Selenium and mercury and eating fish

Ocean fish – salmon, herring, mackerel, and sardines – are good sources of the omega-3 fatty acids EPA and DHA that have been associated with healthy fetal development, healthy cardiovascular function, and healthy ageing.  Whatever mercury there is in these fish has bound with the selenium in the fish.   This chemical binding has rendered the mercury harmless but has also depleted the amount of selenium available for absorption.

Too many of us are missing out on the health benefits of the omega-3 fatty acids available to us from eating certain types of fish a couple of times a week.  Why are we avoiding fish?  Because many of us are afraid of “eating mercury” in the fish.

It turns out, there is research to show that this is a misconception.  Professor Nick Ralston and his colleagues at the University of North Dakota’s Energy and Environmental Research Center have measured and evaluated the molar ratios of selenium in fish to the mercury in fish [Ralston 2007, 2016].

Their studies show that many of the edible ocean fish have an abundance of selenium in relation to mercury.  So, not only are we missing out on the omega-3 fatty acid benefits, we are also missing out on a good source of dietary selenium [Berry 2008].

Here is what Dr. Ralston says happens in many varieties of ocean-caught fish.  The mercury in the fish preferentially binds with the available selenium in the fish and is rendered harmless [Ralston 2010].   In doing so, however, the mercury depletes the amount of selenium available to us for the bio-synthesis of needed selenoproteins.

So the real problem is a problem of selenium depletion more than it is a problem of mercury intoxication.

Let’s examine the question of eating ocean-caught fish step-by-step.

First: Where does the mercury in fish come from?
Large concentrations of the neurotoxin mercury in edible ocean fishes are essentially a man-made problem.  We release great quantities of mercury into the atmosphere from power plants and industrial plants that burn fossil fuels such as coal and oil.  The mercury falls onto the oceans (as well as onto lakes and rivers).  Plankton and other forms of sea life take on the mercury, and the mercury spreads up the food chain as the larger fish eat the smaller fish.

We humans not only breathe in the mercury in the air.  If we eat the wrong kinds of fish, we also eat the concentrated mercury in fish and seafood.  Too much exposure to mercury will cause brain and nervous system damage and developmental problems for humans.  However, if the mercury is bound together with selenium in the fish, it will cease to be harmful to humans [Ralston 2010].

Second: What is the role of selenium in in our bodies?
We need selenium for the formation of selenoproteins (= proteins containing selenium incorporated into the amino acid selenocysteine).  These selenoproteins, many of which act as antioxidant enzymes, are necessary to protect the brain and the nervous system as well as other body systems from oxidative damage.  The body needs a continuing supply of dietary or supplemental selenium to synthesize the selenoproteins that protect against oxidative damage.

Third: How does selenium prevent oxidative stress and damage?
Normal everyday oxygen-dependent metabolism in the body as well as exposures to environmental toxins and radiation produce oxygen reactive species, also known as free radicals.  The condition of oxidative stress, usually a bad condition for us humans, is defined as an imbalance of the numbers of these harmful free radicals and the numbers of antioxidants needed to neutralize the free radicals.

Oxidative damage is the resulting damage to cells and tissues and DNA and lipoproteins that is caused by chain reactions of these reactive oxygen species.

Selenium-dependent antioxidants such as the glutathione peroxidases, the thioredoxin reductases, and the selenoprotein P prevent and even reverse the oxidative damage in the brain and nervous systems and in other body systems.

From this brief description, we see that a lack of selenium for the formation of the antioxidant selenoproteins will have a deleterious effect on human health.  Mercury’s binding with selenium reduces the amount of mercury we get from eating fish but also reduces the amount of selenium available to us from fish.

Fourth: How does selenium disarm mercury?
Mercury, in the common form of methylmercury, is a toxic agent in fish and in humans.  Methylmercury molecules have a great affinity for binding with selenium in the fish and also in humans.  Mercury has up to one million times more affinity for binding with selenium than with sulfur, the second most common substance with which mercury forms chemical bonds.

Fifth: What is the good and the bad of mercury’s binding with selenium?
In an organism, fish or human, when there is selenium and there is mercury, the mercury will bind with the available selenium, tying the selenium up irreversibly in protein complexes.  Mercury sequesters the selenium, so to speak.

This is both good and bad.

On the one hand, – the good side – the ingested methylmercury is inhibited from having toxic effects of its own.  It is bound up with the selenium in the protein complexes and is deactivated.

On the other hand, – the bad side – the selenium is also bound up and is no longer available for the formation of the essential antioxidant selenoproteins.

As a result, more oxidative stress and oxidative damage occur.  There are not enough selenium-dependent selenoproteins available to stop the damage.

Sixth: Can we eat some varieties of ocean fish?
Fortunately, many edible varieties of ocean fish have high selenium-to-mercury ratios.  There is more than enough selenium in these fish to bind with the mercury and still have some selenium left over when we eat fish.  There is still a little selenium for the synthesis of selenoproteins.

But, notice, there is not as much selenium available as there would be if there were not much mercury in the tissues of the fish.  Mercury is a big depleter of the selenium available for selenoproteins to be formed.

Seventh: What are Dr. Ralston’s Selenium-related Health Benefit Value calculations?
Dr. Ralston’s selenium-related health benefit value calculations are intended for use as a risk assessment tool.  The calculations are based on the molar concentrations of selenium and methylmercury in various types of fish.  The more favorable the ratio of selenium to methylmercury in the fish, the more benefit and the less risk there is in eating the fish [Ralston 2016].

Eighth: Fish for pregnant and breastfeeding women
The selenium-related health benefit values are important not just for the general public but also, especially, for pregnant and breastfeeding women.  Which fish should they avoid?  Which fish should they eat?

Methylmercury can cross the placental and blood-brain barriers, so too much mercury during pregnancy can affect the brain development of the fetus.

At the same time, a lack of selenoproteins can also affect the brain development of the fetus [Gilman 2015].

Mothers’ eating the right kinds of fish every week, fish with high selenium-to-mercury ratios, can help to improve the brain development of infants.  Fish is a good source of omega-3 fatty acids, protein, many vitamins and minerals, and the important trace element selenium.

Ninth: Which ocean fish have high amounts of selenium?
The question is, which ocean fish can be safely eaten, and which ones cannot?

Here is a partial list of ocean fish that have high selenium-to-mercury ratios:

  • cod
  • flounder
  • grouper
  • halibut
  • pollock
  • salmon
  • snapper
  • sole
  • tuna (preferably chunk light)

The FDA has warned against eating some ocean fish that do not have a good selenium-to-mercury ratio:

  • king mackerel
  • marlin
  • orange roughy
  • shark
  • swordfish
  • tile fish
  • tuna, big-eye
  • whale fish

Freshwater fish
Freshwater fish are a bit more problematic.  Typically, the selenium-to-mercury ratio varies from region to region.  It is important to search the Web for reports of local studies of the selenium-to-mercury ratio.  A Google search yields many such local reports.

Fish from lakes and rivers in regions with selenium-poor soil are likely to have higher concentrations of methylmercury because there is less selenium in the fish for the mercury to bind to and become inactive.

Farm fish
Farm fish can be a tricky question.  On the one hand, farm fish might have less exposure to mercury because the farm fish are fed a carefully regimented diet that is based on grains and soy rather than on fish meal that might contain high concentrations of mercury.

On the other hand, many of the fish farms are located in the ocean near the shore where there is run-off of industrial pollutants as well as run-off from agricultural herbicides and pesticides.  The farm fish could still ingest mercury as well as absorb toxic PCBs and dioxins.  Moreover, in many instances, the farm fish are exposed to treatment with antibiotics.

Overall, until the opposite is proven, it is best to regard farm fish as less safe to eat than wild ocean fish.  The Seafood Watch website and app produced by the Monterey Bay Aquarium provide a guide to which seafood is safe and which seafood should be avoided [Harvest of fears].

Summing up: selenium and mercury and eating fish

  • Meals from many ocean fish have considerable nutritional value and should be included in a good dietary plan.
  • Meals from freshwater fish should be regarded with caution unless there is local documentation of the fish’s selenium-to-mercury ratio.
  • Meals from farm fish should be viewed with caution.  It should be up to the seller to prove that the fish is safe to eat, not up to the buyer to take a chance.


Berry, M. J., & Ralston, N. C. (2008). Mercury toxicity and the mitigating role of selenium. Ecohealth, 5(4), 456-459. doi:10.1007/s10393-008-0204-y

Gilman, C. L., Soon, R., Sauvage, L., Ralston, N. C., & Berry, M. J. (2015). Umbilical cord blood and placental mercury, selenium and selenoprotein expression in relation to maternal fish consumption. Journal of Trace Elements in Medicine and Biology, 3017-24. doi:10.1016/j.jtemb.2015.01.006

Harvest of fears: farm-raised fish may not be free of mercury and other pollutants.  (2016).  Scientific American.  Retrieved from

Ralston, N. C., Blackwell, J. 3., & Raymond, L. J. (2007). Importance of molar ratios in selenium-dependent protection against methylmercury toxicity. Biological Trace Element Research, 119(3), 255-268.

Ralston, N. C., & Raymond, L. J. (2010). Dietary selenium’s protective effects against methylmercury toxicity. Toxicology, 278(1), 112-123. doi:10.1016/j.tox.2010.06.004

Ralston, N. C., Ralston, C. R., & Raymond, L. J. (2016). Selenium Health Benefit Values: Updated Criteria for Mercury Risk Assessments. Biological Trace Element Research, 171(2), 262-269. doi:10.1007/s12011-015-0516-z

Disclaimer: The information presented in this review article is not intended as medical advice and should not be used as such.

Leave a Reply

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