Mercury in fish a concern

Mercury is a food safety issue for fish in particular (Photo: Gordon).

Methylmercury a hazard in fish (Photo: Gordon).

Just in time for Christmas 2012, the European Food Safety Authority (EFSA) published a new risk assessment of organic and inorganic mercury in food. Fortunately, the traditional Christmas dishes of ham and turkey got the all clear, while fish and other seafood were again highlighted as major culprits. Nothing revolutionary new in this since methylmercury contamination of fish and other seafood has been of concern for a long time. But still an eyeopener to watch your fish consumption. As a matter of fact dietary advice on recommended limits for consumption of certain fish species has been around for a long time.

The culprit – methylmercury

But first a little background. There are both natural and man-made sources of mercury that contaminate the environment and gradually find their way to food. Human activities include coal burning, gold mining and other industrial activities like chlorine and caustic soda production that all contribute to pollution. Old batteries and laboratory waste are other sources. Once in the environment, mercury cycles between air, water and land. It is transformed to organic mercury in the marine environment by the action of anaerobic bacteria that live in lakes, rivers, wetlands, sediments, and the open ocean resulting in the formation of methylmercury. And in here lies the real problem.

Methylmercury is particularly toxic to the developing nervous system including the brain. High levels can result in loss of IQ points, and decreased performance in tests of language skills, memory function and attention deficits. Methylmercury exposure in adults has also been linked to increased risk of cardiovascular disease including heart attack. Some evidence also suggests that methylmercury can cause autoimmune effects in sensitive individuals. Although average exposure to methylmercury in food is unlikely to exceed guidance values, EFSA pointed out that the likelihood of reaching such a level increases for high and frequent fish consumers. This group may include pregnant women, resulting in exposure of the fetus at a critical period in brain development.

Accumulation of methylmercury in the aquatic food chain

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Mercury accumulates in the aquatic chain

Unfortunately, methylmercury accumulates in the aquatic food chain. It moves from bacteria, to plankton, through macro-invertebrates, to herbivorous fish and finally to predatory fish. At each step in the food chain, the concentration of methylmercury increases and in top level aquatic predators it can reach a level a million times higher than the level in water. However, the concentration depends on the species of fish, the age and size of the fish and the type of environment in which it is found. The EFSA opinion found that in particular tuna, swordfish, cod, whiting and pike were major contributors to methylmercury dietary exposure in the adult age groups, either because of high mercury levels or because they are commonly consumed. The same species, with the addition of hake, were the most important contributors in the child age groups. Regular consumption of such fish, including shark in some countries, can put consumers at risk.

In reviewing safe consumption, the beneficial effects on brain development related to long chain omega 3 fatty acids present in fish must be balanced against the negative effects of mercury. EFSA pointed out that the beneficial effects may have previously masked potential adverse effects of methylmercury in fish. The opinion is thus advocating a lowering of safe intake levels by about 20%. Previous fish consumption recommendations might as a consequence need to be slightly lowered for consumers to be on the safe side.

Inorganic mercury is less of a threat

EFSA also looked at exposure to inorganic mercury from food. The critical target for toxicity of inorganic mercury is the kidney. Other targets include the liver, nervous system, immune system, reproductive and developmental systems. However, it is much less of a problem in that it is considered to be less toxic than methylmercury.

Amalgam filling contribute to mercury exposure (Photo: abaveta)

Amalgam fillings contribute to mercury exposure (Photo: abaveta)

Inorganic mercury is also common in fish and other seafood, but is more wide-spread across the food chain at lower levels. Estimated exposure to inorganic mercury from the diet alone is well within given guidelines. However, here we unfortunately have another issue. Inhaled elemental mercury vapour from dental amalgam, which after absorption is converted to inorganic mercury, is an additional source that will increase exposure.

Amalgam used for dental fillings is an alloy of various metals of which half normally consists of mercury.  Concerns about the toxicity of mercury have made its use increasingly controversial with a worldwide plan to phase out its use in dental fillings. It might be the largest source of mercury exposure for most people in developed countries.

What to do?

So the take home message: eat large predatory fish in moderation (particularly pertinent for pregnant women) while smaller fish can be freely consumed. It might not hurt to ask your dentist to use alternative composite materials for your next filling instead of amalgam.

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4 thoughts on “Mercury in fish a concern

  1. Another nice article Stefan.

    I’ve done a quick analysis of the levels of organic mercury in the blood samples of the American NHANES data from 2005 to 2010 (http://www.dazult.com/article/diet_hg_relationships/). Using the dietary recall and food frequency questionnaires to estimate each individual’s dietary preferences, the average levels of mercury in blood depends on diet type in a fairly predictable manner:

    vegan < vegetarian < non-vegetarian (with no fish in previous month) < fish consumers

    While the average blood levels of mercury for all diet types is still well below the US EPA guideline of 5.8 µg/L, there is some concern for about 5% of fish consumers who just consume too much (as well as some vegetarians?!).

  2. Thanks Paul – I am not an avid fish eater but I do worry about people with odd preferences. There have been Asian toddlers fed on fish porridge made of shark (a real predator) resulting in very high mercury values. And an affluent pregnant female eating swordfish four times a day misinterpreting the advice of her doctor. Sky high mercury values. But what about the vegetarians? Could that be from some other sea organism – sea weed?

  3. There are no signs of seaweed for those effected in the two days of recorded consumption. I suppose I should say “those observed to have a broadly vegetarian diet” rather than vegetarian, but at around 3% of the combined surveys (396 out of 13637 (adults only)), this is consistent with data on the number of vegetarians in the US published by the vegetarian society. 8 subjects out of the 396 “vegetarians”, were above the reference level, compared with just 13 of the 3406 non-fish consuming non-vegetarians, so there’s a significant difference. I haven’t looked at any of the other variables, but I not sure which ones should matter.

    Nearly 10% of people who consumed fish on the day before the blood sample was taken exceed the reference level, but I haven’t researched enough material on the effect the closeness of the test to the consumption plays in interpreting the results. Around 4% of fish consumers who didn’t eat fish that day beforehand exceed the level, compared to just 0.4% of non-fish consumers. I love salmon and cod, but have no taste for tuna, one of the big offenders, so hopefully the benefits outweigh the negatives. I’ve probably enough other consumption habits doing me more harm anyway!

    Mercury levels in fish from the Med tend to be higher than those from US waters, so it’s a pity no detailed surveys are publicly available in Europe.

  4. It could also be a supplement. It is possible vegetarians complement their diet by taking some supplements. Heavy metals can be found in some of them and they could even contain fish. But it will be difficult to trace which supplement, if any, could be the culprit. Such details are rarely given in food consumption surveys as you know.

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