What should we know about eating fish and exposure to mercury in fish?
Fact: Most fish contain mercury. Mercury is a toxic substance. Fortunately, most fish contain both selenium and mercury. On a molar basis, most edible fish contain more selenium than mercury. That makes it safe to eat most fish [Ralston 2024; Ralston 2016].
The selenium in fish binds tightly to mercury. The binding forms stable complexes that prevent mercury from damaging our cells. When there is more selenium than mercury (on a molar basis), this binding effectively neutralizes mercury’s toxicity. As a result, the mercury becomes biologically inactive and far less harmful when we eat the fish [Ralston 2023].
Even so, having just enough selenium available to neutralize the mercury exposure is not good enough. Logically, there must be a surplus of selenium. Without a surplus of selenium, there will not be enough selenium to synthesize antioxidant selenoenzymes. And, we need the antioxidant selenoenzymes to counteract the harmful free radicals in the brain. Providing more selenium, via supplementation if necessary, keeps selenoenzymes doing their antioxidant job.
How Do We Know if We Need Supplementary Selenium?
The selenium researcher Nick Ralston has developed a health-benefit value metric to determine whether it is safe to eat certain fish. Basically, Ralston’s metric subtracts moles of mercury in a fish from moles of selenium. Accordingly, if the result is positive, then there is selenium surplus. This corresponds to protective potential. However, if there is a negative result, then there is a higher potential risk [Ralston 2016].
What Fish are Generally Safe to Eat?
Using Ralston’s metric shows that the following fish are generally safe to eat:
- Anchovies
- Atlantic mackerel (not king mackerel)
- Cod
- Herring
- Salmon (especially wild salmon)
- Sardines
What Fish Should We Avoid?
Fish that are more likely to have high mercury content relative to selenium include the following types:
- King mackerel
- Shark
- Swordfish
- Tile fish (Gulf of Mexico)
- Large tuna (e.g., big-eye tuna)
Why is a Surplus of Selenium over Mercury Necessary?
The thing to note here is that Ralston is not basing his health-benefit value solely on the mercury content in the fish [Ralston 2016]. For the most part, he is concerned with the molar ratio of selenium to mercury in fish. In a 2024 study, Ralston et al showed that maternal consumption of typical varieties of ocean fish provided substantial amounts of selenium. The selenium from the fish protects against mercury-binding losses in selenium bioavailability [Ralston 2024].
The important point that Ralston makes is that selenium must be available in surplus to mercury. If there is only enough selenium to neutralize the mercury, that is good but not sufficient. For the synthesis of antioxidant selenoenzymes, there must be selenium available beyond the selenium needed to bind with mercury. Otherwise, there will be oxidative damage to cells and tissues [Ralston 2020; Ralston 2018].
Should Pregnant and Lactating Women Eat Fish Regularly?
An earlier consensus was that mercury from the mother’s eating ocean fish during pregnancy could harm children. Researchers discouraged pregnant women from eating fish. However, studies investigating over 200,000 mother-child pairs show something different [Ralston 2023]:
- Increased consumption of ocean fish during pregnancy boosts the child’s IQ.
- Not consuming ocean fish during pregnancy lowers the child’s IQ by 4-6 points and even up to 10 IQ points.
Conclusion: Selenium and Mercury in Fish
Fortunately, most ocean fish provide enough selenium to keep mother and children from being harmed my mercury toxicity.
However, a surplus of molar selenium over molar mercury is necessary for antioxidant selenoenzymes to form and provide antioxidant protection in the brain.
The selenium:mercury ratio plays a pivotal role in determining the benefits and risks associated with eating seafood.
Sources
Ralston NVC et al. Maternal seafood consumption is associated with improved selenium status: Implications for child health. Neurotoxicology. 2024 Mar;101:26-35.
Ralston NVC, Raymond LJ. Net effects explains the benefits to children from maternal fish consumption despite methylmercury in fish. Neurotoxicology. 2023;99:195-205.
Ralston NVC, Raymond LJ. Mercury’s neurotoxicity is characterized by its disruption of selenium biochemistry. Biochim Biophys Acta Gen Subj. 2018;1862(11):2405-2416.
Ralston NVC, Ralston CR, Raymond LJ. Selenium health benefit values: updated criteria for mercury risk assessments. Biol Trace Elem Res. 2016;171(2):262-269.
Ralston NVC, Raymond LJ. Dietary selenium’s protective effects against methylmercury toxicity. Toxicology. 2010;278(1):112-123.
Raymond LJ & Ralston NVC. Mercury: selenium interactions and health implications. NeuroToxicology. 2020;81:294-299.
The information provided in this review article is not intended as medical advice. It should not be used as such.