Cadmium, a toxic heavy metal with estrogenic properties, is associated with increased risk of breast cancer. Unlike some other heavy metals such as copper, iron and zinc, cadmium has no beneficial effect on health and is not essential to human life. Numerous studies have demonstrated that cadmium interacts with breast cancer cells in ways that promote their growth and proliferation. High cadmium levels in breast tissue and urine are linked to increased rates of breast cancer. Certain foods are potentially significant sources of cadmium for U.S. women.
Cadmium is a heavy metal with industrial uses
Cadmium has toxic properties at relatively low concentrations. The body has no efficient mechanism for excreting it, so cadmium accumulates in tissues. Cadmium itself is not mined; it is a byproduct of the smelting of other metals such as zinc, lead, and copper. Cadmium has wide industrial uses, including in soldering and metal plating, in the manufacture of alloys, plastics, and pigments, in the stabilization of phosphate fertilizers, and as a component of batteries and electrical conductors.
Laboratory studies show cadmium promotes breast cancer development & growth
Cadmium functions as an endocrine disruptor, stimulating estrogen receptor (ER) activity and promoting uterine and mammary gland growth in mice. Cadmium influences gene expression, modifying signaling in both normal and cancerous breast cells. Exposure to cadmium leads to increased cell survival, reduced DNA repair capacity, and genomic instability. The increased proliferation of cells with genomic instability is one possible viable mechanism for cadmium-induced cancer.
Long-term exposure to cadmium in MCF-10A breast cancer cells (which are ER- but can convert to ER+ during malignant transformation) has been shown to increase colony formation and invasion, both typical of cancer cells. Injection of such cadmium-treated cells into mice produces tumors with characteristics of triple negative breast cancer. Hence, cadmium appears capable of transforming normal human breast cells into a triple negative or basal-like cancer phenotype. Cadmium has also been shown to stimulate metastasis-associated processes (cell migration, invasion, adhesion, and proliferation) in triple negative breast cancer cells, as well as increasing their growth.
Cadmium has been found to modify the cell cycle, suppress apoptosis (programmed cell death), and alter the expression of various genes in ways that reduce the treatment effectiveness of 5-Fluorouracil (5-FU) chemotherapy in hormone receptor positive (ER+/PR+) breast cancer cells.
Human studies show link between cadmium and breast cancer risk
While not all studies are in agreement, there is ample evidence that cadmium promotes breast cancer in women:
- A 2013 Japanese study reported that women in the highest third of urinary cadmium levels had more than six times the risk of breast cancer compared to those in the lowest third. Japanese women have potentially high exposure to cadmium through their rice and seafood consumption. The risk of breast cancer increased with increasing levels of cadmium.
- A large Swedish prospective study reported that women with the highest third of cadmium intake had a 21% higher risk of breast cancer than women with the lowest intake. The risk was highest among slender and normal weight women with high intake, for whom it was 27%. The risk was also higher risk for ER+ tumors than ER- tumors (for which the elevated risk did not reach statistical significance). The risk of breast cancer was found to decline with increasing consumption of whole grain/vegetables within each level of cadmium exposure, suggesting that whole grain/vegetable consumption protects against such exposure.
- A study that measured urinary cadmium levels of women living on Long Island, New York (a region with a high rate of breast cancer) and in a representative sample of the general U.S. population reported that both groups of women had increasing likelihood of breast cancer for increasing levels of urinary cadmium. Urinary cadmium is a measure of exposure to cadmium over the prior 20 to 30 years.
- A Lithuanian study that compared cadmium levels of breast cancer patients and women with benign breast tumors reported higher concentrations of cadmium in the breast tumors and urine of breast cancer patients. Breast cancer patients with ER+ disease had significantly higher levels of cadmium in their breast tissue than patients with ER- tumors.
- A U.S. study that matched breast cancer patients and cancer-free controls reported that women in the highest fourth of urinary cadmium levels had twice the breast cancer risk of those in the lowest quartile. The study also found a statistically significant increase in breast cancer risk with increasing cadmium level.
- Several human and animal studies have reported that cadmium exposure is associated with increased breast density, a strong marker of breast cancer risk.
Non-food sources of exposure to cadmium
Cadmium is very easily absorbed through the lungs, less easily absorbed in the digestive tract, and difficult to absorb through the skin. Cadmium is released into the environment from making and using phosphate fertilizers, automobile exhaust, mining and metal smelting operations, burning coal, oil or garbage, iron and steel production, and disposal of metal products. While U.S. Environmental Protection Agency 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. Since cadmium is found in car exhaust, soils near roads can have high levels of cadmium.
Crafts, toys and jewelry
Artisans or small business owners who make jewelry or stained glass, or work with paints containing cadmium must take steps to minimize exposure to it. Silver solder contains cadmium and should be handled carefully. Long-term exposure to cadmium plating baths should be avoided. Artists who work with cadmium pigments (commonly used in strong oranges, reds, and yellows) must also take precautions.
Because it is used in pigments, cadmium is sometimes found in plastic toys and food containers, especially those manufactured in Asia. In February 2010, high levels of cadmium were found in a line of Miley Cyrus jewelry sold at Wal-Mart. Cadmium was also detected in paint used on promotional drinking glasses made for McDonald's Restaurants in 2010 (glasses that were recalled).
Cigarettes are also a significant source of cadmium exposure. The tobacco plant is particularly efficient at taking up and storing cadmium from the soil. Smokers have approximately double the cadmium in their bodies as nonsmokers. However, breathing secondhand smoke is not believed to be a significant source of cadmium exposure.
Food sources of cadmium
Although cadmium is more easily absorbed through the lungs, most U.S. nonsmokers get their exposure from food, often food that is imported. Cadmium particles settling from the air can result in high levels of cadmium in surface soils. Once on the ground, cadmium moves through soil layers and is taken up by certain plants. Cadmium that lands in water tends to sink and accumulate in the sediment at the bottom. Hence, shellfish are more likely to have high levels of cadmium than fish.
Cadmium levels vary by country and region
In countries with lax environmental standards, untreated cadmium-containing industrial waste and sewage may be released directly into rivers, lakes and coastal waters and may also contaminate underground sources of water used for irrigation. Countries reported to have significant problems with such pollution include China, India, Pakistan, Thailand, Nigeria and Russia. Phosphate fertilizers and manure used to fertilize crops can also be a significant source of cadmium. However, developed countries also have problems with cadmium contamination of 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.
Uptake of cadmium in plants and people varies
Certain plants take up and incorporate cadmium more readily than others. These plants include rice and other cereal grains, flaxseed, potatoes, sunflowers, peanuts, spinach and other leafy vegetables, and tobacco. Generally speaking, cooking is of very limited value as a means of reducing cadmium concentrations in food.
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. Smokers with high fruit consumption have lower blood cadmium concentrations of than those with low fruit consumption. Women with iron deficiency have increased uptake of ingested cadmium. In other words, nutritional status can be a more important determinant of cadmium uptake than the actual amount of cadmium ingested.
Specific foods of concern for U.S. women
Most U.S. women can limit their cadmium exposure by avoiding the small group of foods below that could significantly increase intake. Sometime we fall into the habit of regularly consuming a particular food that appears to be healthy, or at least not harmful. If one of the foods below falls into this category for you, make the necessary adjustments to reduce or eliminate this source of cadmium:
- Various types of shellfish from locations around the world have been found to contain cadmium as a result of industrial pollution. 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, the problem is not limited to such countries. For example, oysters from the both the West and East coasts of Canada have been found to contain high levels of cadmium. 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 flaxseed 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 that are stated to have low levels of cadmium. Refined flaxseed oil has negligible levels of cadmium and other heavy metals.
- Sunflower plants have a tendency to accumulate cadmium and sunflower seeds and kernels are a dietary source of cadmium. Sunflower seeds are grown either for sunflower oil production or for consumption as seeds (confectionary sunflower seeds). U.S. confectionary sunflower seeds are grown primarily in North and South Dakota, a region with relatively high cadmium levels in the soil. Stricter European rules on cadmium in sunflower imports from the U.S. have caused confectionary sunflower seeds grown in U.S. soils with lower levels of cadmium to be diverted to the European market. 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.
- 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, 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. 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.
- 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.
- 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. 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. In the process, they ingest soil. Snails can also become contaminated by roadside car exhaust fumes.
- 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 in 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 and quality.
Below are links to recent studies on this topic. For a more complete list of studies, please click on cadmium.