By "shellfish", we mean edible crustaceans and mollusks, including abalone, clam, crab, crayfish, lobster, mussel, octopus, oyster, prawn, scallop, shrimp, langoustine, and squid. Small crustaceans such as shrimp are scavengers, feeding on very small shellfish and other zooplankton, as well as detritus that has fallen to the ocean floor.
Large crustaceans such as lobster are more likely to be active predators, consuming smaller fish and shellfish. Bivalves such as clams generally feed by pumping water across their gills and trapping phytoplankton.

Shellfish are linked to some digestive cancers

Persistently high colorectal cancer rates in the European Union are thought to be the result of shellfish consumption, in particular shellfish incorporating diarrhetic shellfish poisoning toxins such as okadaic acid. A report from 73,224-participants in the Shanghai Women's Health Study also found high shellfish consumption to be associated with increased risk of colorectal cancer. Environmental pollution was thought to be a possible contributing factor in that case. Some, but not all, studies have also found an association between fish and shellfish consumption and thyroid cancer.

Breast cancer-related effects of eating shellfish

Shellfish contain compounds favorable for breast cancer prevention

Shellfish are a dietary source of marine omega-3 fatty acids (incorporating approximately 5% to 30% of the level of omega-3 fatty acids found in a similar size serving of salmon). Such fats have been found in numerous studies to be associated with reduced risk of breast cancer and its recurrence. (However, note that some U.S. shrimp farmers have started to use soybean feed instead of fish-based feed. While this may improve the water quality of the shrimp farm and surrounding region, it reduces the level of omega-3 fatty acids in the shrimp.)
The pink coloration of some shellfish is due to astaxanthin, a carotenoid that has also been shown to have anti-oxidant and chemopreventive properties. In addition, shellfish are a good source of dietary iodine and selenium, both of which have been linked to reduced risk of breast cancer. However, these favorable attributes appear to be outweighed on balance by the potential of shellfish to increase breast cancer risk or interfere with treatment.

Shellfish can promote breast cancer

Few epidemiological studies have examined the associations between shellfish consumption and breast cancer risk. One study of 9 to 12 year old Korean girls reported that breast development was significantly positively associated with the consumption of shellfish, a finding that was ascribed to shellfish contamination. In the absence of relevant population studies, the rationale for limiting or avoiding shellfish is based primarily on their heavy metal and other contaminant content.

Heavy metals in shellfish linked to breast cancer risk

Generally speaking, shellfish are a significant source of copper and iron. While these are are essential nutrients, high levels are associated with increased breast cancer risk. Shellfish may also incorporate relatively high levels of cadmium, a known breast carcinogen with no beneficial effects on health. Most low-priced shrimp sold in the U.S. are sourced from parts of Asia known to have heavy metal pollution of coastal waters. While the problem is more acute in regions with limited regulation and pollution control, it is not limited to such locations.
Shellfish, particularly oysters, lobster and squid (calamari), can contain high levels of copper, which could contribute to angiogenesis and metastasis of breast cancer, especialy in women with inflammatory breast cancer (IBC) or triple negative (ER-/PR-/HER2-) disease.
Shellfish such as clams, squid, mussels, oysters, and abalone also tend to be a significant source of heme iron (meat-based iron). While iron deficiency anemia is to be avoided, the contribution of significant iron in the diet as a result of regularly consuming shellfish could be detrimental for some women. For example, relatively high levels of iron in benign breast tissue was found in one prospective study to be associated with increased risk of breast cancer. In addition, excess iron has been reported to interfere with the treatment effects of the chemotherapy drugs Adriamycin and cisplatin. Iron depletion has been shown to lead to significant inhibition of breast cancer cell growth in the laboratory.
Cadmium is a fairly common contaminant in shellfish. Cadmium functions as an endocrine disruptor, stimulating estrogen receptor (ER) activity and promoting uterine and mammary gland growth in mice. 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 tends to sink and accumulate in the sediment at the bottom of water bodies. Hence, shellfish are more likely to have high levels of cadmium than fish found in the same waters.

Other harmful contamination of shellfish is common

Reports of shellfish contamination that could potentially increase breast cancer risk are not uncommon. The following list represents a small fraction of available examples (location - shellfish affected - contaminant):
  • St. Lawrence maritime estuary, Canada - mussels and clams - cadmium
  • Normandy, France - oysters - various pesticides
  • Moroccan Atlantic coast - mussels - untreated chemical wastes from industrial processing of phosphates
  • Bangladesh - dried shrimp - DDT (readily available in spite of official bans) applied by fishermen and merchants to help keep dried shrimp from spoiling
  • Southwest Louisiana - shrimp, oysters, crayfish, crabs - mercury, lead and cadmium from local petrochemical plants
  • Catalonia, Spain - clams, mussels, and shrimp - arsenic, cadmium, mercury, and lead
  • Malaysian coasts and rivers - mollusks and oysters - lead, zinc and cadmium from manufacturing, agriculture, sewage and motor vehicle emissions.
Generally speaking, continuous monitoring of water conditions and shellfish health is rare, even in the U.S. When unsafe levels of industrial or agrochemical contamination are suspected, they first must be studied and then, if possible, ameliorated. Unless consumers become acutely ill from affected shellfish, this process can take several years, during which time the problem might not be well publicized.
In recent years, there have also been reports of microplastic contamination of shellfish, especially in Asia, with unknown health consequences.

Shellfish aquaculture can introduce harmful chemicals

Aquaculture supplies over half of the world's seafood, including at least 55% of shrimp and most oysters. Leaving aside the environmental degradation typical in aquaculture, farmed shellfish usually incorporate much higher levels of natural and man-made toxic substances (antibiotics, pesticides, and persistent organic pollutants) than wild species.
Captive shrimp are very susceptible to viral and bacterial infection. Farmers use antibiotics and pesticides to deal with this problem. Top non-U.S. producers routinely use chemicals banned in the U.S. Dried shrimp and salted shrimp paste, in particular, tend to incorporate concentrated levels of pesticides.

Shellfish toxins can cause illness

Toxins produced by a variety of waterborne microorganisms (often dinoflagellates) can produce shellfish poisoning when shellfish consume them and store the toxins. Shellfish poisoning can have diverse effects: paralytic, neurotoxic, and diarrhetic, among others. Below are examples of such outbreaks:
  • Central California coast - rock crabs - paralytic shellfish toxins from the dinoflagellate Alexandrium catenella
  • South Norway - brown crabs and mussels - diarrhetic shellfish poisoning from a bloom of the dinoflagellate, Dinophysis acuta.
Acute shellfish poisoning is often well publicized, making it easier to avoid than other forms of shellfish contamination. However, even in such cases, consumer protections may be inadequate or not timely enough for breast cancer patients undergoing treatment.
For example, in one case, the FDA advised retail and foodservice operations to be aware that raw oysters shipped in containers bearing a For Cooking Only label might contain harmful levels of the bacterium Vibrio parahaemolyticus (which can cause diarrhetic illness). The For Cooking Only labeling had been worked out as a compromise to allow the continued sale of oysters when harvesting conditions did not meet specific criteria for reduced risk of Vibrio parahaemolyticus contamination.
Some shellfish toxins have the potential to increase breast cancer risk directly in addition to causing illness. For example, Yessotoxin, an algal toxin that can accumulate in edible mollusks, has been shown to interfere with the tumor suppressive functions of E-cadherin (a cell adhesion molecule) in breast cancer cells.

Breast cancer patients should avoid shellfish during treatment

Breast cancer patients are advised not to consume shellfish (especially raw preparations) during radiation treatment or chemotherapy because impaired immunity makes such patients more susceptible to shellfish toxins, which could make them ill and delay treatment.

Additional comments

For those who love shellfish

Of the possible shellfish contaminants, heavy metals such as cadmium appear to be most directly linked to increased risk of breast cancer, followed by some pesticides. Please see our article on cadmium and breast cancer. Note that cooking is not effective in removing heavy metal from shellfish. Shellfish caught in the open ocean are not exempt; excess levels of heavy metals have been found in king crabs from the Barents Sea and waters near Australia and South America.
Shellfish from Louisiana is known to have persistently high levels of heavy metal contamination (long before, and not caused primarily by, recent oil spills). Shellfish should not be purchased without knowing the country of origin — in the case of shrimp, it will likely come from China, Ecuador, India, Indonesia, Thailand, or Vietnam, none of which are safe sources, generally speaking. It makes sense to avoid inexpensive restaurant shrimp.

Sources of information provided in this webpage

The information above, which is updated continually as new research becomes available, has been developed based solely on the results of academic studies. Clicking on any of the underlined terms will take you to its tag or webpage, which contain more extensive information.
Below are links to 20 recent studies concerning shellfish and their components. For a more complete list, including less recent studies, please click on shellfish.