The health benefits of eating fish outweigh the health risks. This is especially true when we have sufficient dietary selenium intake and sufficient selenium status to 1) give a protective effect against methylmercury and 2) serve other biological purposes [Jorge 2024].
To varying degrees, all fish contain some mercury. The mercury in fish and seafood is primarily in the form of methylmercury. Unchecked, the methylmercury targets the nervous system. It disrupts cellular processes. Too much exposure can lead to a loss of muscular coordination and to a loss in cognitive function. Accordingly, the presence of mercury could be a health risk for people who eat fish regularly [Jorge 2024].
Fortunately, many edible fish contain selenium in molar concentrations adequate to sequester the methylmercury in the fish and to render it less harmful. Ralston & Raymond have developed criteria for the calculation of selenium health based values. The calculations are based on the relative molar concentrations of mercury and selenium in various types of fish [Ralston 2016].
Selenium Binds with Mercury
Selenium has an extraordinary ability to form stable complexes with mercury. Consequently, selenium can reduce the extent of mercury’s targeting and disrupting cellular activity. Selenium can induce increased excretion of mercury. Thus, selenium, which is a common component in fish and other seafood, is a mitigating factor against mercury toxicity [Jorge 2024].
Note that the US Food and Drug Administration has prepared a chart showing which varieties of fish are best choices, good choices, and choices to avoid when eating fish.
Selenium and Methylmercury Interaction
For several years now, nutrition scientists have thought that an especially harmful effect of mercury is that the mercury binds up the available selenium and produces a selenium deficiency state in the body. The selenium deficiency results in a lack of selenoproteins for necessary biological roles in the body. These biological functions include but are not limited to involvement in DNA synthesis, reproduction, thyroid hormone metabolism, and protection against oxidative damage and infection. Given the mercury-induced deficiency state, selenium supplementation may be necessary to restore adequate selenium status and to mitigate the toxicity of mercury [Spiller 2018].
In 2024, Jorge et al conducted a review of the available research relating to the interaction between selenium and mercury. They have focused on peer-reviewed English-language studies that discuss selenium’s protective mechanisms against methylmercury toxicity [Jorge 2024].
The major findings from the literature review are the following:
- People are exposed to methylmercury when they eat fish and other seafood.
- Too much exposure to mercury can have a toxic effect on the nervous system and the immune system of adults.
- Too much exposure to mercury can adversely affect the development of the fetus and the child early in life. Methylmercury passes through the placental barrier and accumulates in the fetus.
- Selenium has a high chemical affinity for mercury and binds easily with mercury to form harmless stable compounds. The selenium-mercury complexes that form are much less toxic than mercury alone.
- Even so, the binding of selenium with mercury makes selenium less available for incorporation into selenoproteins via selenocysteine.
- The lack of selenium that results from the binding of selenium with mercury deprives the cells of the ability to synthesize needed selenoproteins such as the glutathione peroxidases, the thioredoxin reductases, and the selenoprotein P.
- Ergo, individuals who eat fish regularly may well need a selenium supplement to compensate for the loss of the selenium that is bound to mercury and not available for other purposes.
Effect of Cooking on Mercury Bioavailability and Toxicity
Interestingly, frying or boiling fish can remove some mercury, thus rending the mercury from fish somewhat less harmful.
However, frying and boiling fish will also affect adversely the selenium content of the fish. This is important because it is the selenium in the fish that reduces the intake of methylmercury from fish.
Furthermore, cooking also depletes the supply of selenium in other foods as well, e.g., in potatoes and in cereals.
Conclusion: Selenium Protects Against methylMercury in Fish
Yes, it is important to be alert to mercury contamination in fish and seafood.
However, adequate selenium intake and status protects against the risk of methylmercury toxicity.
Selenium has an unusual ability to bind to mercury and to reduce mercury’s negative impact on human health.
But, the binding of selenium to mercury reduces the bioavailability of selenium and selenoproteins for other biological functions in the body.
Thus, selenium supplementation may be necessary to ensure an intake greater than the amount of selenium needed to sequester the methylmercury in fish and seafood.
Sources
Jorge AO, Chamorro F, Carpena M, Echave J, Pereira AG, Oliveira MB & Prieto MA. Protection of selenium against methylmercury in the human body: a comprehensive review of biomolecular interactions. Biol Life
Sci Forum. 2024;35:8.
Ralston NVC, Ralston CR, Raymond LJ. Selenium health benefit values: updated criteria for mercury risk assessments. Biol Trace Elem Res. 2016 Jun;171(2):262-269.
Spiller HA. Rethinking mercury: the role of selenium in the pathophysiology of mercury toxicity. Clin Toxicol (Phila). 2018 May;56(5):313-326.
US Food and Drug Administration. Advice about Eating Fish. 5 March 2024. Retrieved from https://www.fda.gov/food/consumers/advice-about-eating-fish.
The information presented in this review article is not intended as medical advice. It should not be used as such.