Mercury is well known for its toxic effects on humans, particularly in terms of cardiovascular disease and prevention of proper brain development in the fetus. The organic form, methyl mercury is widely found in the environment due to pollution. Inevitably, significant quantities of methyl mercury are swept into the world’s oceans where they are easily assimilated into the marine food chain. Typically, levels in fish flesh are in the region of 30 to 1000 ppb. (Click here for more.)

The highest methyl mercury residues will thus be found in lean tissue of the longest-lived, carnivorous marine animals, such as shark, tuna, marlin and swordfish. As such, governmental advice has suggested restricting consumption of these fish to 1 or 2 portions a week to vulnerable humans. (Click here for more.)

Pregnant women are considered particularly vulnerable, as the methyl mercury can easily cross the placenta into the fetus where it can impede neural development. Fish oil supplements contain much lower levels of mercury, indeed the levels are often below those of detection. The key reason for this is that most fish oil supplements are made from smaller fish, lower down the food chain, that contain less methyl mercury. Also, the methyl mercury binds to tend to bind to protein rather than the oil; and finally the refining process used on fish oils will reduce or remove levels completely. Examples of mercury levels in fish oils can be seen in the following FDA survey.

Oily fish also represent our best source of the omega 3 fatty acids docosahexaenoic Acid (DHA) and eicosapentaenoic Acid (EPA), which are widely acknowledged as providing a whole host of health benefits. Indeed since 2012, health claims are permitted by statute in Europe for DHA and maintenance of brain and visual health, and for EPA and DHA for the maintenance of heart health.
 
Two fairly recent, large scale cohort studies, ALSPAC (Hibbeln et al 2007) and project Viva (Oken et al 2008 a,b) conducted in the U.K. and Denmark, respectively, demonstrated that the children of mothers who had consumed the highest levels of fish during pregnancy had IQ gains of 4.2 and 3.8 points per 100 mg of DHA per day, respectively. In both cases, the methyl mercury intake of the mothers eating the most fish was also determined, and was not shown to effect the IQ.

The obvious inference from these studies is that the relative risk of lack of fish intake outweighs the relative risk of methyl mercury in terms of infant brain development. This topic has already been investigated in 2007 by the Institute of Medicine in the U.S. (Click here for more.) In 2010, the European Food Safety Authority (EFSA) reported on the risk-benefit assessment of fish consumption and exposure to methyl mercury as an example study in its risk-benefits assessment in foodstuffs report. Further studies on the subject with regard to cardiovascular disease can be found here. 

In December 2012, the EFSA Panel on Contaminants in the Food Chain (CONTAM) established revised TWI (tolerable weekly intake) for methyl mercury of 1.3 µg/kg body weight. However, the panel also stated that that “n-3 long-chain polyunsaturated fatty acids in fish may counteract negative effects from methyl mercury exposure” and hence a full risk-benefit analysis is required.
 
Therefore, it is no surprise that EFSA has just agreed to a request from the Directorate-General Fisheries of the European Commission to conduct a full risk-benefit analysis of fish, balancing nutritional gains against possible harms, particularly from mercury. (Click here and see Question 6 for more.) EFSA’s Contaminants and Nutrition panels will jointly carry out this work.

While this development is gratifying, it remains possible there is too little study data available for the panel to come to a final concrete conclusion.
 
References:
 
Hibbeln, J.R., Davis, J.M., Steer, C.P., Emmett, I., Rogers, C., Williams, C. & Golding, J. 2007.
Maternal seafood consumption in pregnancy and neurodevelopmental outcomes in childhood
(ALSPAC study): an observational cohort study. Lancet, 369:578–585.
 
Oken, E., Radesky, J.S., Wright, R.O., Bellinger, D.C., Amarasiriwardena, C.J., & Kleinman,
K.P. 2008a. Maternal fish intake during pregnancy, blood mercury levels, and child cognition at age 3
years in a US cohort. American Journal of Epidemiology, 167:1171–1181.
 
Oken, E., Østerdal, M.L., Gillman, M.W., Knudsen V.K., Halldorsson, T.I., Strøm, M. &
Bellinger, D.C. 2008b. Associations of maternal fish intake during pregnancy and breastfeeding
duration with attainment of developmental milestones in early childhood: a study from the Danish
National Birth Cohort. American Journal of Clinical Nutrition, 288:789–796.
 
Rob Winwood, @RobWinwood1, is Manager of Scientific Communications at DSM Nutritional Products LLC. He chairs the GOED omega3 scientific committee. The author gratefully acknowledges the assistance of friends and colleagues in the preparation of this blog.

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The ideas and opinions expressed in this article are those of the author and do not necessarily reflect views held by Nutraceuticals World.