The heavy metal cadmium is a known breast cancer risk factor, although not all population studies have reported an association. Numerous animal and cell studies have demonstrated the harmful contributions of cadmium to breast cancer development. Breast cancer patients tend to have higher levels of cadmium in their breast tissue and urine than cancer-free women. Exposure to cadmium has been linked to increased breast density in premenopausal women. Cadmium exposure can occur through non-food sources, however for most U.S. women it occurs primarily through the diet. Now a new study has reported that cadmium stimulates metastasis-associated processes in triple negative breast (ER-/PR-/HER2-) cancer cells.
Cadmium exposure can occur through non-food sources
Cadmium is released into the environment from mining and metal smelting operations, burning coal, oil or garbage, making and using phosphate fertilizers, iron and steel production, and disposal of metal products. While EPA regulations have been successful in reducing the cadmium exposure of the general population to levels considered safe, people living near or working for industries that conduct any of these activities might be exposed to high levels of cadmium in the air. Cadmium is also found in car exhaust; soils near roads can have high levels of cadmium from this source. Cigarette smoke is also a significant source of cadmium exposure.
Most cadmium exposure occurs through diet
Phosphate fertilizers and manure used to fertilize crops can also be a significant source of cadmium. Many countries (developed as well as less developed) cadmium contaminated coastal waters. In addition, there are large areas of some countries, including the U.S., Canada and China, in which significant levels of cadmium are found naturally in the soil.
Certain plants take up and incorporate cadmium more than others. Included are rice and other cereal grains, flaxseed, potatoes, sunflowers, peanuts, spinach and other leafy vegetables, and tobacco. However, some of these plants contain compounds that tend to offset the deleterious effects of cadmium (e.g., the iron in spinach). Also, other dietary components influence the impact of cadmium intake on health. For example, curcumin, piperine (found in black pepper), selenium, zinc and iron all appear to protect against cadmium toxicity.
Food sources of cadmium
Most U.S. women can limit their cadmium exposure by avoiding regular consumption of a small group of foods that could significantly increase intake:
- Flaxseed. Some but not all regions 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 high soil cadmium levels. The other major flaxseed producers are China and India, both known for heavy metal pollution in some regions. The "organic" label does not necessarily mean a low-cadmium product. Consumers should look for flaxseed that are stated to have low levels of cadmium. Refined flaxseed oil has negligible levels of cadmium and other heavy metals.
- Shellfish. Various types of shellfish from locations around the world have been found to contain cadmium. Most low-priced shrimp sold in the U.S. are sourced from parts of Asia known to have heavy metal pollution in coastal waters. The problem is more acute in shellfish from countries with limited regulation and control of such pollution. However, it is not limited to such countries. For example, oysters from the northwest coast of Canada have been found to contain high levels of cadmium.
- Sunflower seeds. U.S. sunflower seeds are grown primarily in North and South Dakota, a region with relatively high cadmium levels in the soil.
- Rice. Southern Louisiana is home to numerous oil refineries and petrochemical plants that have introduced cadmium to some of the soil used for rice production, in addition to cadmium contributed in previous decades by phosphate fertilizers. 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 which may be 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.
- Dried apricots. Significant levels of cadmium have been detected in dried apricots from Turkey. Most fresh apricots sold in the U.S. are grown in California and Washington state. However, the U.S. imports significant quantities of dried apricots from Turkey.
- Escargot. Free range escargot snails have been found to accumulate cadmium in their tissues from soil containing cadmium. Commercially farm-raised snails typically are fed a diet of ground cereals and do not have high levels.
- Indian mustard. Indian mustard (black mustard) is known for its tendency to incorporate heavy metals, including cadmium. Heavy metal contamination of agricultural soils and stream sediments have been reported in many countries, including China, India, and Russia. 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.
Latest research finds cadmium promotes metastasis
The study referenced at the beginning of this news article was designed to investigate the effects of cadmium on molecular processes involved in the progression of triple negative breast cancer to metastasis. To conduct the study, the authors evaluated the impact of cadmium on cell adhesion, migration, and invasion in MDA-MB-231 triple negative breast cancer cells.
Treatment of triple negative cells with 1 μM cadmium was found to increase cell spreading and cell migration. This was associated with the activation of genes and signaling associated with cancer migration and invasion (integrin β1, FAK, Src, and Rac1,as well as β-catenin). Additional experimentation demonstrated that prolonged cadmium treatment reorganized the cytoskeleton (the cell skeleton, consisting of microfilaments and other cellular structural components), which aided malignancy, including cell invasion. Prolonged cadmium treatment also was found to promote cell growth. The authors conclude that cadmium modifies important signaling processes involved in the regulation of cytoskeleton in ways that enhance triple negative cell migration, invasion, adhesion, and proliferation.
Please see our article on cadmium for more information.