Cadmium is a known breast cancer risk factor. 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 than cancer-free women. Cadmium exposure can occur through non-food sources, however for most U.S. women it occurs primarily through the diet.

Many plant-based foods, including some that are otherwise considered healthy, contribute to cadmium exposure. However, some of these plants contain compounds that tend to offset the deleterious effects of cadmium. Consumers can limit their cadmium by avoiding regular consumption of a small group of foods that could significantly increase exposure intake. Recently, the first large prospective study reported that cadmium increases the risk of breast cancer in postmenopausal women.

Animal and cell studies demonstrate harmful effects of cadmium

Numerous studies have demonstrated that cadmium interacts with breast cancer cells in ways that promote their growth and proliferation. 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 reduced DNA repair capacity and genomic instability. The increased proliferation of cells with genomic instability is one possible mechanism for cadmium-induced breast cancer.

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. 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 high cadmium levels in the soil. 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.
Consumers growing their own vegetables should avoid using roadside plots, which can be contaminated by cadmium from car exhaust.

Latest research finds cadmium increases postmenopausal breast cancer risk

The Swedish prospective study referenced at the beginning of this news article was designed to investigate the association between dietary cadmium exposure and risk of breast cancer. To date, no prospective population studies have been performed to examine the link between cadmium intake and breast cancer risk. The study included 55,987 postmenopausal women for whom dietary cadmium exposure was estimated at baseline in 1987. The women were followed for an average of 12.2 years, during which time 2,112 were diagnosed with breast cancer (1,626 with ER+ tumors and 290 with ER-disease). The analysis was performed while adjusting for other factors that influence breast cancer risk, including intake of whole grains and vegetables (which account for 40% of the cadmium exposure, but also contain phytochemicals that are thought to protect against breast cancer).

The women were divided into three groups, according to their estimated level of dietary cadmium intake. Women with the highest cadmium intake were found to have a 21% higher risk of breast cancer compared to women with the lowest intake. The risk was highest among slender and normal weight women with high intake, for whom the risk 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. The authors conclude that dietary cadmium might have a role in postmenopausal breast cancer development.