Mercury (Hg) is a well known heavy metal that was historically used in thermometers, barometers and sphygmomanometers.  Unfortunately, mercury (especially methylmercury) has been linked to varying degrees of toxicity if ingested or inhaled in large amounts.1-5  More recently, the risk has been linked to the ingestion of certain types of fish and shellfish, which has led some regulatory authorities to recommend that women who are pregnant, planning to become pregnant or breastfeeding as well as young children should avoid certain types of fish and shellfish.6,7  All fish and shellfish contain methylmercury with some species containing more than others.6,7  Conversely, the consumption of fish has also been linked to reduced cardiovascular disease which has led to the development of both prescription and nonprescription omega-3 fatty acids supplements that are now used to treat hyperlipidemia as well as reduce cardiovascular-related mortality.8-11 

Why do fish contain mercury (specifically methylmercury)?
One of the main mechanisms of contamination of fish and shellfish with mercury is industrial sources (various factories, waste incinerators, coal fired power plants, etc).12  The emissions from these sources produces airborne mercury  that eventually falls to the ground and contaminates the water.  Natural sources as well as microorganisms (bacteria and fungi) are known to convert mercury to methylmercury (one of the more toxic forms).13  The fish and shellfish who feed in these water sources will then absorb the methylmercury.  Fish and shellfish who are larger and older will have greater amounts of methylmercury as compared to smaller and younger fish.14  In addition, large predatory fish who consume smaller fish will also absorb greater amounts of methylmercury.  As such, regulatory agencies recommend women and children avoid eating shark, swordfish, king mackerel and tilefish and consider eating fish that are known to have lower amounts of mercury such as shrimp, canned light tuna, salmon, pollock and catfish.6,7  The United States Environmental Protection Agency's National Listing of Fish Advisories is available online for anyone interested in knowing information about the fish in their geographic location (simply go to: EPA NLFA). 

Since all fish contain some amount of mercury and also contain the omega-3 fatty acids that can be helpful in cardiovascular disease, do fish oil supplements then contain mercury?
It makes sense to assume that fish oil supplements derived from fish containing methylmercury would stand a good chance of containing methylmercury as well.  It is important to note that the above information from many regulatory agencies is directed at the consumption of fish and not fish oil supplements.6,7  To our knowledge, none of these regulatory agencies take a position on fish oil supplements.  Unfortunately, the answer to this question is dependent on the manufacturing process used by the company making the fish oil supplement.  This is due to the fact that nonprescription omega-3 fatty acid supplements are not regulated by the Food and Drug Administration (FDA) and as such do not have to follow criteria for manufacturing as outlined by the United States Pharmacopeia (USP).15-17  This not only means that the safety and efficacy for most nonprescription omega-3 fatty acids have not been evaluated by the FDA, but the identity, strength, quality, purity, packaging and labeling for many of these supplements has also not been verified.17  This is not the case for the prescription omega-3 fatty acid medication marketed as Lovaza which does meet the standards for both the FDA and USP.18  As it relates to the prescription omega-3 fatty acid formulations, clinicians can confidently inform their patients that the prescription formulation of omega-3 fatty acids does not have mercury or mercury metabolites present.18  However, clinicians cannot do the same for all nonprescription versions of the omega-3 fatty acids unless the manufacturer has chosen to undergo the strict criteria and testing as set by the USP and the product label has the USP Verified mark present.  The USP Verified mark basically ensures the clinician or consumer of the following: 1) the claims the manufacturer have made about the active and inactive ingredients provided in the dosage form are accurate, 2) the supplement does not contain harmful levels of contaminants, 3) the supplement will break down and release the ingredients in the body, and 4) that the supplement was made following good manufacturing practices.  For a list of those manufacturers who have chosen to get USP Verified marks on their products, go to: The USP Dietary Supplement Verification Program. 

Interestingly, analyses of several nonprescription fish oil (omega-3 fatty acid) supplements for the presence of mercury found that of all the brands tested, none of them contained mercury and may even be safer than eating fish.19,20  It is possible that the manufacturing process alone and isolation of fish oil eliminates mercury.  While this independent analysis provides some evidence that fish oil supplements may be safe, this information cannot be applied with certainty to all nonprescription omega-3 fatty acid supplements.  As indicated above, prescription omega-3 fatty acids alone have the greatest level of consistency and protection for the clinician and consumer against environmental contaminants followed by a select few of the nonprescription omega-3 fatty acid supplements with the USP Verified mark on their labels.

References:

1. Harada M.  Minamata disease: methylmercury poisoning in Japan caused by environmental pollution.  Crit Rev Toxicol  1995;25:1-24.
2. Grandjean P, Weihe P, White RF et al.  Cognitive deficit in 7-year-old children with prenatal exposure to methylmercury.  Neurotoxicol Teratol  1997;19:417-28.
3. Rusyniak DE, Furbee RB, Pascuzzi R.  Historical neurotoxins: what we have learned from toxins of the past about diseases of the present.  Neurol Clin  2005;23:337-52.
4. Davidson PW, Strain JJ, Myers GJ et al.  Neurodevelopmental effects of maternal nutritional status and exposure to methylmercury from eating fish during pregnancy.  Neurotoxicity  2008;29:767-75.
5. Despres C, Beuter A, Richer F et al.  Neuromotor functions in Inuit preschool children exposed to Pb, PCBs, and Hg.  Neurotoxicol Teratol  2005;27:245-57.
6. United States Environmental Protection Agency.  Mercury: Fish Consumption Advisories.  Last accessed on 08/08/09.  
7. United States Food and Drug Administration.  Food: What you need to know about mercury in fish and shellfish - March 2004.  Version 05/07/2009.  Last accessed on 08/08/2009.  
8. He K, Song Y, Daviglus ML et al.  Accumulated evidence on fish consumption and coronary heart disease mortality: a meta-analysis of cohort studies.  Circulation  2004;109:2705-11.
9. Kim DN, Eastman A, Baker JE et al.  Fish oil, atherogenesis, and thrombogenesis.  Ann N Y Acad Sci  1995;748:474-80.  
10. Sirtori CR, Crepaldi G, Manzato E et al.  One-year treatment with ethyl esters of n-3 fatty acids in patients with hypertriglyceridemia and glucose intolerance: reduced triglyceridemia, total cholesterol, and increased HDL-c without glycemic alterations.  Atherosclerosis  1998;137:419-27.  
11. Omega-3-acid ethyl esters (Lovaza) product package insert.  GlaxoSmithKline; Research Triangle Park, NC.  November 2008.
12. United States Environmental Protection Agency.  Mercury: Controlling Power Plant Emissions: Overview.  Last accessed on 08/08/2009.  
13. Agency for Toxic Substances & Disease Registry (ATSDR).  Public Health Statement for Mercury.  March 1999.  Atlanta, GA., U.S. Department of Health and Human Services.  Last accessed on 08/08/2009.  
14. Sakamoto M, Kubota M, Liu XJ et al.  Maternal and fetal mercury and n-3 polyunsaturated fatty acids as a risk and benefit of fish consumption to fetus.  Environ Sci Technol  2004;38:3860-3.  
15. Busti AJ, Lehew DS, Nuzum DS et al.  Why are natural or herbal medicines not regulated by the Food & Drug Administration (FDA) when many have mechanisms of action similar to prescription medications?   PW Nat Med Newsl 2009;1(11):1-4.  
16. U.S. Food & Drug Administration.  Dietary Supplement Health and Education Act of 1994: Public Law 103-417.  Last accessed on March 24, 2009:  FDA
17. U.S. Pharmacopeia.  United States Pharmacopeial Convention.  2009.
    
18. Omega-3-acid ethyl esters (Lovaza®) product package insert.  GlaxoSmithKline; Research Triangle Park, NC.  November 2008.
19. ConsumerLab.com, LLC.  Product Review: Fish oil/Omega-3 Supplements and EPA/DHA Fortified Foods and Beverages.  Version 07/07/2009.  Last accessed on 08/08/2009.
    
20. Melanson SF, Lewandrowski EL, Flood JG et al.  Measurement of organochlorines in commercial over-the-counter fish oil supplements: implications for dietary and therapeutic recommendations for omega-3 fatty acids and a review of the literature.  Arch Pathol Lab Med  2005;129:74-7.

• Pharmacology: Fish Oil (Omega-3 Fatty Acids) Supplements and Mechanism for Increased Risk of Bleeding
  

Omega-3 Fatty Acids, Fish Oil Supplements, Mercury, Hg, Methylmercury