Good for your heart but bad for your baby?

Headlines such as “Mercury warning for children, pregnant women” and “Danger of too much fish” appeared in March throughout Australian newspapers. The media blitz was triggered by the release of revised advice from Food Standards Australia New Zealand (FSANZ) on health risks associated with consuming fish with high methylmercury (MeHg) content (Box).1 The warnings come after a Food and Agriculture Organisation of the United Nations/World Health Organization Expert Committee halved the “provisional tolerable weekly intake” of MeHg in pregnancy from 3.3 µg to 1.6 µg per kilogram bodyweight to protect fetal development.2

Fetal neurotoxicity of MeHg was discovered in the 1960s in Japan. It was named “fetal Minamata disease” after 25 cases of cerebral palsy were found in newborns whose mothers had high levels of MeHg exposure from eating fish contaminated by industrial pollution,3 while the expected number of cases in that population was less than one. Subsequent cohort studies following children from birth to 14 years in New Zealand4 and in the Faeroe Islands5 reported associations between maternal MeHg exposure from fish consumed during pregnancy and deficits in psychological performance or in neurophysiological testing. One prospective study in the Seychelles did not find such effects.6

The Minamata case and subsequent studies indicate that there may be a shift to the left in IQ distribution as a result of excessive MeHg exposure from fish, even at levels too low to produce overt mental retardation. However, a robust debate is continuing about the toxic level of exposure and the “safety margin” required to protect the fetus. The potential risk to children in Australia needs to be carefully considered.

The new advice from FSANZ is welcome, as some commonly consumed ocean fish (such as shark) often have natural MeHg concentrations sufficient to cause high weekly exposures. Interestingly, no recommendation was made for tuna. Although canned tuna is usually sourced from smaller, younger fish and is relatively low in mercury, some tuna (albacore, bluefin) has higher concentrations. The United States Environmental Protection Agency advises vulnerable groups against consuming any fish with high mercury content.7

Fish is well established as a “healthy” food. Evidence for cardiovascular benefits from regular fish consumption emerged in the 1990s, as low rates of cardiovascular disease were found in populations with high levels of fish consumption. A number of studies indicate that omega-3 fatty acids reduce cardiovascular risk by improving lipid profiles, inhibiting atherosclerotic plaque, improving arrhythmia, improving vascular function, and reducing damage from ischaemia.8 Curiously, one study reported that high levels of MeHg exposure from fish increased the incidence of myocardial infarction.9

We are therefore faced with the difficult public health challenge of avoiding the health risks from MeHg intake in fish in vulnerable groups while taking advantage of the health benefits of fish consumption. The National Heart Foundation recommends fish be consumed at least twice a week, consistent with advice from FSANZ for most kinds of fish, but this is two to four times the latest recommendations for consumption of fish containing high levels of mercury. While some species of fish have high levels of MeHg, others, such as salmon and hake, have relatively low levels. Expecting consumers to change their understanding that “fish is good” to “some fish are good, sometimes”, and “some fish are not so good, sometimes” introduces a level of complexity into consumer health education that has rarely been seen. Parallels might be drawn with fats and oils, with important shifts in understandings from “all fats are bad” to “some fats are good”, or with alcohol consumption, where some patterns of moderate drinking might be more beneficial to health than abstinence.10

Patterns of fish consumption are highly variable, so ensuring advice reaches those most at risk is essential. For example, shark is frequently unintentional “by-catch”, often used in cheaper meals such as fish and chips and fishcakes, which are consumed fairly regularly by some groups. People more likely to rely on these products may also be less aware of, and less able to respond to, the health advice from FSANZ. While some relatively expensive fish are also high in mercury (swordfish, orange roughy), these are perhaps less likely to form a regular part of the diet.

Mercury has a half-life of about 9 weeks, so that women who stop all consumption of fish on becoming pregnant may still be exposing their fetuses to high levels of mercury well into pregnancy. Therefore, we suggest that all healthcare professionals make women of child-bearing age aware of the revised FSANZ recommendations and the potential risks to the developing fetus associated with even moderate consumption of some types of fish during pregnancy. Standards of fish nomenclature should also be developed and enforced to reduce confusion and to ensure consumers are getting what they expect. Further, epidemiological research on actual levels of exposure and the efficacy of the FSANZ health advice is much needed.