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.

Leave a Reply