202 Mercury in Seafood
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Dynamic Chiropractic – July 30, 2007, Vol. 25, Issue 16

Mercury in Seafood

By G. Douglas Andersen, DC, DACBSP, CCN

It took me a few years following graduation to come to the conclusion that trying to provide a patient with true nutritional guidance while also delivering chiropractic care diverts the attention of both parties, resulting in suboptimal information exchange.

That was the case recently when a new patient was seen for a nutritional consultation in my office. I discovered this gentleman's diet included six to eight cheeseburgers a week. I asked him if he liked fish, and he answered that he did, but that his previous health care provider had warned him of the risks of mercury exposure from seafood. Thus, he had stopped eating his favorite fish, which he normally consumed twice a week.

What type of fish was his favorite and what he would consume instead of fish were questions not raised during his therapy with his previous chiropractor. It turned out he was trading salmon for cheeseburgers to reduce mercury exposure. And at the molecular level, he was trading heart-healthy, mind-healthy, anti-inflammatory fats for artery-clogging, pro-inflammatory fats. Even worse, the elimination of salmon does little to lower mercury, because neither wild nor farmed salmon accrues much mercury.

Mercury Sources

Mercury is a heavy metal that can come from coal-fueled power plants, chlorine production, waste incineration, gold mining, and electrical-switch manufacturing. It also may be present in antiseptics, disinfectants and fungicides, although the number of products in these categories that contain mercury has declined significantly in the past 10 years. Mercury enters the environment from natural sources as well. Active volcanoes, forest fires, weathering of rocks, and soil erosion all release mercury.

Mercury Cycle

When released into the atmosphere, mercury will return to earth via rain or snow, some of which will eventually enter streams, lakes and the sea. Trace amounts of mercury can also come from the effects of weather eroding soil and rocks. When mercury reaches the water, it normally does so in its elemental or inorganic forms. Microorganisms will convert elemental and inorganic mercury into organic methylmercury, which is highly absorbable in biologic tissues. Methylmercury enters all organisms by internal ingestion or external absorption. In turn, they are eaten by small fish, which are then eaten by medium fish. This cycle continues until reaching large fish at the top of the food chain. Once there, the oldest and largest are the most likely to have the highest levels of mercury, due to greater amounts consumed over their lifetime.

However, this does not guarantee a specific individual (fish) will be toxic. In my previous article, "Omega-3 Fatty Acids in Seafood" [www.chiroweb.com/archives/25/14/02.html], I explained that omega-3s in fish of the same species may vary by up to 300 percent. Mercury and other chemical contaminants can vary by more than 3,000 percent.1 For example, FDA shark surveys tested 351 fish. Mercury levels in sharks ranged from nondetectable to a whopping 4.54 parts per million. The mean mercury level in shark is 0.99 parts per million, which is the second-highest tested, behind only tilefish from the Gulf of Mexico, and just ahead of swordfish and king mackerel.2 The likelihood that these species contain high levels of mercury prompted a joint FDA and EPA statement on March 19, 2004,3 which recommended that sexually active women of childbearing age, along with pregnant and nursing mothers, completely avoid these species. They went on to recommend that the general population limit consumption (tilefish, shark, swordfish or mackerel) to one serving per month. Fish that contained high levels of mercury in a single study, but not over an average of studies, included Spanish mackerel from the Gulf of Mexico, snapper, tuna and grouper. Since the fish on our plates remain untested, there are no assurances that a given serving will be either toxic or safe, regardless of type.

The Effects of Mercury

All people contain small amounts of mercury. Occasional mercury ingestion is not a problem in most healthy adults. The body can naturally cleanse and remove mercury, but it will take a month or so. Most people are not aware that very low levels of mercury are present in most fruits and vegetables.

Methylmercury is the form of mercury contained in seafood. It is a neurotoxin and can affect cognition, visual-spatial ability, speech, concentration and recall. Women planning to become pregnant or who are pregnant increase the risk for mercury problems in their children.

Acute toxicity (poisoning) can result in paresthesias of the hands, feet and mouth. Difficulty with speech, sound, muscle coordination, and muscle strength is also noted. In the 17th and 18th centuries, the phrase "mad as a hatter" originated because mercury was used in manufacturing the lining in hats, which led to high mercury levels in those who wore them.4 Levels of exposure considered acute when incurred during pregnancy can magnify the above impairments to levels that may cause frank disability.

Not all studies on humans eating seafood high in mercury yielded negative results. We do not know why prenatal mercury intakes of the Seychellois (from the Seychelles archipelago in the Indian Ocean) correlated with elevated neurological function in their children.5 We also do not know why, in a recently published paper, mercury improved dexterity in adults.6 Some speculate these findings may be due in part to selenium, which is rich in seafood.1 If consumed in adequate amounts by either fish or humans, selenium allows more elimination and less accumulation of methylmercury in biologic tissue.


It appears the risk of cardiovascular and neurological problems caused by mercury in seafood is much less than the potential for developing cardiovascular and neurologic problems as a result of the typical Western/standard American diet minus seafood. As for my patient, he has reduced his weekly cheeseburger intake and increased his consumption of seafood, including salmon.


  1. Mozaffarian D, Rimm EB. Fish intake, contaminants and human health: evaluating the risks and benefits. JAMA, 2006;296(15):1885-99.
  2. www.epa.gov/mercury/faq.html.
  3. www.cfsan.fda.gov/~frf/sea-mehg.html.
  4. Bralley JA, Lord RS. Laboratory Evaluations in Molecular Medicine. Institute for Advances in Molecular Medicine, 2001:60.
  5. Davidson PW, Palumbo D, et al. Neurodevelopmental outcomes of Seychellois from pilot cohort at 108 months following prenatal exposure to methylmercury from maternal fish diet. Environ Res, 2000;84:1-11.
  6. Weil M, Bressler J, et al. Blood mercury levels and neurobehavioral function. JAMA, 2005;293:1875-82.

Click here for previous articles by G. Douglas Andersen, DC, DACBSP, CCN.

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