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
Why do fish contain mercury (specifically
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
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.
- Harada M. Minamata disease: methylmercury poisoning in Japan caused
by environmental pollution. Crit Rev Toxicol 1995;25:1-24.
P, Weihe P, White RF et al. Cognitive deficit in 7-year-old children
with prenatal exposure to methylmercury. Neurotoxicol Teratol
DE, Furbee RB, Pascuzzi R. Historical neurotoxins: what we have
learned from toxins of the past about diseases of the present. Neurol
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.
C, Beuter A, Richer F et al. Neuromotor functions in Inuit preschool
children exposed to Pb, PCBs, and Hg. Neurotoxicol Teratol
- United States Environmental Protection Agency. Mercury: Fish Consumption Advisories. Last accessed on 08/08/09.
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.
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.
- Kim DN, Eastman A, Baker JE et al. Fish oil, atherogenesis, and thrombogenesis. Ann N Y Acad Sci 1995;748:474-80.
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.
- Omega-3-acid ethyl esters (Lovaza) product package insert. GlaxoSmithKline; Research Triangle Park, NC. November 2008.
States Environmental Protection Agency. Mercury: Controlling Power
Plant Emissions: Overview. Last accessed on 08/08/2009.
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.
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.
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
Food & Drug Administration. Dietary Supplement Health and
Education Act of 1994: Public Law 103-417. Last accessed on March 24,
- U.S. Pharmacopeia. United States Pharmacopeial Convention. 2009.
- Omega-3-acid ethyl esters (Lovaza®) product package insert. GlaxoSmithKline; Research Triangle Park, NC. November 2008.
LLC. Product Review: Fish oil/Omega-3 Supplements and EPA/DHA
Fortified Foods and Beverages. Version 07/07/2009. Last accessed on
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.