The Mercury Content of Seafood: Should you worry?

August 11, 2012 in Categories: , by

Print Friendly

We are often warned not to consume too much seafood over fears of the mercury contamination in these foods building up in our systems and leading to mercury poisoning.  Pregnant women are advised to limit seafood consumption to just two 6oz-servings per week over fears that mercury will cause brain damage to the developing fetus.  It certainly sounds scary, but is this concern well founded?

Mercury is present in all foods.  Concentrations are quite low in fruits and vegetables because mercury uptake by plants from soil is low.  In contrast, mercury levels can be quite high in certain types of fish because fish absorb mercury from the water and from the organisms that they consume.  Methylmercury, an organic form of mercury, is the predominant form of mercury in fish.  It is concentrated in the muscle of the fish (not in the fat, which is why paying for low-mercury fish oil is quite pointless) and, because it binds so tightly to certain proteins in fish, it accumulates over time.  Fish at the lower end of the food chain tend to contain very low levels of methylmercury; but fish that eat other fish tend to have a higher concentration of methylmercury (this process is called biomagnification). 

The concern over consumption of methylmercury is that, when ingested, it is almost completely absorbed from the gastrointestinal tract and distributed to all tissues (elemental or inorganic forms of mercury are not easily absorbed and some methylmercury is converted into elemental forms by your gut microflora).  Methylmercury also readily crosses both the blood-brain barrier and the placenta.  High levels of methylmercury are known to cause damage to the central and peripheral nervous system (the term “mad as a hatter” comes from felt makers in the 18th and 19thcenturies eventually going crazy due to chronic mercury exposure; mercury was used in felt production which was a common material in hats).  Yep, this definitely all sounds scary.

However, studies evaluating the effects of seafood consumption on the developing fetus have varied tremendously in their results.  On one end of the spectrum is brain damage (albeit subtle) from methylmercury exposure and on the other end of the spectrum is enhanced cognitive ability attributed to a maternal diet rich in omega-3 fatty acids (DHA, in particular).  So, should we eat more seafood or less?  The answer lies is in the selenium content of the seafood being eaten.

Selenium is a mineral that is required for activity of 25–30 different enzymes (selenoenzymes), whose job it is to protect the brain from oxidative damage.  Methylmercury irreversibly binds to selenium.  This is bad if you are exposed to methylmercury because it renders selenoenzymes inactive.  In fact, this is the mechanism through which methylmercury is believed to damage the brain and nervous system, by inhibiting the ability of selenoenzymes to protect these tissues from oxidants.  Very importantly, most typically consumed varieties of ocean fish contain much more selenium than methylmercury.  This is good for the fish (they don’t die from mercury exposure), but it is even better for us.  Selenium-bound methylmercury is not efficiently absorbed by our bodies.  What methylmercury is absorbed is already bound to selenium so it can’t interfere with our selenoenzymes.  The only fish that need to be avoided are those which contain more methylmercury than selenium (which is a fairly short list).

Fish that tend to contain very low levels of methylmercury include shellfish (for example oysters, clams, scallops, mussels), salmon, crab, shrimp, trout, herring, haddock, pollock (Boston bluefish), sole, flounder, lobster, Atlantic mackerel and lake whitefish.  However, any fish that has a higher selenium content than methylmercury contamination are perfectly safe to consume.  This includes the vast majority of ocean fish and approximately 97% of fresh water fish.  In fact, the only fish that you need to avoid are:  pilot whale, tarpin, swordfish, shark, marlin, king mackerel, and tilefish.  These recommendations are based on various small scale studies (see links below).  The EPA is currently undergoing a comprehensive survey of fresh and salt water fish and assigning what is called the Selenium-Health Benefit Value (Se-HBV) to each type of fish, which is essentially a ratio of selenium to methylmercury content of each fish.  The results of the project should be available in April 2013 which may alter the safe versus avoid list of fish. 

Increasing your dietary intake of selenium is one way to protect yourself from mercury exposure from food sources or from environmental factors (broken compact fluorescent light bulbs or amalgam fillings).  Selenium is found abundantly in seafood, seaweed, mushrooms, onions, Brazil nuts, sunflower seeds, and meat and poultry (especially the liver).  The vast majority of the fish that you are likely to find in a store or restaurant are perfectly safe to eat (swordfish being the obvious exception).  In fact, the dietary selenium that you will gain from including fish and shellfish in your diet (not to mention all the good DHA and EPA omega-3 fatty acids, iodine and very easily digested protein) will promote better health.  

No, you don’t need to worry about the mercury content of (most) fish.  Actually, I recommend increasing your consumption of fish rather than decreasing, even for pregnant women!  Aiming for three 6oz servings of oily cold-water fish per week (on top of other varieties of fish and types of seafood) will provide you with your recommended intake of DHA and EPA.  And if you have unresolved inflammation, eating even more fish is a good idea.  I personally aim to eat fish 5-6 times per week.  I find that canned sardines and salmon make for inexpensive lunches during the week, frozen pink salmon can be quite inexpensive year-round (my local grocery store routinely has it on sale for $4/pound), and salmon is in season in the late summer and early fall during which time it can typically be found on sale.  So, enjoy!

More information:

Ralston, NVC. “Selenium Health Benefit Values as Seafood Safety Criteria” EcoHealth 5, 442–455, 2008

Ralston, NVC et al. “Dietary and tissue selenium in relation to methylmercury toxicity” Neurotoxicology. 2008 Sep;29(5):802-11.

Western Pacific Regional Fishery Management Council http://wpcouncil.org/ 

Selenium and Mercury Ratio Graphic URL:  http://www.wpcouncil.org/councilmtgs/145th/Selenium_Poster_final.pdf

Energy & Environmental Research Center, University of North Dakota (EERC).” EERC Research Finds Mercury Levels in Freshwater and Ocean Fish Not as Harmful as Previously Thought”. June 22, 2009. Accessed at http://www.undeerc.org/news/newsitem.aspx?id=343

Health Canada,Bureau of Nutritional Sciences, Food Directorate, Health Products and Food Branch “Human Health Risk Assessment of Mercury in Fish and Health Benefits of Fish Consumption.” March 2007. Accessed at http://www.hc-sc.gc.ca/fn-an/pubs/mercur/merc_fish_poisson-eng.php

http://chriskresser.com/is-eating-fish-safe-a-lot-safer-than-not-eating-fish#fn-672-1

http://cfpub.epa.gov/ncer_abstracts/index.cfm/fuseaction/display.highlight/abstract/9503

Comments

Thanks for doing the research on this & passing it on to those of us who don’t have the knowlege or the time.
Im enjoying reading your blog. trying your banger recipe tomorrow.

Thank you so much Sarah! I absolutely love fish , in fact eating it almost everyday but feeling guilty because I thought I was exposing myself to more mercury. I did not know that mercury is in the meat/muscles of the fish as opposed to the fat. It will be easier and less expensive for me to buy fish oil supplements. THANKS FOR SHARING THIS INFORMATION!!!
I vacationed in the Philippines and was eating a lot of fish belly( from milk fish) YUMMY!!!

Your site is my favorite!
Rosanna

Sarah thanks so much for doing the research and taking the time to do the write up. I had some testing which showed that I have high levels of mercury. I eat fish 2-3 times a week and my functional medicine doctor told me to back off for now. I didn’t think it was from the fish but possibly dental fillings. Do you have any thoughts on this?

Well, I definitely think a selenium supplement sounds like a good idea (sorry, I don’t know what brand or dose to recommend). From everything I’ve read, the seafood you are eating isn’t likely to be contributing (unless you are a swordfish fiend), but I would recommend sticking to the low mercury fish for a while. Good luck!

I will do some research on a selenium supplement. I think I’m going to look into testing of old fillings and begin the ugly process of having them removed safely too. I’m not a big swordfish fan. I’ve kept to salmon, halibut, ahi, yellow fin and albacore tuna as well as shellfish. Thanks for all you do lady!

I just went through the process of having mercury fillings removed. I used an experienced dentist who knew what he was doing and had nutritional support also. I had 6 removed over a matter of months. I didn’t realize until the last removal that I was having acute symptoms of mercury poisoning throughout the process until things got downright scary. The form of mercury is different then what’s found in seafood but nonetheless, do your homework. And you might think twice about adding the seafood back in your diet until the burden is lighter on your body. The effects of the mercury can be cummulative and you don’t know if you’ve gone over the top until you’re on the other side!

Are you worried about other toxins in fish? With pastured and free-range meat, it seems we don’t have to worry about this so much, but with farmed and wild caught fish, I do wonder…?

I adore your blog, btw, and am starting Paleo today with my resistant family. Thanks so so much!! We started green smoothies about 1-2 months ago, but we have a long way to go (especially my carb obsessed 8 year old)!

Hi TPM, Firstly, thank you for providing such informative information. It really is eye-opening! I have a question for you regarding fish: would Hoki be okay to consume? I am not sure its selenium levels, but assume they are high considering it is a cold water fish of the Southern Hemisphere. Thank you.

Polyunsaturated fats are much more stable when still part of the whole food (and not isolated like fish oil or flax oil). It should say on the tin how much is in it.

Hi Sarah,

personally, I am not convinced that the high levels of mercury in fish such as tuna and swordfish are safe if they exist in the presence of equimolar or higher concentrations of selenium.

The definitive study in this issue is Chang’s, in which16 kittens were fed a diet of tuna containing 0.3 to 0.5 ppm mercury, plus supplementary nutrients and vitamins. After a period of 7 months, 2 of the cats had mild ataxia and 1 had severe ataxia. The cats were sacrificed at 11 months and autopsy revealed extensive liver damage including damage to the mitochondria. In this case the selenium in the fish was not protective.

In his review on Mercury: selenium interactions and health implications Dr Ralston, a supporter of the protective effect of selenium, says that “the ‘protective effect’ of selenium against mercury exposure may actually be backwards. Mercury’s propensity for selenium sequestration in the brain and endocrine tissues may inhibit formation of essential Se-dependent proteins (selenoproteins). Hence selenium’s ‘protective effect’ against mercury toxicity may simply reflect the importance of maintaining sufficient free selenium to support normal selenium-dependent enzyme synthesis and activity.” But even that may not be enough to completely nullify mercury’s toxic effects.

In the same review he mentions the study by Friedman into the protective effects of dried swordfish on methylmercury toxicity in rats. He states that rats fed a diet of swordfish and methylmercury showed no signs of neurotoxic effects, while rats fed a control diet spiked with methylmercury without swordfish did. Dr Ralston attributes this to the protective effects of selenium. However, the protective effect is not that impressive.
In spite of the proposed protective effect of selenium, both the control group and the experimental groups died, at 4.6 and 5.3 weeks respectively. If the experimental rats had lived longer, there would have been more chance for any neurological effects to manifest. It should also be noted that the control diet included 15% casein which has been shown to reduce mercury excretion in rats, and thus may have exacerbated the effects of mercury toxicity in the control rats.

These studies do not take into account the long-term effects of mercury exposure. The lower the dose of mercury, the greater the delay in the manifestation of symptoms. Deborah Rice fed monkeys a diet of 50 micrograms of methylmercury for 7 years. After cessation, blood levels quickly dropped to normal levels. When the monkeys were tested at 13 years of age they displayed clumsiness and loss of fine motor skills as well as decreased sensitivity to touch. Humans are exposed to mercury for decades and have longer to develop overt signs of mercury toxicity.

One of the problems with the studies from the Seychelles and the Faroe Islands on the effect of methylmercury on neurodevelopment is that they rely on hair testing of mercury levels. This is often accurate, but does not take into account the fact that mercury disrupts cellular transport due to its affinity for sulfhydryl molecules. These molecules often form the active site in cellular transport proteins. Mercury binds to these active sites, altering mineral transport. This can result in hair readings for mercury and other toxic elements that are artificially low. Thus children with high exposure may actually be classed as having low exposure. Hair analysis actually provides a measure of how much mercury is being excreted. The most important factor is how much mercury is being retained in the body and that is difficult to measure.

Amy Holmes found that autistic children, even though they had higher exposures to mercury through their mothers’ dental amalgams and Rhogam injections, had lower levels of mercury in their hair, implying a reduced ability to excrete mercury. The following hair test illustrates the ability of mercury to disrupt mineral transport – http://www.livingnetwork.co.za/files/hairtest_564.pdf

This is clearly an abnormal distribution of elements (all except one of the essential elements are below the 50th percentile) associated with a low reading for mercury.

It may be safe for some individuals with optimal antioxidant and metallothionein status to consume tuna and other high mercury fish, but I think for many it would be safer to stick to fish such as sardines and salmon, which also have high levels of omega 3s, but much lower levels of mercury.

David Hammond
author – Mercury Poisoning: The Undiagnosed Epidemic.

References

Chang, L. W., & Yamaguchi, S. (1974). Ultrastructural changes in the liver after long-term diet of mercury-contaminated tuna. Environmental Research, 7(2), 133-148.

Raymond, L. J., & Ralston, N. V. (2004). Mercury: selenium interactions and health implications. Seychelles Medical and Dental Journal, 7(1), 72-77.

Rowland, I. R., Robinson, R. D., & Doherty, R. A. (1984). Effects of diet on mercury metabolism and excretion in mice given methylmercury: role of gut flora. Archives of Environmental Health: An International Journal, 39(6), 401-408.

Rice DC. Delayed neurotoxicity in monkeys exposed developmentally to methylmercury. Neurotoxicology. 1989 Winter; 10(4):6450-50..

Holmes, A. S., Blaxill, M. F., & Haley, B. E. (2003). Reduced levels of mercury in first baby haircuts of autistic children. International journal of toxicology , 22 (4), 277-285.

Several years ago I had a hair analysis done & it showed very high mercury levels. I have no amalgams but was eating lots of canned albacore tuna. I can only guess that my subclinical mercury poisoning was from the tuna…

There are likely many people suffering from subclinical mercury poisoning – unfortunately most doctors are not aware that common symptoms such as anxiety, fatigue, attention deficit and insomnia can be caused by mercury poisoning. The following case was reported in the Wall Street Journal.

SAN FRANCISCO — One by one, Matthew Davis’s fifth-grade teachers went around the table describing the 10-year-old boy. He wasn’t focused in class and often missed assignments, they said. He labored at basic addition. He could barely write a simple sentence.

“Our jaws dropped,” says his mother, Joan Elan Davis, describing a teachers’ meeting she had requested in late 2003, when her son abruptly lost interest in homework. Matthew had always excelled in school. In the fourth grade, he had written and illustrated a series of stories about a superhero named Dog Man.

Ms. Davis noticed something else: Her son’s fingers were starting to curl, as if he were gripping a melon. And he could no longer catch a football.

A neurologist ordered tests. They showed Matthew’s blood was laced with mercury in amounts nearly double what the Environmental Protection Agency says is the safe level for exposure to the metal. Matthew had mercury poisoning, his doctors said.

The Davises had pinpointed the suspected source: tuna fish. For a year or so, starting in late 2002, Matthew had gobbled three to six ounces a day of white albacore tuna. Based on Food and Drug Administration data for canned albacore, he was consuming a daily dose of mercury at least 12 times what the EPA considered a safe level for a 60-pound child. The Davises’ doctors’ prescription was simple: Matthew should stop eating canned tuna.

See the rest of the article at http://online.wsj.com/news/articles/SB112268169016100484

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>

Sign up for my FREE weekly newsletter!

Stay up-to-date, never miss a post, and get exclusive content and coupons! Sign up now and you'll get a FREE Paleo Quick-Start Guide!

We will never share your information with anyone.