By "shellfish", we mean edible crustaceans and bivalves, including abalone, clams, crab, crayfish (crawdads, crawfish), lobster, squid, mussels, oysters, prawns, scampi, scallops and shrimp. In nature, smaller crustaceans such as shrimp usually are scavengers, feeding on very small shellfish and other zooplankton, as well as plant detritus and parts of creatures that have fallen to the ocean floor. Larger 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.
Generally speaking, shellfish are a significant source of copper, iron, selenium and vitamin B12 and contain some marine omega-3 fatty acids (approximately 5% to 30% of the omega-3 fatty acids found in a similar size serving of salmon). The pink coloration of some shellfish is due to astaxanthin, a carotenoid derived from algae and phytoplankton that has been shown to have anti-oxidant and anti-atherogenic properties and may also have chemopreventive properties.
Persistently high colorectal cancer rates in the European Union are thought to be the result of shellfish consumption, specifically shellfish incorporating diarrhetic shellfish poisoning toxins such as okadaic acid. A 2009 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. Some but not all studies have found an association between fish and shellfish consumption and thyroid cancer.
Breast cancer-related effects of eating shellfish
One study of Korean 9 to 12 year old girls found that breast development was significantly positively associated with consumption of shellfish and processed meat. Yessotoxin, an algal toxin that can accumulate in edible mollusks, has been shown to interfere with the tumor suppressive functions of E-cadherin in breast cancer and other cancer cells.
The heavy metal cadmium, which is found frequently in shellfish, has been shown to increase the risk of breast cancer. Shellfish, particularly oysters, lobster and squid (calamari), can also 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, especially clams, squid, mussels, oysters, and abalone, also tend to be a significant source of heme iron (meat-based iron). While iron deficiency anemia obviously 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. Iron depletion has been shown to lead to significant inhibition of breast cancer cell growth in the laboratory. Relatively high levels of iron in benign breast tissue was found in one prospective study to be associated with an increase in risk of subsequent breast cancer. Excess iron can interfere with the treatment effects of the chemotherapy drugs Adriamycin and cisplatin.
While individual components of shellfish, such as omega 3 fatty acids, have been shown to be preventative against breast cancer, shellfish consumption is more likely to increase the risk of breast cancer and other cancers (especially digestive cancers) than to be protective. Health risks are derived from a combination of natural toxins from algal blooms, infection with pathogenic bacteria, heavy metals and other contaminates derived from agricultural and industrial pollution of coastal regions, and various unhealthful conditions under which shellfish typically are farmed. Generally speaking, continuous monitoring of water conditions and shellfish health is rare, even in the U.S. When unsafe levels of toxins or other unhealthful conditions are suspected, they first must be studied and then, if possible, ameliorated. This process can take several years, during which time consumers often are not aware of any problem.
Shellfish aquaculture can introduce risk
Aquaculture supplies approximately 40% of the world's fish food, including at least 30% 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, and farmers use antibiotics and pesticides to deal with this problem. Top non-U.S. producers normally use chemicals banned in the U.S. For example, chloramphenicol, a potent antibiotic banned in the U.S. in 1986 because of associated adverse health effects in humans, is widely used in parts of Asia. Shrimp farm standards imposed by the Aquaculture Certification Council, a nonprofit, nongovernmental body established to certify safety standards at aquaculture facilities around the world, provides an interesting list of banned practices, which, by implication, are commonly used overseas. 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.
Shellfish contamination is common
When researching consumption of shellfish as it relates to cancer risk, we found few population studies that assessed this risk, but many studies describing reports of shellfish contamination. The following summary list (with format: location - shellfish affected - contaminant) represents a small sample: St. Lawrence maritime estuary, Canada - mussels and clams - cadmium; Central California coast - rock crabs - paralytic shellfish toxins from the dinoflagellate Alexandrium catenella; 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; South Norway - brown crabs and mussels - diarrhetic shellfish poisoning from a bloom of the dinoflagellate, Dinophysis acuta.
For those who want to keep eating shellfish: Of the possible contaminants, heavy metals such as cadmium appear to be most directly linked to increased risk of breast cancer, followed by some pesticides. Note that cooking is not effective in removing heavy metal from shellfish. Avoid shellfish from Louisiana, which is known to have high levels of heavy metal contamination (long before the 2010 Deepwater Horizon oil spill). Shellfish caught in the open ocean are not exempt from contamination; excess levels of heavy metals have been found in king crabs from the Barents Sea and waters near Australia and South America. Do not buy shellfish without knowing the country of origin - in the case of shrimp, it will likely come from Mexico, Ecuador, Brazil, China, India, Thailand, Vietnam, or Bangladesh, none of which are safe sources. Know your supplier and avoid inexpensive restaurant shrimp.
In August 2008, the FDA advised retail and foodservice operations to be aware that raw oysters shipped in containers bearing a "For Cooking Only" label may have a greater likelihood of containing harmful levels of the bacterium Vibrio parahaemolyticus (which may cause illness). "For Cooking Only" labeling had been worked out as a compromise to allow the continued sale of oysters when harvesting conditions do not meet specific criteria for reduced risk of Vibrio parahaemolyticus contamination. Do not buy oysters with this label.
Dried shrimp and salted shrimp paste (concentrated finely ground fermented shrimp in sea salt) imported from Asia sometimes incorporate concentrated levels of pesticides.
Individuals with hepatitis C and others with impaired immunity (e.g., those undergoing chemotherapy) should not eat raw shellfish.
Below are links to recent studies concerning this food. For a more complete list, including less recent studies, please click on shellfish.