Shellfish should be avoided for breast cancer

By "shellfish", we mean edible crustaceans and bivalves, including abalone, clams, crab, crayfish (crawdads, crawfish), lobster, 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. Shellfish are a rich source of protein and they 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. Heavy metals such as cadmium found in some contaminated shellfish, have been shown to increase the risk of breast cancer.

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.

Additional comments

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.

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 - year of study - shellfish affected - contaminant) represents a small sample: St. Lawrence maritime estuary, Canada - 2009 - mussels and clams - cadmium; Central California coast - 2009 - rock crabs - paralytic shellfish toxins from the dinoflagellate Alexandrium catenella; Normandy, France - 2008 - oysters - various pesticides; Moroccan Atlantic coast - 2008 - mussels - untreated chemical wastes from industrial processing of phosphates; Bangladesh - 2008 - dried shrimp - DDT (readily available in spite of official bans) applied by fishermen and merchants to help keep dried shrimp from spoiling; Southwest Louisiana - 2007 - shrimp, oysters, crayfish, crabs - mercury, lead and cadmium from local petrochemical plants; Catalonia, Spain - 2006 - clams, mussels, and shrimp - arsenic, cadmium, mercury, and lead; Malaysian coasts and rivers - 2006 - mollusks and oysters - lead, zinc and cadmium from manufacturing, agriculture, sewage and motor vehicle emissions; South Norway - 2005 - 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. 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. Don't 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. 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. Don't 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.

Tags: Australian, Indian, Korean, Scandinavian, abalone, algae, antibiotics, cadmium, carotenoids, chemotherapy, clams, crab, omega3, processedMeat, salmon, salt, shellfish, southernEurope, soyProteinIsolate

Selected studies

Human dietary exposure to heavy metals via the consumption of greenshell mussels (Perna canaliculus Gmelin 1791) from the Bay of Islands, northern New Zealand Science of The Total Environment, July 2009

The present study was designed to investigate the exposure to heavy metals resulting from human consumption of greenshell mussels (Perna canaliculus) from the Bay Islands in northern New Zealand. Cadmium, mercury, arsenic, lead and tin concentrations were measured in soft tissues from whole greenshell mussels gathered from Urapukapuka-Rawhiti Island, Opua Marina, Waitangi Bridge and Opua Wharf. All the samples were found to have metal concentrations well below the Food Standards Australia and New Zealand maximum limits. Based on the average values detected, the concentrations of heavy metals consumed in a "typical diet" containing greenshell mussels were below the provisional tolerable weekly intake. However, since Māori, Pacific Islanders and Asians consume a far greater quantity of seafood than the general public of New Zealand, they could consume enough shellfish to exceed the provisional tolerable weekly intake for cadmium (but not for mercury, arsenic, lead or tin). In addition, individual differences in human susceptibilities to various heavy metals could increase the risk of harm for those with low tolerances. The authors conclude by suggesting that a survey of the frequency, amount and species consumed by groups whose diet may be largely shellfish-based is required to make a more comprehensive risk assessment.

Occurrence, source diagnosis, and biological effect assessment of DDT and its metabolites in various environmental compartments of the Pearl River Delta, South China: A review Environmental Pollution, June 2009

The current review summarizes and analyzes available data concerning the occurrence of dichlorodiphenyltrichloroethane (DDT) and its main metabolites, dichlorodiphenyldichloroethane (DDD) and chlorodiphenyldichloroethylene (DDE) -- collectively referred to as DDTs -- in the environment of the Pearl River Delta in South China. In general, total levels of DDTs have not declined appreciably since 1983. However, the composition of residues has altered over time. DDTs in soil, freshwater sediment and freshwater fish species were primarily residues from chronological use. However, there is evidence that new point sources (such as dicofol (a miticide) and anti-fouling paint) may have contributed DDTs to various environmental compartments since 1983. Risk assessments referencing existing criteria show that current levels of DDTs in water and some fish species may pose adverse health effects to human beings or wildlife, and DDTs in the soil and sediment may also harm the eco-environment of the Pearl River Delta. The occurrence, possible input sources, and biological effects of DDTs in the Pearl River Delta are reviewed.

Frequency and Type of Seafood Consumed Influence Plasma (n-3) Fatty Acid Concentrations Journal of Nutrition, December 2008

The present study was designed to evaluate the relationship between different seafood meals and marine omega-3 fatty acid (eicosapentaenoic acid and docosahexaenoic acid) biomarkers. The study cohort, the Multi-Ethnic Study of Atherosclerosis (MESA), included white, Chinese-American, black, and Hispanic participants. Dietary intake from a food frequency questionnaire and plasma phospholipid fatty acids were determined for 900 MESA participants who were not taking any fish oil supplements. Simultaneously adjusting for all seafood groups, concentrations of eicosapentaenoic acid and docosahexaenoic acid in plasma phospholipids were found to be positively associated with nonfried fish consumption in all four ethnic groups (r = 0.24–0.46; P < 0.01) but not with fried shellfish, fried fish, or fish in mixed dishes. This correlation was reduced by up to 67% when type of seafood was not taken into account. After further adjusting for demographic characteristics and other dietary characteristics, the association of plasma marine omega-3 fatty acid levels with nonfried fish consumption remained significant (P-trend < 0.001). The data were suggestive of a plateau effect at a nonfried fish intake of approximately two times per week. The association of nonfried fish consumption was not modified by intake of (n-6) polyunsaturated fatty acids or α-linolenic acid. The authors conclude that this study highlights the importance of cooking methods (i.e., nonfried versus fried fish), types of seafood (fish versus shellfish), and overall seafood consumption when assessing the health effects of long-chain (n-3) fatty acids of seafood consumption.

Cadmium induces mitogenic signaling in breast cancer cell by an ERalpha-dependent mechanism Molecular and Cellular Endocrinology, January 2007

The present study was designed to evaluate whether cadmium, a toxic heavy metal pollutant, regulates MCF-7 breast cancer cell proliferation by activating ERK1/2, Akt and PDGFRα kinases. Breast cancer is known to be linked to estrogen exposure and hormone-related cancers have been linked to estrogenic environmental contaminants. Estradiol (E2) stimulates breast cancer cell proliferation by binding the estrogen receptor (ER). Cadmium was found to act as an estrogen in breast cancer cells, increasing cell proliferation, whereas the estrogen receptor-antagonist ICI 182,780 blunted the effect of cadmium. To characterize the estrogen receptor-dependent mechanism, the impact of cadmium on ERα/β expression was evaluated. Cadmium was found to reduce ERα expression, but not ERβ. Cadmium also increased ERK1/2, Akt and PDGFRα phosphorylation while ICI 182,780 blocked it. Since stimulation of phosphorylation was actually slower than expected, c-fos and c-jun proto-oncogenes, and PDGFA also were analyzed. Cadmium was found to rapidly increased c-jun, c-fos and PDGFA expression. MCF-7 breast cancer cells were also co-incubated with cadmium and specific kinases inhibitors, which were found to block the cadmium-stimulated proliferation. The authors conclude that cadmium increases breast cancer cell proliferation in vitro by enhancing Akt, ERK1/2 and PDGFRα kinases activity, in all likelihood by activating c-fos, c-jun and PDGFA by an ERα-dependent mechanism.

Dietary patterns are associated with sexual maturation in Korean children British Journal of Nutrition, April 2006

The present cross-sectional South Korean study was designed to investigate the association between dietary patterns and sexual maturation among children in Seoul. The study included 422 boys and 365 girls aged nine to 12 years old. Food records covering three days were obtained, pubertal stages were determined using Tanner stages, and body fat and bone mass were measured. Four distinct dietary patterns using twenty-four food groups were developed: (1) rice and Kimchi; (2) shellfish and processed meat; (3) pizza and drinks; and (4) milk and cereal. Twenty-six percent of the boys were found to have had genital development, and 79% of the girls were found to have breast development. Genital development was found to be weakly positively associated with "shellfish and processed meat" dietary factor scores (odds ratio (OR) = 1.65, CI 0.95 - 2.89, P for trend 0.07) among the boys after adjusting for confounders. On the other hand, breast development was found to be significantly positively associated with shellfish and processed meat consumption among the girls (OR = 1.88, CI 1.08 - 3.26, P for trend 0.05). The authors conclude that dietary patterns are related to body composition and sexual maturation among the Korean children.

Certification Issues for Some Common Aquaculture Species Reviews in Fisheries Sciences, October 2005

The present study identifies issues to be considered in developing certification standards for catfish, tilapias, rainbow trout, oysters, mussels, clams, scallops, abalone, and seaweed aquaculture. Interest in certification of aquaculture facilities has grown in response to concerns about environmental, social, and food safety issues. Common concerns include land and water use, water pollution, effects on other bottom dwelling species, effects on biodiversity, use of antibiotics and other chemicals, and relationships with workers and surrounding communities. Fish meal use in feeds is likely to be a contentious concern in finfish certification. Use of groundwater, removal and disposal of dead fish, and medicated feed should be given special attention in catfish farming. Raising tilapias (an African fish) can introduce these species into watersheds, resulting in serious competition with native species. Environmentalists and consumers have been known to object to hormone treatment of tilapia to produce all male fry. Widespread use of antibiotics in trout culture will be a major concern. Shellfish certification should focus on water use conflicts and public health risks associated with shellfish consumption. The introduction of non-native oyster species and genetically modified oysters also will need to be addressed. Introduction of non-native species should also be a major focus of discussions of abalone and seaweed culture. The sources of wild seaweed to use as food in abalone aquaculture likely also will be an issue.

Selective disruption of the E-cadherin–catenin system by an algal toxin British Journal of Cancer, March 2004

The present study was designed to investigate the effect of yessotoxins on MCF-7 breast cancer cells. Yessotoxins are algal toxins that can accumulate in edible mollusks. Yessotoxin treatment of MCF-7 cells was found to cause the accumulation of a 100 kDa fragment of E-cadherin, which the authors named ECRA100. A collapse of the E-cadherin system was detected after two to five days of yessotoxin treatment. An analysis of the structure of ECRA100 revealed that it consists of an E-cadherin fragment lacking the intracellular domain of the protein. However, ECRA100 was not released into the culture media of the yessotoxin-treated cells. Accumulation of ECRA100 was also observed in other epithelial cells, such as human Caco-2 intestine and MDCK dog kidney cancer cells after treatment with yessotoxin. In turn, it was found that yessotoxin could not induce accumulation of fragments of other members of the cadherin family, such as N-cadherin in the PC12 pheochromocytoma tumor cell line and K-cadherin in sensitive cells (MCF-7, Caco-2, MDCK). The authors conclude that the results show that yessotoxin causes the selective disruption of the E-cadherin–catenin system in epithelial cells, and that this raises some concern about the potential for an algal toxin found in seafood to disrupt the tumor suppressive functions of E-cadherin.

Consumption of Salted Fish and Other Risk Factors for Nasopharyngeal Carcinoma (NPC) in Tianjin, a Low-Risk Region for NPC in the People's Republic of China Journal of the National Cancer Institute, February 1990

The current case-control study was designed to assess the association between diet and lifestyle factors and nasopharyngeal cancer in Tianjin City, People's Republic of China. The study participants included 100 patients with histologically confirmed nasopharyngeal carcinoma and 300 neighborhood controls who were individually matched to the patients with respect to age, sex, and race (i.e., Han). The dietary habits, alcohol and tobacco use, occupational exposures, and medical histories were collected from study participants. Exposure to salted fish was found to be significantly associated with an increased risk of nasopharyngeal cancer. Four characteristics of salted fish consumption independently contributed to the increased risk: (1) earlier age at first exposure; (2) increasing duration of consumption; (3) increasing frequency of consumption; and (4) cooking the fish by steaming it rather than grilling, frying, or boiling it. Significant associations were also observed for consumption of salted shrimp paste during childhood (increased risk) and consumption of carrots (reduced risk). The three dietary effects (consumption of salted fish, salted shrimp paste, and carrots) were found to be independent of each other. None of the nondietary factors studied were found to be significantly associated with nasopharyngeal cancer risk.