What you and your patients need to know about Fish and Mercury

Many patients are familiar with at least some of the health benefits of fish oil supplementation and dietary consumption of fatty fish, especially species higher in the n-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). There are a multitude of benefits, including improved cardiovascular health, better functioning immune system, help with arthritis, atherosclerosis, depression, adult-onset diabetes mellitus and some cancers. There is reduced risk for stroke, asthma, dementia and kidney disease. In recent research, it was noted that modest consumption of fish (e.g., 1-2 servings/wk), especially species higher in the n-3 fatty acids EPA and DHA, reduces risk of coronary death by 36 percent (95 percent confidence interval, 20-50 percent; P<.001) and total mortality by 17 percent (95 percent confidence interval, 0-32 percent; P=.046) and may favorably affect other clinical outcomes.1

Many of the benefits are attributed to the anti-inflammatory effects of these fatty acids. There are thousands of research articles touting the benefits of polyunsaturated fatty acids or omega-3’s obtained from our diet as well as supplementation. (A search of Medline with the key word “omega-3” results in just under 10,000 articles.) Many well-known organizations, such as the American Heart Association, recommend eating fish as a regular part of a healthy diet. However, we are being made increasingly aware that, along with these benefits, come risks. As a result, consumers are rightly concerned about health risks of contaminants from eating. Many Americans have dangerous levels of methylmercury in their bodies, including 5- 8 percent of American women of childbearing age.

In 2004, the Food and Drug Administration and the Environmental Protection Agency issued a joint consumer advisory about mercury in fish and shellfish. The significance of this becomes apparent when one considers that this is the first time these two organizations have combined their advice to form a single uniform advisory.2 Their advice was directed toward women who might become pregnant, women who are pregnant, nursing mothers, and young children. In July of 2003, the Food Standards Agency in the United Kingdom issued a similar precautionary warning, however they also included children under sixteen years of age.3

In a 2004 article published in the Annals Of Internal Medicine, Wilson states that “studies have linked over consumption of certain fish (particularly popular ones such as swordfish, tuna steaks, Chilean sea bass, and some kinds of salmon) to neurologic deficits, cancer, autoimmune and endocrine disorders, and even some heart disorders. The risks stem mainly from two toxins: mercury, which accumulates over the lifetime of larger, longer-living fish, and polychlorinated biphenyls (PCB’s), which are found in fish living in polluted waters and in some farmed fish.”4

In adults, methylmercury poisoning causes tremor, difficulty with concentration, vision deficits, and numbness and tingling. In children, numerous health problems may result, including brain damage, mental retardation, blindness, and seizures. With lower levels of methylmercury exposure in the womb, there may be subtle but irreversible deficits in learning ability as well as altering the male reproductive organs and increased risk for cancer.4

Mercury is an element that is naturally found in the environment. It is also released into the air through industrial pollution, including the burning of fossil fuels and solid wastes. This mercury accumulates in streams and oceans where bacteria cause chemical changes that transform mercury into methylmercury. Fish and shellfish absorb the methylmercury as they feed in these waters and tend to build up in the larger predatory species. Of these, shark, tilefish, tuna and bottom-feeders, such as certain crab, tend to accumulate the highest levels. Essentially, the higher up the food chain, the higher the bioaccumulation of toxins. According to Wilson, exposure to methylmercury comes almost solely from eating fish. She also notes that “methylmercury is absorbed from the gastrointestinal tract and binds readily with proteins; the highest levels in the body are found in the kidneys.”

Methylmercury crosses the blood-brain barrier, affecting the brain. According to some researchers, the health effects of low-level methylmercury in adults are not clearly established.1 However, we suggest it is best to err on the side of caution and limit our exposure to toxic elements.

In addition to the dangers posed by methylmercury, fish are also the main source of concentrated polychlorinated biphenyls (PCB’s) with the highest dietary levels found in farmed salmon. PCB exposure is associated with liver and breast cancer, neurologic and endocrine problems, and possibly even increased risk for heart disease. “In children, PCB exposure in utero and from breast milk consumption has been linked with neurodevelopmental delays, impaired cognition, immune problems, and alterations in male reproductive organs…. Generally, freshwater fish that live in inland lakes, such as bluefish, lake trout, and smelt, are more likely to be contaminated.”4

Farmed salmon sounds innocent enough until you know a little more about it. Wilson states that “any more than a single eight-ounce portion of farmed salmon a month posed an ‘unacceptable cancer risk’ to consumers.”

Foran, et al., analyzed farmed vs. wild salmon and reported that organic arsenic was significantly higher in farmed than in wild salmon, whereas cobalt, copper, and cadmium were significantly higher in wild salmon. In addition, he stated that none of the contaminants exceeded federal standards or guidance levels.5 In a follow-up article, he stated that “health risks (based on a quantitative cancer risk assessment) associated with consumption of farmed salmon contaminated with PCB’s, toxaphene, and dieldrin were higher than risks associated with exposure to the same contaminants in wild salmon.”6

In the journal Science, Hites, et al., note, “The annual global production of farmed salmon has increased by a factor of forty during the past two decades.” He and fellow researchers analyzed over two metric tons of farmed and wild salmon for organochlorine contaminants and discovered that concentrations of these contaminants are significantly higher in farmed salmon than in wild.7

This data is frightening, considering over 90 percent of the salmon sold in the United States is farmed and that farmed salmon is sometimes labeled as “wild” in grocery stores.

Many researchers recommend purified fish oils that have been tested and found to be free of contaminants. We noted previously, “Untainted fish oils containing abundant levels of omega-3 fatty acids should be a routine supplement during pregnancy and lactation.”8 Our suggestion is that you consider for your patients a high quality purified fish oil as a daily supplement to help reduce exposure to environmental toxins and yet reap the benefits of a diet high in polyunsaturated long chain fatty acids.

After reviewing a number of articles and studies including information from the EPA, FDA and the Food Standards Agency in the United Kingdom, we have produced a guide of fish and shellfish to avoid and those that are lower in mercury. In addition, we list some commonly eaten fish with their mercury levels. Note that this list does not include PCB and other contaminants.

Our recommendation is, “Do Not Eat: shark, swordfish, king mackerel, tilefish, farmed salmon and marlin.” (The FDA recommends that pregnant and breast-feeding women, and women who intend to become pregnant limit their consumption of tuna to no more than two medium size cans or one fresh tuna steak per week.)

Five of the most commonly eaten fish that are low in mercury are shrimp, canned light tuna, salmon, pollock, and catfish. Others include flounder, rainbow trout, sole, anchovies and clams. Lower levels of mercury are also found in lobster (spiny), oysters and sardines.

For those of your patients that are fishermen, it is recommended that they check local advisories about the safety of locally caught fish. If no advice is available, eat one average meal per week of fish from local waters, but don’t consume any other fish during that week.

“Lean ocean fish, such as cod, flounder, and haddock, are the least likely to be contaminated with PCB’s.”4

Earlier this year in the Journal Environmental Science and Technology, Oregon State University and the EPA published the results of a survey of over 2700 fish involving 600 rivers and streams in the western United States. The fish were analyzed and found to contain alarmingly high levels of mercury. According to Associate Professor Alan Herlihy, mercury was literally in every single fish sampled. Based on this and other information presented, it would be wise to exercise caution in our dietary selection of fish. In addition, our suggestion for you and your patients is to consider a high quality purified fish oil as a daily supplement to help reduce exposure to environmental toxins and yet reap the benefits of a diet high in polyunsaturated long chain fatty acids.

Dr. Dan Murphy graduated magna cum laude from Western States Chiropractic College in 1978. He received Diplomat status in Chiropractic Orthopedics in 1986. Since 1982, Dr. Murphy has served part-time as undergraduate faculty at Life Chiropractic College West, currently teaching classes to seniors in the management of spinal disorders. He has taught more than 2000 postgraduate continuing education seminars. Dr. Murphy is a contributing author to both editions of the book Motor Vehicle Collision Injuries and to the book Pediatric Chiropractic.  

Michael L. Underhill, D.C., C.C.S.P., C.C.S.T., is a 1981 graduate of Western States Chiropractic College. He is certified in Chiropractic BioPhysics® as well as being certified as a sports chiropractor and in spinal trauma. He holds a diplomate in thermography. Dr. Underhill is also a contributing author to both editions of the book Motor Vehicle Collision Injuries: Mechanisms, Diagnosis, and Management. He has been in private practice in Beaverton, Oregon, for the past twenty-five years and has taught both chiropractic postgraduate and undergraduate classes. Dr. Underhill can be reached by email at [email protected].


1. Mozaffarian D, Rimm EB. Fish intake, contaminants, and human health: evaluating the risks and the benefits. JAMA. 2006 Oct 18;296(15):1885-99

2. US Food and Drug Agency website: http://www.fda.gov/fdac/features/2004/304_fish.html

3. United Kingdom Food Standards Agency, www.food.gov.uk/multimedia/faq/mercuryfish/

4. Wilson JF. Balancing the Risks and Benefits of Fish Consumption. Annals of Internal Medicine 2004 Dec;141:12, pp977-980

5. Foran JA, Hites RA. A Survey of Metals in Tissues of Farmed Atlantic and Wild Pacific Salmon. Environmental Toxicology and Chemistry: 2004 Sep;23, pp. 2108–2110.

6. Foran JA, Carpenter DO, Hamilton MC, Knuth BA, Schwager SJ. Risk-based consumption advice for farmed Atlantic and wild Pacific salmon contaminated with dioxins and dioxin-like compounds. Environ Helath Perspec. 2005 Oct;113(10):p655-6

7. Hites RA, Foran FA, Carpenter, Hamilton M, Knuth B, Schwager SJ. Global Assessment of Organic Contaminants in Farmed Salmon. Science 92004 January: Vol. 303. no. 5655, pp. 226-229.

8. Murphy DJ, Underhill ML. Omega-3 Fatty Acids in Pregnant Women and Infants. The American Chiropractor. Jan 2007:1, pp16-20.

9. US Food and Drug Administration website: http://www.cfsan.fda.gov/~frf/sea-mehg.html




Daniel J. Murphy, DC and Michael L. Underhill, DC

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