Maternity Care Calendar and Guidelines: mercury in fish

Literature Search:

Medline was searched using the MeSH *Mercury/ or mercury in fish.mp. or *Methylmercury Compounds/ or *Mercury Poisoning/ AND pregnancy.mp. or PREGNANCY/ limited to English and human from 1998-2005. This search yielded 132 articles from which 51 articles were selected and reviewed. An earlier search using the MeSH *Food Contamination/ or *Fishes/ or *Methylmercury Compounds/ or *Mercury/ or Mercury Poisoning/ or *Water Pollutants, Chemical/ or Fish Products/ AND *PREGNANCY (MeSH) limited to English and human from 1990-2003. This search yielded 33 articles. Clinical practice guidelines and review articles were selected. The Canadian Task Force on Preventive Health Care, the US Preventive Services Task Force, the CMA Infobase of Clinical Practice Guidelines and the US National Guideline Clearinghouse were also reviewed for guidelines on fish consumption and pregnancy. The websites for national associations of obstetricians and gynecologists, pediatricians and family physicians (ACOG, SOGC, RCOG, AAFP, CFP, CPS, AAP) were also checked for clinical practice guidelines. Google was searched using the terms mercury in pregnancy for consumer messages.

Summary of Evidence:

Mercury in the environment occurs both naturally and as a result of pollution. Organic methylmercury binds to proteins in fish tissue and is particularly concentrated in fish that are long-lived or higher up in the food chain.[1-3] (http://www.fda.gov/fdac/reprints/mercury.html) For most people food, especially fish, is the principal source of exposure to mercury. Nearly all fish contain trace amounts of methylmercury usually ranging from 0.01 ppm to 0.5 ppm. [2, 3] However in areas with industrial mercury pollution the levels can be much higher. As well, certain large predator fish including shark (0.96 ppm), swordfish (1.00 ppm), tilefish (white snapper) (1.45 ppm), king mackerel (0.73 ppm) and large tuna (usually sold as steaks or sushi) (0.32 ppm) have been found to have much higher levels. The methylmercury levels in various seafood species can be found at the following FDA website: http://www.cfsan.fda.gov/~frf/sea-mehg.html

The WHO adopted 5ppm (measured in the mother's hair) as the international standard for upper tolerable level of mercury. [4] The WHO guidelines for mercury intake are 0.47 痢/kg daily. [4] The Environmental Protection Agency in the US set the dose much lower at 0.1 痢/kg per day. [4]Health Canada set the limit for mercury at 0.5 ppm in commercial fish.[1] The limit for methylmercury in fish in the United States is 1.0 ppm. Current estimates are that 6- 8% of pregnant women have levels of mercury exposure exceeding the most conservative safe limits.[5, 6]

Two studies of mercury poisoning in Japan and Iraq demonstrate the health effects of high levels of mercury exposure. [2, 5] Symptoms included parasthesias, abnormal gait and speech, constriction of visual fields, muscle weakness, fatigue, headache, irritability, deafness, tremors, coma and death. In Iraq, infants born to mothers exposed to high levels of mercury showed neurologic abnormalities including developmental delay. [2, 4] These outcomes were linked to consumption of fish that levels of > 10ppm of methylmercury (from severe environmental contamination).

Methylmercury easily crosses the placenta leading to mercury concentrations 30% higher in the fetal red blood cells than those of the mother. [2] Children exposed to increased levels of mercury in utero may have abnormal reflexes, problems with suckling and swallowing, gait and speech abnormalities and mental retardation. [1]

In the 1980s, 2 well-designed cohort studies were conducted examining the impact of prenatal mercury exposure on neurodevelopment in young children in different populations with conflicting results. In one study, done in the Seychelles, only 1 of 46 primary endpoints showed a positive adverse association with prenatal mercury exposure. [7, 8] The other cohort, conducted in the Faeroe Islands, examined a population with increased mercury exposure due to maternal consumption of pilot whale meat. [9] These children exhibited mercury-related neuropsychological dysfunctions most pronounced in the domains of language, attention, and memory, and to a lesser extent in visual-spatial and motor functions. One possible explanation for the difference in the findings is that the population in the Seychelles had a nearly daily exposure to fish, while the Faeroe Island population consumed pilot whales episodically. A daily low dose exposure may result in different effects than an intermittent higher dose. [3]

Recommendations of Others:

The FDA recommends that pregnant women and women who may become pregnant, as well as breastfeeding women and young children avoid consuming tilefish, shark, swordfish and king mackerel.( http://www.cfsan.fda.gov/~lrd/tphgfish.html ) [10] FDA recommendations allow for consumption of other fish (including canned fish and shellfish) up to an average 12 oz per week of cooked fish.

The FDA has published a consumer advisory available at:
http://www.cfsan.fda.gov/~dms/admehg.html

The Washington State Department of Health has more stringent recommendations:
http://www.doh.wa.gov/fish/FishAdvMercury.htm As well, as adopting the FDA recommendations, they also suggest that women of childbearing years and children avoid tuna steaks. They suggest limits to the intake of canned tuna based on bodyweight -women of average weight should limit consumption to about one can per week.

Health Canada recommends that women of childbearing age and young children limit their intake of swordfish, shark, fresh or frozen tuna to 1 meal per month. The restriction does not apply to canned tuna. The recommendations are outlined in a Canadian Food Inspection Agency handout available at http://www.inspection.gc.ca/english/corpaffr/foodfacts/mercurye.pdf

Food Standards Australia and New Zealand posts the following information:
http://www.foodstandards.gov.au/mediareleasespublications/factsheets/
factsheets2004/mecuryinfishupdatedi2819.cfm They recommend that pregnant women, women planning pregnancy and young children should limit their intake of shark (flake), broadbill, marlin and swordfish to no more than one serve per fortnight with no other fish consumed during that fortnight. For orange roughy (also sold as sea perch) and catfish, the advice is to consume no more than one serve per week, with no other fish being consumed during that week.

Another useful consumer handout is available at:
http://www.obfocus.com/nutrition/Seafood.htm

Canadian Task Force on Preventive Health Care (CTFPHC):
website searched using mercury and fish- no guideline available

U.S. Preventive Services Task Force (USPSTF):
website searched using mercury and fish- no guideline available

American College of Obstetricians and Gynecologists (ACOG):
no guideline available

Society of Obstetricians and Gynaecologists of Canada (SOGC):
no guideline available

Institute for Clinical Systems Improvement (ICSI):
Site searched using the term "mercury" -no guidelines found

Cochrane Collaboration:
No systematic review available

Motherisk:
In a recent Motherisk bulletin, the authors concede that a great deal of uncertainty still exists regarding the potential risks and benefits of seafood consumption.[4] They recommend that pregnant women, women of childbearing age and children under 15 eat no more than 4 meals per month of fish designated safe by provincial regulatory bodies. Fresh shark, swordfish or tuna should be limited to 1 meal per month. They state that eating canned tuna is allowed because mercury levels are much lower than guideline level.

Conclusions:
The new guideline on the MCC will be as follows:

avoid eating shark, swordfish, king mackerel, tilefish, tuna steaks due to high levels of mercury (other fish, including canned tuna can be eaten in moderation-about 1-2 meals per week)

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The guideline will be located under issues for discussion ideally at the preconception visit or alternatively in early pregnancy

Reviewer:
Colleen Kirkham, MD, CCFP, FCFP - April 2003
Updated - March 2005

3/30/2005 4:38:00 PM

1. Wooltorton, E., Facts on mercury and fish consumption. Cmaj., 2002. 167(8): p. 897.
2. Mercury in fish: cause for concern? FDA Consumer., 1994 http://www.fda.gov/fdac/reprints/mercury.html.
3. Davidson, P.W., G.J. Myers, and B. Weiss, Mercury exposure and child development outcomes. Pediatrics, 2004. 113(4 Suppl): p. 1023-9.
4. Yagev, Y. and G. Koren, Eating fish during pregnancy. Risk of exposure to toxic levels of methylmercury. Canadian Family Physician., 2002. 48: p. 1619-21.
5. Bolger, P.M. and B.A. Schwetz, Mercury and health.[comment]. New England Journal of Medicine., 2002. 347(22): p. 1735-6.
6. Centers for Disease, C. and Prevention, Blood mercury levels in young children and childbearing-aged women--United States, 1999-2002. MMWR. Morbidity & Mortality Weekly Report, 1018. 53(43): p. 1018-20.
7. Davidson, P.W., et al., Effects of prenatal and postnatal methylmercury exposure from fish consumption on neurodevelopment: outcomes at 66 months of age in the Seychelles Child Development Study.[comment]. Jama., 1998. 280(8): p. 701-7.
8. Myers, G.J., et al., Prenatal methylmercury exposure from ocean fish consumption in the Seychelles child development study. Lancet, 2003. 361(9370): p. 1686-92.
9. Grandjean, P., et al., Cognitive deficit in 7-year-old children with prenatal exposure to methylmercury. Neurotoxicology & Teratology., 1997. 19(6): p. 417-28.
10. Formanek, R., Jr., Highlights of FDA food safety efforts: fruit juice, mercury in fish. FDA Consumer., 2001. 35(2): p. 15-7.