Methylmercury in Fish

Research shows that fatty fish containing the long chain marine polyunsaturated fatty acids eicosapentanoic acid (EPA, C20:5 (n-3)) and docosahexanoic acid (DHA, C22:6 (n-3)) are beneficial to the health, and may reduce the risk of diabetes, cardiovascular disease and dementia. These fish oils are also required for the correct development of the central nervous system of the foetus during pregnancy. However, the recommendation to eat more fatty fish is controversial because of increasing contamination with methylmercury. Methylmercury is a highly toxic metal found in most fish, but is particularly prevalent in the high order predators such as mackerel, tuna, shark and swordfish. Fish is now the primary route of exposure for methylmercury in Western populations. The deleterious effects of methylmercury result in neurological problems, which are also one of the major effects of low intakes of EPA and DHA contained within the fish.

Mercury occurs naturally in the environment at low levels in soil, water and rock, but levels can increase with the burning of fossil fuels, medial waste and other household rubbish. This mercury accumulates in water where bacterial metabolism causes the formation of methylmercury which is subsequently absorbed by low order marine organisms. Bioaccumulation of the methylmercury occurs within the food chain until the high order consumers, the large predator marine fish, become highly contaminated. Although some methylmercury can be consumed in small amount without causing noticeable neurological symptoms, the exact safety level of consumption is controversial. Recommendations vary, but research suggests that 58 micrograms per litre of blood is the level below which no neurological problems can be detected in humans. The Environmental Protection Agency in the United States recommends less than 0.1µg per kg of bodyweight per day as safe.

However, the Federal Drug Administration (FDA) in the United States recommends that pregnant women avoid eating large predator fish such as sword fish, shark or tuna fish. Similar recommendations are in place in the United kingdom. Fish intake is also suggested to be limited in children and infants. However, despite the Environmental Protection Agency recommendation to maintain intake below 0.1µg per kg of bodyweight per day, no dietary guidelines exist for healthy non-pregnant adults. In addition, some research has shown that lower fish consumption during pregnancy may actually result in lower intelligent quotient (IQ) scores as well as lower motor, communication and social development scores in offspring. This has called into question the wisdom of limiting fish at all, even in pregnant women. The conflicting exaggeration of dangers by consumer groups, and dismissal of dangers by industrial lobbies, further obfuscates the true picture.

Studies looking at the toxicological effects of methylmercury poisoning have shown mixed results. The Nutritional Research Council (NRC) performed a literature review on existing research examining the toxicity of methylmercury, relying on 3 epidemiological papers: the Seychelles Child Development Study, ongoing research on the Faroe Islands and a study looking at children in New Zealand. The Seychelles study found no adverse effects from methylmercury consumption, whereas the data from the Faroe Islands found correlations between blood mercury concentrations at age 7 and deficits in language. Results from the New Zealand study may be unreliable due to outlying data affecting the statistics. The NRC concluded that the risk of poisoning from methylmercury in the general population was low, but higher risk was present in pregnant women, infants and young children. The National Health and Nutritional Examination Survey 1999-2000 also found that efforts to reduce methylmercury exposure in women of child bearing age were warranted.

Evidence suggests that consumption of fatty acids from fish is beneficial to the health, but that contamination from methylmercury may counteract any benefits. The consumer is therefore left with a dilemma, which is exacerbated by conflicting recommendations and propaganda from lobby groups. One solution to attain the benefits of the long chain fatty acids EPA and DHA might be to avoid the contaminated fish but consume the fish oil as a supplement. However, the methylmercury can be transferred to the supplement during processing and so care must be taken with supplement choice. Recent advances in the manufacture of fish oil supplements has allowed purification of the fatty acids by removal of toxic compounds such as methylmercury. Alternatively, DHA is now available from tank grown algae and this offers a clean sources of long chain fatty acids that is also suitable for vegetarians.


Hughner, R. S., Maher, J. K. and Childs, N. M. 2008. Review of food policy and consumer issues of mercury in fish. Journal of the American College of Nutrition. 27(2): 185-194

About Robert Barrington

Robert Barrington is a writer, nutritionist, lecturer and philosopher.
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