Healthy fish consumption and reduced mercury exposure

Health benefits of eating fish

The health benefits of eating fish are related to the presence in fish of 2 long-chain omega-3 fatty acids, docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA). Besides being obtained directly from fish, both DHA and EPA can be derived from a plant-based omega-3 fatty acid, α-linolenic acid (ALA); ALA is an essential fatty acid that cannot be synthesized by the body, so dietary sources are required. Fish in the mother’s diet is thought to provide the optimal source of DHA and EPA, as the fetus and infant appear to have limited ability to synthesize DHA from dietary ALA.3

Health benefits for pregnancy

Docosahexaenoic acid is incorporated into the brain and retinal tissues during fetal development and into the developing human brain for the first 2 years of life. Randomized controlled trials of DHA-enriched infant formula demonstrate a modest positive effect on intelligence quotient, which is more pronounced in premature infants (level I evidence).4 The relationship between maternal fish consumption during pregnancy and the child’s long-term cognitive development has recently been studied in cohorts in the United States and United Kingdom.5,6 In general, beneficial effects on child cognitive development were found when mothers ate fish for more than 2 meals per week.

Prenatal dietary requirement for DHA

There has been much debate in the literature concerning the prenatal dietary requirement for DHA.7,8 A recent consensus statement confirms the importance of DHA in the maternal diet, and recommends that pregnant and lactating women consume at least 200 mg of DHA daily.9 Citing a lack of intervention studies and conflicting results from observational studies, Motherisk concluded in 2007 that until evidence accumulated, clinicians should not encourage women to take omega-3 fatty acid supplements during pregnancy.10 In recently released prenatal nutrition guidelines, Health Canada states that fish oil supplements should not be considered equivalent to eating fish, and recommends that a food-based approach should be emphasized for women who do not eat fish.11

Mercury in fish and its health effects

Fish consumption is the main source of exposure to methylmercury, which is bound to protein in the muscle of the fish.12,13 It is almost completely absorbed from the gastrointestinal tract, accumulates in the mother’s body, and is transferred across the placenta and the fetal blood-brain barrier.14 Levels in cord blood are approximately 1.7 times higher than in maternal blood,15,16 and levels in the fetal brain are higher than in the mother.17 Levels in breast milk, on the other hand, are substantially lower than in maternal blood.

At high doses, methylmercury is neurotoxic to adults. Evidence of the effects on the developing fetal brain of lower doses of methylmercury (from fish in the maternal diet or from maternal body stores) comes from both animal and human studies.17 Animal models of in utero exposure indicate dose-related deficits in sensory, motor, and cognitive neurodevelopment in exposed offspring. Evidence in humans derives mainly from 3 longitudinal cohort studies of fish-eating populations, studies that reached conflicting results (Table 1).17–23 These divergent findings have been variously explained in terms of differences in dietary composition as it relates to fish species, patterns of neurotoxicant intake, dietary intakes of “protective” omega-3 fatty acids and selenium, and possible genetic differences among the different populations studied.17,24

The Harvard Center for Risk Analysis23 estimated that prenatal mercury exposure that raises maternal hair mercury measured at birth by 1 μg/g reduces intelligence quotient by 0.7 points in the exposed child. This finding, although not clinically observable in individual patients, would have considerable societal effects at the population level.25

“Safer” levels of mercury: intake and tissue levels

There are no consistent guidelines for an “acceptable” or “tolerable” level of mercury for the developing brain.12 Health Canada has recently developed a provisional interim blood guidance value of 8 μg/L based on the existing provisional tolerable daily intake of 0.2 μg/kg daily for children, pregnant women, and women of childbearing age.26 Brodkin et al14 recommend that at blood mercury levels of 10 μg/L (50 nmol/L), the source of exposure to mercury should be assessed and reduced. The US Environmental Protection Agency (US EPA)27 set the reference dose (RfD) at 0.1 μg/kg daily, equivalent to a maternal blood mercury level of 5.8 μg/L (29 nmol/L).17 Mergler and colleagues point out, however, that in the risk assessment upon which the US EPA’s RfD is based, the US National Research Council used cord blood mercury measures and did not account for the bioconcentration of methylmercury across the placenta when determining a maternal reference blood mercury level.16 Mahaffey and colleagues suggest, therefore, that maternal blood mercury levels exceeding the range of values from 3.5 μg/L (15 nmol/L) to 5.8 μg/L (29 nmol/L) should be the reference point for preventing fetal neurotoxicity.28 A recent Motherisk systematic review of the literature identified 0.3 μg/g of hair mercury to be the lowest observable adverse effect level for adverse effects on child neurodevelopment resulting from fetal exposure.29 Motherisk advises that hair analysis of high fish-consuming populations might be warranted before pregnancy, as dietary modifications can decrease body burden of mercury.30

Mercury in fish

Mercury in the environment can come from natural sources, but anthropogenic emissions have increased environmental mercury by a factor of 2 to 4.31 Atmospheric mercury is converted by bacteria in lakes into organic methylmercury, which is taken up by microorganisms, such as plankton, in water. It is then biomagnified, being found at highest concentrations in longer-living and larger predatory fish species at the top of the aquatic food chain,31 including tuna, shark, swordfish, marlin, orange roughy, and escolar.32 Mercury levels also vary with the age and size of fish, the water body from which they originate, and even the season of capture. Species such as salmon, herring, sardines, Arctic char, Atlantic mackerel, and rainbow trout are lower in mercury and high in omega-3 fatty acids.33

Health Canada has set a regulatory limit, banning the sale of fish containing more than 0.5 ppm of total mercury, except for the predatory high-mercury fish listed above, for which a new limit of 1 ppm has recently been set.32 In a survey of fish from retail outlets in Vancouver, BC, Toronto, Ont, and Halifax, NS, mercury was found in all samples; the highest concentrations were found in swordfish, followed by shark, fresh tuna, and marlin.34 Tuna deserves special mention, as it is so widely available and it is an economical protein source (Table 2).

Exposure from eating fish: groups at higher risk

Average levels of mercury in the diet of a representative sample of Canadian adults and children were found to be low and below Health Canada guideline levels.35 The dietary intake estimates, however, were based on data from a 1977 Nutrition Canada survey, and consumption practices have likely changed over the past 3 decades. Other Canadian studies have documented higher methylmercury exposure in subgroups with higher fish consumption, such as anglers fishing in the Great Lakes in Ontario (with levels higher in Asian Canadians than in Canadians of European descent),36 Chinese-Canadian schoolchildren in Vancouver,37 and residents of the Canadian Arctic, where 3% of Inuit maternal blood and 56% of umbilical cord blood samples contained methylmercury levels above the 5.8 μg/L (29 nmol/L) reference level.38

Preliminary data from the 2007 to 2008 Canadian Health Measures Survey39 indicate that Canadian blood mercury levels might be slightly higher or about the same as those of the American general population. Data are not currently available for the upper percentiles, which would provide an estimate of blood mercury for those who consume large amounts of fish. Recent data from the US National Health and Nutrition Examination Survey40 indicated that blood mercury levels and fish consumption were higher in women living within 40 to 80 km of the Great Lakes, Asian women, and higher-income women, with more than 3 million American women (4.7%) estimated to have blood mercury levels exceeding 5.8 μg/L (29 nmol/L).28 The average mercury levels of adult women in New York City, NY, were more than 3 times those found in a national study. New Yorkers of Asian descent had higher blood mercury than did other racial or ethnic groups.41

Fish consumption advisories and advice

Fish consumption advice and advisories have been issued in many jurisdictions.32,42–44 The fish consumption advice issued by Toronto Public Health33 (summarized in a patient handout available from CFPlus*) deserves highlighting for 2 reasons. First, the risk calculations are based on the US EPA RfD,27 a more conservative measure than Health Canada’s provisional tolerable daily intake.26 We consider this caution appropriate in a country with diverse population groups, many of which consume large amounts of fish for cultural reasons.

Second, it identifies what fish species are the best choices and how often and in what quantity they can be eaten, rather than simply advising vulnerable groups to limit high-mercury fish. This is important, in light of research demonstrating that pregnant women might respond to fish advisories warning about the intake of high-mercury fish by reducing or eliminating fish intake.45

The Toronto Public Health resource33 lists more than 80 species of fish commonly found in stores or fish markets in Canada and recommends eating a variety of fish. It also lists species caught or raised in environmentally unsustainable ways and provides a list of fish species that can be safely eaten every day, which is helpful for providing guidance to those subpopulations that consume large amounts of fish.

Environment Canada provides a listing of provincial and territorial advisories46 for anglers who catch and eat their own freshwater sport fish. These fish could contain higher levels of mercury and other contaminants such as polychlorinated biphenyls.

Resources

The fish consumption resources detailed in the handout* give accessible advice on what fish can be eaten how often, the number of servings per week for each age group, and the species of fish to avoid or eat rarely. A summary in the form of a wallet card is also available from Toronto Public Health.33 Referral to a registered dietitian is recommended if there are more detailed dietary issues. Families who catch and eat their own fish should be directed to sport fishing advisories.46