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
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.
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.
- What You Need to Know About the Different Forms of Mercury, the Next Generation of Mercury Testing, and How to Detox Safely (healthimpactnews.com)
- Counting The Cost Of Mercury Pollution (eurasiareview.com)
- Extracting/Disposing of Methyl Mercury (methylmercury) from Canned Tuna (ask.metafilter.com)
- Mercury released into air contaminates ocean fish (sciencedaily.com)
- Eating Fish During Pregnancy Safe: Study (medicaldaily.com)
- Mercury in food – EFSA updates advice on risks for public health (thehumanimprint.typepad.com)
- EFSA mercury opinion (EFSA website)