significant risk," McElroy told The Huffington Post. "Of course, there have only been a few."
Given that this is still an emerging area of research and that the etiology of breast cancer is not fully understood, the exact association between cadmium consumption and breast cancer risk is not yet understood. Experts favor the hypothesis that cadmium mimics estrogen, possibly contributing to women's risk of developing breast cancer.
"It may act like estrogen," said Dr. John S. Kovach, a professor in the department of preventive medicine at Stony Brook who recently authored a study looking at environmental cadmium and breast cancer risk. "It binds to estrogen receptor and presumably turns on genes, just like estrogen does."
Kovach added that cadmium has a particularly long half-life, meaning it can stay in the body for years. The Centers for Disease Control's Agency for Toxic Substances and Disease Registry says that is because humans lack effective pathways for cadmium elimination.
Experts caution that the current study has its limitations and only points to a possible association. McElroy said researchers relied on estimated levels of exposure, not actual values, which could lead to justifiable skepticism about the amount of increased risk researchers found.
The American Cancer Society's web site states that while research has probed this question of possible environmental influences on breast cancer risk -- particularly substances with estrogen-like properties -- the existing data does not show a clear link.
In the meantime, McElroy said there are a few steps people can take to try and limit their cadmium consumption.
"The first thing people can do to reduce their risk is not smoke," she said, explaining that cigarette smoke is a source of cadmium. "If it's something people are concerned about, they might also want to think about limiting the frequency of eating crustaceans, as well as liver and kidney."