A new study presented at the 2011 Era of Hope breast cancer conference has reported preliminary findings that prenatal exposure to cadmium or arsenite (a type of arsenic compound) increases mammary tumor risk factors in rats. Exposed rat pups experienced earlier puberty, more dense mammary fat pads, and increased estrogen receptor (ER) expression. To conduct the study, pregnant rats were given injections of 5 μg cadmium or arsenite per kg body weight on days 12 and 17 of their pregnancies.

The offspring of these rats experienced earlier onset of puberty, as indicated by earlier vaginal opening. The exposed offspring also had altered mammary gland development; there was a significant increase in the number of branches in the mammary glands of exposed animals. Control animals had fewer branches/unit length (approximately 1.69 branches/unit length) compared to arsenite (approximately 2.5 branches/unit length) or cadmium (approximately 2.66 branches/unit length) exposed animals. Exposed animals also had greater mammary gland density (43%) than controls (35%) as of the 15th day after birth. It is possible that in utero exposure to arsenite and cadmium increased the number of branches by increasing the number of mammary stem and/or progenitor cells along the ducts, according to the author. Finally, overexpression of ER was observed in the mammary glands of exposed prepubertal offspring, with a 2.5-fold increase in animals exposed to arsenite and a 3.1-fold increase in animals exposed to cadmium compared to controls. The overexpression of ER persisted throughout development into adult life.

Sources of cadmium exposure

Arsenic is widely present in the environment. Drinking water is one avenue of exposure, as are shellfish and fish. However, there is little evidence from human population studies that exposure to arsenic increases risk of breast cancer. On the other known, exposure to cadmium is associated with increased breask cancer risk. Although exposure to cadmium can occur among certain industrial and chemical workers, those living near waste sites, and cigarette smokers, food (especially imported food) is the greatest source of exposure for most U.S. consumers. Below is a brief discussion of some foods that have the potential to incorporate unacceptably high levels of cadmium:

• Various types of shellfish from locations around the world have been found to contain cadmium as a result of industrial pollution of the waters in which they were harvested. The problem is more acute in shellfish from countries with limited regulation and control of such pollution. Continuous monitoring of water conditions and shellfish health is rare, even in the U.S.
• Flaxseed can absorb unacceptable levels of cadmium. Areas of Canada, which is the world's largest producer of flaxseed, have high levels of cadmium in the soil. North and South Dakota, the two largest producing U.S. states, also are known to have cadmium in the soil. The other major producers are China and India, both known for heavy metal pollution in some regions. It is possible that flaxseed from high-cadmium areas are reaching the U.S. consumer. The "organic" label does not necessarily mean a low-cadmium product. We recommend that consumers use flaxseed and flaxseed oil that are stated to have low levels of cadmium.
• Rice represents a path of cadmium exposure when the rice is grown in contaminated irrigation water or soil. Southern Louisiana is home to numerous oil refineries and petrochemical plants that have introduced cadmium to some of the soil used for rice production. Although the U.S. is a net rice exporter, it imports specialty rice varieties such as jasmine and Basmati rice from countries such as Thailand and India. This rice is often grown under conditions that would be unacceptable inside U.S. borders. Rice grown in California is most likely to have low levels of heavy metal contaminants.
• Sunflower seeds and kernels are a dietary source of cadmium. U.S. sunflower seeds grown for consumption as seeds (confectionary sunflower seeds) are produced primarily in North and South Dakota, a region with relatively high cadmium levels in the soil. Efforts have been under way for several years to breed sunflower hybrids that will take up less cadmium, however it is not clear to what extent U.S. consumers are benefiting from such efforts.
• Significant levels of cadmium have been detected in dried apricots from Turkey. Most U.S. apricots are grown in California and Washington state. However, the U.S. imports significant quantities of dried apricots from Turkey.
• One study that examined the levels of mercury and other trace metals in canned fish samples purchased in the states of Georgia and Alabama found that cadmium levels were unacceptably high in one of the tuna samples.
• Commercially farm-raised escargot snails typically are fed a diet of ground cereals. Free range snails (which are considered superior in taste and texture) consume a variety of decayed matter, dead animals and insects, and a wide variety of leaves. Such snails have also to found to accumulate cadmium in their tissues from soil containing cadmium.
• Indian mustard (black mustard) is known for its tendency to incorporate heavy metals, including cadmium, from its soil. Heavy metal contamination of agricultural soils and stream sediments have been reported in many countries, including China near coal and copper mines, in India near tanneries, and in Russia near uranium plants and heavy metal smelting complexes. Much of the imported Indian mustard consumed in the U.S. is imported from Canada, but it also comes from parts of the old Soviet Union, India and China. Buyers of Indian mustard from specialty markets should be aware of its source and assure themselves of its safety.