(Published January 2010)
This chapter focuses on methylmercury, an environmental contaminant that has documented adverse effects on fetal development.
The Reproductive Impact of Methylmercury
Methylmercury is considered a developmental toxicant that is found primarily in predatory marine and freshwater fish. However, there are many health benefits from consumption of fish and seafood, which can make providing guidance to patients on fish and seafood consumption complicated. The National Academy of Sciences, in its 2000 review, supports continued fish intake.1 The report states, “Because of the beneficial effects of fish consumption, the long-term goal needs to be a reduction in the concentrations of methylmercury in fish rather than a replacement of fish in the diet by other foods. In the interim, the best method of maintaining fish consumption and minimizing mercury exposure is the consumption of fish known to have lower methylmercury concentrations.”
Sources of Methylmercury
The most common source of methylmercury exposure in the United States is seafood that has become contaminated with the heavy metal.2 There are both geophysical and human causes of environmental mercury contamination, in particular, pollution from coal-fired power plants.3 Airborne mercury from these power plants and other sources falls to the earth and accumulates in streams, lakes, oceans, and wetlands. Inorganic mercury is converted to organic methylmercury by bacteria in aquatic sediments. Methylmercury is a particularly toxic form of the chemical that bioaccumulates, or collects in greater concentration than in the surrounding environment, as smaller fish are consumed by larger fish that are consumed by even larger fish in the food chain.4 In general, methylmercury concentrations are highest among large predatory marine fish that have lived longer, because of the greater accumulation of methylmercury in their bodies compared with younger, smaller fish.4 However, some small predatory freshwater fish can be highly contaminated with methylmercury as well.
As shown in Figure 8, which is based on National Health and Nutrition Examination Survey (NHANES) data collected from 1999 to 2004, there is a significant positive correlation between reported intake of seafood and blood mercury level.2
For example, children born to women living in the Faroe Islands who consumed a heavy diet of contaminated seafood during pregnancy were found to have lower scores in IQ, language development, visual–spatial skills, gross motor skills, memory, and attention.5 When those children reduced their consumption of the heavily contaminated seafood, several of the observed neurological deficits improved.
Seafood intake—and blood mercury levels—vary across the United States. Figure 9 demonstrates that both intake and blood mercury levels are highest in the Northeast region of the country.2 A study based on NHANES data found that Asian ethnicity and higher income also were associated with greater seafood intake and higher blood mercury levels. A 2008 New York Times journalist reported that sushi purchased in Manhattan was found to have high levels of methylmercury.6 Sushi obtained at five of the 20 restaurants tested had mercury levels high enough to meet criteria for the Food and Drug Administration (FDA) to take legal action to remove the products from the market. This example illustrates how important it is for providers to consider the risk of methylmercury exposure in all women.
|Minamata Disease, which was first identified in Minamata, Japan, in 1956, demonstrated a direct link between high-dose mercury exposure and severe neurological symptoms. Methylmercury discharged from a chemical factory into the Yatsushiro Sea contaminated fish and shellfish in the local area.7 Local residents and fishermen began exhibiting symptoms that suggested mercury poisoning, such as paresthesias, blurred vision, concentric vision, deafness, dyskinesia, seizures, coma, and in some cases, death.8 Deficits in neurologic development were seen in children whose mothers were exposed when pregnant. Urine tests revealed high levels of methylmercury in affected individuals, and testing of wastewater from the factory showed methylmercury contamination. The company—Chisso Corporation—only ceased polluting in 1968, when the method of mercury production previously used became outdated. In 1969, the company was forced into court. Later, researchers determined that the company had consistently released methylmercury into the bay from 1932 until 1968, despite the growing evidence of adverse effects.9 The Minamata tragedy led scientists to explore the possibility that adverse health effects would be seen at far lower exposures than those experienced in Minamata.|
Recommendations for Fish Intake
|Table 4: Recommendations for Seafood Species10|
|Species to Avoid||Safer Species|
Recommendations for fish intake must balance two factors: the nutritional benefits of seafood and the risks associated with methylmercury exposure. The FDA advises that children and pregnant women avoid eating shark, swordfish, king mackerel, and tilefish, because they are large predatory marine fish that are excessively contaminated with methylmercury.10 In some locations, freshwater fish also are highly contaminated. Safer choices are trout, shrimp, salmon, tilapia, and sardines (see Table 4). The FDA also recommends that pregnant women and children eat no more than 12 ounces per week of these fish.10 The Natural Resources Defense Council, an environmental action group with a strong scientific foundation, advises that in addition to the guidelines on species selection and servings per week, pregnant women and children eat no more than two cans of light tuna per week, or two-thirds of a can per week of white albacore tuna.11 The Environmental Working Group, a consumer advocacy group, recommends that pregnant women choose fish species carefully, and it uses scientific guidelines from the FDA—not the FDA consumption advisory—to calculate the maximum amount of tuna that can be consumed safely.12 The group proposes that the FDA consumption advisory could expose women to unsafe levels of mercury, if their only intake of seafood is tuna.
Counseling Patients on Fish Consumption
In many states, freshwater fish have extremely high levels of methylmercury. Clinicians should be familiar with the situation in their region to better advise all women of reproductive age—whether pregnant or not—about safe fish consumption. They can access information about the safety of fish in local waters through fish advisories from the Environmental Protection Agency and state health departments.
Providers can recommend the following points about safe fish consumption:
Additional Provider Resources on Fish Consumption:
Additional Patient Resources on Fish Consumption:
Case Study: Lori
Lori is a 32-year-old woman who is 30 weeks pregnant. She has two small children at home and is a school teacher. At her prenatal visit, she asks you about an article she read recently in a women’s magazine. The article stated that children of mothers who ate fish during pregnancy had higher IQ scores than children of mothers who avoided fish. Lori is confused. She has avoided all fish since she learned she was pregnant because of concern about mercury contamination. What do you tell her?
You could begin by telling Lori that scientific reports have continued to show the value of omega 3 fatty acids, which are abundant in seafood. These fats appear to be especially important to healthy neurological development. For this reason, Lori may want to add seafood back into her diet, although with caution. She also could obtain these nutrients by taking distilled fish oil capsules. Other sources of omega 3 fatty acids include many green vegetables, canola oil, walnuts, flaxseed, and flaxseed oil. In addition, wild Alaskan salmon is a good source of omega 3 fatty acids and low in methylmercury contamination.
You might ask Lori to describe her fish intake prior to pregnancy. Did she eat canned tuna? If so, what kind? Did she eat sushi? Swordfish? Did she eat fish locally caught for sport? How many servings of seafood did she eat each week? The answers to these questions can help you guide Lori in making safer choices about seafood species and amounts. Finally, give Lori resources, like those provided in this monograph, to help her plan and monitor her seafood intake and that of her children.
The next chapter covers exposures to synthetic chemicals in the workplace.
- Available at: http://nap.edu/openbook.php?record_id=9899&page=R1. Accessed December 3, 2009.
- Mahaffey KR, Clickner RP, Jeffries RA. Adult women’s blood mercury concentrations vary regionally in the United States: association with patterns of fish consumption (NHANES 1999-2004). Environ Health Perspect. 2009;117:47–53.
- Chen CY, Serrell N, Evers DC, et al. Meeting report: methylmercury in marine ecosystems—from sources to seafood consumers. Environ Health Perspect. 2008;116:1706–12.
- Environmental Protection Agency. What you need to know about mercury in fish and shellfish. March 2004. Available at: http://www.epa.gov/waterscience/fish/advice/. Accessed August 31, 2009.
- Grandjean P, Weihe P, White RF, et al. Cognitive deficit in 7-year-old children with prenatal exposure to methylmercury. Neurotoxicol Teratol. 1997;19:417–28.
- Burros M. High Mercury Levels Are Found in Tuna Sushi. New York Times. January 23, 2008. Available at: http://www.nytimes.com/2008/01/23/dining/23sushi.html. Accessed November 4, 2009.
- National Institute for Minamata Disease. Minamata Disease Archives. Available at: http://www.nimd.go.jp/archives/english/tenji/e_corner/etop.html. Accessed December 11, 2009.
- Bolger PM, Schwetz BA. Mercury and health. N Engl J Med. 2002;347:1735–6.
- The Trade & Environment Database. American University. Case study: Minimata disaster. 1997. Available at: http://www1.american.edu/TED/MINAMATA.HTM. Accessed December 11, 2009.
- Food and Drug Administration. What you need to know about mercury in fish and shellfish: advice for women who might become pregnant, women who are pregnant, nursing mothers, and young children. March 2004. Available at: http://www.fda.gov/Food/ResourcesForYou/Consumers/ucm110591.htm. Accessed August 31, 2009.
- Natural Resources Defense Council. Mercury contamination in fish: protect yourself and your family. 2008. Available at: http://www.nrdc.org/health/effects/mercury/protect.asp. Accessed July 27, 2009.
- Environmental Working Group. EWG tuna calculator. 2009. Available at: http://www.ewg.org/tunacalculator. Accessed December 3, 2009.
- Environmental Protection Agency. Polychlorinated biphenyls (PCB) update: impact on fish advisories. September 1999. Available at: http://www.epa.gov/waterscience/fish/files/pcbs.pdf. Accessed August 31, 2009.