In a finding that strengthens the link between environmental pollutants and rising rates of breast cancer, new research finds that women whose diets contain higher levels of cadmium are at greater risk of developing breast cancer than those who ingest less of the industrial chemical in their food.
Cadmium, a heavy metal long identified as a carcinogen, leaches into crops from fertilizers and when rainfall or sewage sludge deposit it onto farmland. Whole grains, potatoes, other vegetables and shellfish are key dietary sources of cadmium, which also becomes airborne as a pollutant when fossil fuels are burned, and is likely inhaled as well as ingested.
The new study, published by the American Assn. for Cancer Research and released Thursday, found that among 55,987 post-menopausal women, the one-third with the highest cadmium intakes were 21% more likely to develop breast cancer than the one-third with the lowest intakes.
Among obese women, the study found no increase in breast cancer rates with higher cadmium exposures.
The study offers new evidence in a large human population that environmental chemicals that mimic the effects of the female hormone estrogen may contribute to women's risk of certain cancers, including endometrial and breast cancers.
The finding comes just three months after the Institute of Medicine, a prestigious body of independent biomedical researchers, concluded that a host of other factors — most within a woman's power to control, such as obesity and hormone-replacement medication — were the most important sources of breast cancer risk.
The panel of experts had called it "biologically plausible" that estrogen-like pollutants promote breast cancers, but noted that evidence that they contribute significantly was inconclusive. By contrast, studies in human populations strongly point to fattening foods, hormone-replacement drugs, alcohol and cigarettes as having roles in boosting a woman's breast cancer risk.
Even this study, while showing a correlation, did not prove cause and effect, experts noted.
UC Davis epidemiologist Irva Hertz-Picciotto, chairwoman of the Institute of Medicine panel that issued its findings in December, said the study "does not move us beyond" the panel's overall conclusions.
"At this point, we have not identified the major drivers of the increase in breast cancer," Hertz-Picciotto said. If cadmium pollution truly turns out to be a cause, she added, "it's probably a small part" of a very large picture.
Each year, about 230,000 women in the United States are newly diagnosed with invasive breast cancer. Breast cancer rates are rising worldwide, with 1.6 million cases in 2010.
A woman's lifetime exposure to estrogen — a hormone made by her own body and present in medications such as birth control pills and hormone-replacement treatments — powerfully influences her risk. In animals and in laboratory tests, cadmium has been shown to exert estrogen-like effects more powerfully than other environmental pollutants, and so suspicion has fallen on the heavy metal as a possible promoter of breast cancer.
"Cadmium is receiving a lot of attention these days because of its estrogenic properties," said Rudolph Rull, a research scientist at the Cancer Prevention Institute of California in Berkeley.
But Rull, who was not involved in the current study but is researching cadmium's effects, said that scientists were unsure how best to measure women's exposure to the chemical. That fact, he said, had made it difficult to show whether and how strongly it might drive cancer incidence.
The current study gauged ingested cadmium only. It estimated a woman's exposure to the chemical on the basis of food logs, extrapolating her likely dietary exposure to cadmium on the basis of national estimates of cadmium in crops.
Looking for a link between dietary cadmium and breast cancer is tricky enough for researchers. But for women who have increased their intake of whole grains and vegetables in hopes that doing so will ward off cancers, finding such a link poses a difficult dilemma: Should they eat fewer of those?
"I wouldn't recommend to anyone to stop eating vegetables and whole grains," Hertz-Picciotto said. "If you were to sit down and do cost-benefit analysis, my intuition is you wouldn't want to sacrifice the great benefits that we already know about and are quite well established for what at this point needs to be looked at in greater detail."
The latest research follows two other studies, published in 2006 and 2010, that first singled out cadmium as a factor in breast cancer. Those studies measured cadmium in the urine of smaller groups of pre- and post-menopausal women, and found that those who had high cadmium exposures were more than twice as likely to develop breast cancer as those with the smallest exposures.
In 2010, consumer watchdog groups warned that toys and costume jewelry manufactured in China and marketed to children throughout the United States contained high levels of cadmium, as well as lead.