Updated April, 2019
There’s no shortage of fear-mongering around the evils of mercury and mercury toxicity.
We hear about its use in vaccines and dental amalgams but where we hear it the most, is in any discussion about eating fish.
To be clear, I’m referring to methyl-mercury not ethy-mercury; there is a difference and they are metabolized differently.
The focus of this post is to see whether or not there’s any grounds for the broad stroke recommendation for everyone to minimize (or avoid) how much fish and seafood they eat.
People ask me about this routinely whether it’s a friend, private practice client or patients at the hospital. They’re worried about mercury in fish but know that omega 3 fats are not only essential but healthy.
Is it just certain people in the population or should everyone avoid fish? Is there a downside to such generalized advice?
Fish consumption guidelines
Health Canada, like most health branches of governments, recognize the many health benefits of omega-3 fats. This is why they recommended 2 servings of fish per week in the former Food Guide.
The bulk of the evidence shows that getting more omega-3 fats (EPA, DHA and DPA) improves overall health, there’s no denying this. However, there’s also a general sense of concern among the public about eating fish and seafood due to mercury content.
FUN FACT: In Canada, the average intake of omega 3 fats is 125 to 140 mg/day. Only 10% get more than the minimum recommended 250 mg/day.
As well, Health Canada, like other agencies around the globe, including the EPA and FDA in the US, has also issued recommendations on fish and seafood consumption as it relates to mercury exposure.
In 2008, Health Canada updated its 2002 dietary advice about mercury in fish. In a nutshell, they suggest that most Canadians shouldn’t be concerned about the amount of mercury in fish. Also, most of the species of fish that are consumed in Canada have low levels of mercury.
They do state that there are a few species that have unacceptable amounts mercury and should therefore be avoided. At the very least, consumption of the following sepses should be extremely restricted. These include:
- Fresh or frozen tuna
- Shark
- Swordfish
- Marlin
- Orange roughy
- Escolar
- Tile fish
Because canned albacore tuna is higher in mercury than canned light, skipjack, tongol or yellowfin tuna, Health Canada cautions that there is a potential for exposure to higher levels of mercury.
For this reason, puts limits for specific groups; children and women who are, or may become, pregnant or are breastfeeding.
They recommend limiting albacore tuna to 75 g or 2.5 ounces, or 125 ml or 1/2 cup which is equal to about half a 170 g can, per week.
While these recommendations were and still are intended for women during pregnancy, breastfeeding and for children, they’ve been erroneously extrapolated to all members of the public. The general word on the street when it comes to eating fish and seafood where mercury is concerned?
Proceed with extreme caution…….Sigh
Are mercury concerns in fish exaggerated?
As stated above, people don’t need to worry about the trace amounts of mercury in the vast majority of fish Canadians eat.
The levels just aren’t high enough to pose a health risk. Regardless, to only hear about the mercury content of fish means the public is only getting half the story – so what else is new? Typical, simple nutritional “avoid” statements, but what about the positive spin?
The more important part of the equation is the health protective nutrient in fish and seafood when it comes to concerns about mercury.
Can I get a drum roll?
Enter selenium
Did you know that selenium plays a huge role in preventing mercury toxicity and the GOOD NEWS is that many of the fish that people routinely eat are high in selenium 🙂 (1).
The SAD NEWS is, this fact has been known about for decades with some of the earliest research as far back as the late 1960s!! Yet the public remains unaware and in the dark; scared of eating fish as a consequence.
Getting enough selenium in your diet is one of the best ways to prevent mercury toxicity. Easier said than done? Maybe, suboptimal selenium deficiency can occur if you’re not getting it in your diet.
The content of selenium in Brazil nuts is very high. A few nuts a week is a good strategy to boost your selenium intake. You can also choose a good multivitamin with minerals that includes selenium or consider selenium supplements. Be sure to talk to a nutrition professional for guidance on that.
Selenium protects against mercury toxicity.
Mercury loves selenium, so much so, that it preferentially binds to it. So long as there’s plenty of selenium in your diet, mercury toxicity is prevented in one of two ways.
Firstly, there’s enough selenium to bind to mercury to sequester it and secondly, the extra selenium is the critical mineral needed to maintain optimal levels of glutathione peroxidase (GSH).
If there’s excess mercury and too little selenium, the selenium gets ‘used up’. Without enough selenium, your body can’t make GSH in adequate amounts. Over the long run, mercury exposure may become a problem.
GSH is referred to as the master antioxidant and detoxifier. GSH’s job is to protect vital tissues from oxidation and inflammation, not the least of which is the brain. The organ most are worried about when it comes to mercury. GSH is also one of your best allies for liver health where it helps to eliminate toxins like mercury.
Dietary and tissue selenium in relation to methyl mercury toxicity
Mercury in fish studies
Studies that found negative health effects of mercury-containing fish were those species that were also low in selenium .
With depleted bodily selenium stores, methyl-mercury is indeed potentially toxic. Simply looking at the mercury content of any given fish species misses the mark – big time – when it comes to advising people on fish consumption. This is evident in the guidelines put out by most governments which, in turn, influences the advice from doctors and nutrition professionals. This misinformed viewpoint is further distorted in the media.
Fortunately, most of the fish found in the food supply have a lot more protective selenium than mercury as evidenced in the diagram below.
The benefits of eating fish far outweigh any theoretical risks
The bulk of the evidence is clear: regular fish consumption, [ideally from sustainable stocks], benefits overall health.
Fish Intake, Contaminants, and Human Health. Evaluating the Risks and the Benefits
This is still true for pregnant women and their developing child.
One study found that women who ate more seafood did have higher levels of mercury in their umbilical cord blood but that did not result in poorer health in children. In fact quite the opposite, the children of the fish & seafood eaters had better motor development and higher verbal and total IQ (2).
This study is consistent with other studies that looked at omega-3 fats & fish consumption and fetal and child development (3, 4).
Telling pregnant women, or anyone else for that matter, to avoid their consumption of fish and seafood is ill-advised. If this was about avoiding a toxin, like asbestos, then yes, it would be straightforward but it isn’t.
The bulk of the research have demonstrated that lower intakes of omega 3 fats during pregnancy is associated and predicative of poorer fetal and child health. This includes growth retardation, reduced fine motor skills, IQ, eye development with implication in neurotransmitter metabolism (3).
The benefits of long chain omega 3 fats (EPA, DPA and DHA) are also well established for adults. Modest fish consumption (One to two, 3 oz or 85 g of cold-water fatty fish like salmon per week) are associated with better health and overall lower mortality (5).
Bottom line
Advising pregnant women, children and adults of any age to reduce their fish consumption as a way to avoid mercury is ill-informed.
In fact, pregnant women should be eating more oily fish than most currently are. While it’s true that some species of fish such as swordfish, shark, tile fish, marlin and others already mentioned, should be limited because they lack selenium, the vast majority of fish are safe to eat by all groups of people.
As a general rule, if a fish species contains more selenium than mercury, it’s safe to eat. You can also protect yourself from mercury but ensuring you include other food sources of selenium in your diet.
On a completely different side note, I was curious to know what my blood mercury level was so I had my doctor test it.
I rarely to never eat raw tuna only because I don’t have a lot of opportunity to do so. I do eat canned salmon (sockeye and pink) and skipjack tuna regularly; two to three times a week. My mercury was 5.8 nmol and according to the lab reference range, it should be less than 18 nmol.
For reference, 500 nmol/L is the cut off for acute toxicity.
My MD said he has seen 80-100 nmol in those who eat a lot of sushi and sashmi [higher mercury containing fish] several times a week. Those consuming high mercury-containing fish almost daily can have levels as high as 998 nmol/L.
You can be sure, I will confidently continue to include selenium-rich fish and seafood in my diet knowing that by doing so, my overall, long-term health will benefit
To learn more about which fish stocks are a better choice with respect to the environment and sustainability, check out Sea Choice for more information.
Doug Cook RDN is a Toronto based integrative and functional nutritionist and dietitian with a focus on digestive, gut, mental health. Follow me on Facebook, Instagram and Twitter.