Mercury, fish oils and the risk of cardiovascular diseases
Mercury is a poisonous environmental pollutant whose greatest source is the emission from lithosphere. Other major sources are industry and burning of wastes and fossil fuels. Mercury exists in three forms: elemental mercury, inorganic mercury compounds and organic mercury. Mercury eventually settles in waterways where it is converted to the toxic (organic) methylmercury by bacteria and algae. This is especially pronounced in Finland where the lakes are shallow and have large catchment areas. The large variation in water levels between seasons washes the humic material and mercury from the soil surface into the lakes. The soil and lakewaters are also relatively acidic (especially in Eastern Finland), which enhances the availability of soil mercury and the drying of swamps by ditching has lead to drainage of high-mercury water from swamps into lakes. Low selenium content of Finnish soil and lake and ground waters is yet another unfavourable characteristic, which together with the above mentioned leads to the high mercury content.
Sources of mercury
Methylmercury is rapidly accumulated by aquatic biota. Since fish is at the top of the aquatic food chain, it also has the highest concentrations of methylmercury. This is especially well seen in large (and hence old) predatory fish, such as Northern pike. In Finland the mercury concentrations of Northern pike generally exceed 0.5 µg/g (w/w), which in many countries is used as the highest acceptable level for edible fish. In humans the major exposure to methylmercury occurs through food and in food the major source is fish. Dental amalgam fillings may be a source of inorganic mercury in humans.
Mercury has no known physiological role in human metabolism. At high concentrations, mercury is known to cause liver and kidney damage as well as neurological symptoms. Mercury is a transition metal and it can thus promote free radical formation by acting as a catalyst in Fenton-type reactions. Mercury also has a very high affinity to thiol groups, which may lead to the inactivation of antioxidative thiolic compounds, such as glutathione, catalase and superoxide dismutase. It can also bind selenium to form an insoluble complex that cannot serve as a cofactor for glutathione peroxidase. All of these could reduce the antioxidative capacity and promote free radical stress and lipid peroxidation. In addition, mercury can also promote platelet aggregation and stimulate proliferation of arterial smooth muscle cells. These effects have raised interest in the adverse effects of mercury on cardiovascular diseases (CVD).
The adverse effects of mercury
In the Kuopio Ischaemic Heart Disease Risk Factor Study high hair mercury content (which is a direct measure of accumulation of methylmercury in body over a period of several months) has been shown to promote progression of carotid atherosclerosis and to be a significant risk factor for acute myocardial infarction and mortality from coronary heart disease and CVD in middle-aged Eastern Finnish men. Also, in this same study population the beneficial effects of fish oil were attenuated by high mercury content in fish. Similar results were observed also in the recent EURAMIC study.
Mercury is said to be the most dangerous environmental poison of all heavy metals. The long half-life and the fact that human body has no way of excreting mercury actively mean that mercury keeps accumulating in the human body throughout life. Does this mean that we should not eat fish, contrary to the current recommendations for a healthy diet? No, but we should vary the type of fish we eat (fatty fish is usually low in mercury) and avoid the regular use of large fish from lakes with known high mercury content. Also, the scientific community should put more emphasis on the possible negative effects of transition metals and pro-oxidants on human health.
In conclusion, if you happen to catch the fish of your dreams, the best you can do is to stuff it and put it on display above the fireplace!
The biggest perch caught by Jyrki (116 0 g, 45 cm). Back then he didn't know about the possible high mercury content in these large predatory fish, so he ate it...
More information about our fish studies jyrki.virtanen at uef.fi, tomi-pekka.tuomainen at uef.fi
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