Selenium’s protective role against mercury

We are all exposed to mercury to varying degrees. Mercury is particularly damaging to the nervous system and make us more vulnerable to several diseases. Fetuses and small children are especially at risk, as mercury exposure can also cause lower IQ. A review published in Biology and Life Sciences Forum shows that selenium protects against mercury. However, this protection depends on a sufficient selenium intake, as this vital trace element is also required for vital body functions such as energy metabolism, thyroid function, immune defense, fertility, and much more.

Mercury comes from many sources such as air pollution (from coal combustion, waste incineration, crematoria, gold mining, etc.), industrial wastewater, amalgam fillings, certain vaccines, thermometers, and batteries. Once released into the atmosphere, mercury circulates globally and is converted by bacteria in aquatic environments into methylmercury - a compound that accumulates in the food chain and is particularly harmful to health. Large predatory fish and whales are known to contain high levels of methylmercury. Concentrations can also vary greatly within the same species depending on age and habitat.
In higher doses, methylmercury can damage the central nervous system, the circulatory system, and the immune system. The current review article describes why mercury is among the most dangerous environmental toxins. A key reason is that mercury interacts with the trace element selenium, which is part of about 25-30 selenoproteins of key importance to energy metabolism, thyroid function, immune defense, fertility, and more. Selenium is also a component of important antioxidants that protect cholesterol and cells from damage caused by free radicals and oxidative stress. Due to these properties, selenium also helps protect against atherosclerosis and cancer.
Mercury toxicity occurs because certain sulfur-containing molecules can transport mercury into the cells, where it disrupts or blocks the function of selenium-dependent proteins. Therefore, symptoms of mercury poisoning are believed to arise mainly because cells cannot utilize selenium. Mercury interacts, for instance, with the selenoprotein thioredoxin reductase (TrxR), which is crucial for cellular energy metabolism, thyroid regulation, and antioxidant defense. Mercury also binds to other key selenoenzymes such as glutathione peroxidases (GPXs).
This means that mercury can represent an underlying factor in many diseases and symptoms, both by disturbing cellular metabolism and by increasing vulnerability to oxidative stress. Long-term exposure to mercury from various sources therefore represents a major health problem, potentially leading to fatigue, headaches, poor concentration, mood swings, nervousness, depression, anxiety, recurrent infections, chronic inflammation, abdominal pain, thyroid imbalances, muscle and joint pain, and allergic reactions. Children are at risk of lower IQ if the mother is exposed during pregnancy. Yet mercury poisoning is rarely diagnosed, and many cases are misdiagnosed.

How selenium protects against mercury

The review explains the molecular mechanisms by which mercury accumulates in the body and interacts with selenium. The authors also describe how selenium protects against mercury by binding to methylmercury, forming a less toxic compound - mercury selenide - which the body can excrete via the urine. However, selenium bound in this way is no longer available as a component for essential selenoproteins and antioxidants. Therefore, mercury exposure increases the body’s selenium requirement so that there is enough both for detoxification and for maintaining normal selenium-dependent processes.
Studies on zebrafish exposed to mercury showed that selenium supplementation reactivated the TrxR enzyme and reduced mercury accumulation by half. Selenium supplements have also been found to lower mercury levels in the kidneys and promote excretion through the blood while boosting GPX antioxidant activity. Other animal studies suggest that organic selenium forms such as selenium yeast are significantly more effective than inorganic selenite.

Mercury exposure and selenium deficiency - a dangerous combination

As mentioned, we are all exposed to mercury to some extent. This problem is aggravated by widespread selenium deficiency in the EU and other regions due to depleted soils and reduced consumption of fish and organ meats. The combination of mercury exposure and selenium deficiency is therefore a dangerous cocktail.
The official daily reference intake (RI) for selenium is 75-90 micrograms, but studies show that about 100 micrograms per day are needed to saturate selenoprotein P, which is a marker of selenium status in the blood that is converted into other selenium compounds in the cells. Mercury detoxification may require higher amounts. According to EFSA (the European Food Safety Authority), the upper safe intake level is 300 micrograms per day, while WHO sets it at 400 micrograms.

Be aware of the selenium-to-mercury ratio in fish

Because mercury accumulates up the food chain, seafood such as shellfish, herring, plaice, cod, salmon, and mackerel, all of which are lower on the food chain, generally contain less mercury. These species tend to have a favorable selenium-to-mercury ratio and provide enough selenium to bind mercury and support selenoprotein function. Previous studies have shown that the selenium content in most fish is sufficient to make them safe to eat, even for pregnant women.
Larger fish such as tuna, pike, and perch, as well as seals and whales, generally contain far more mercury relative to selenium and should therefore be limited or avoided. Health authorities recommend that pregnant and breastfeeding women avoid large predatory fish like tuna altogether. Still, most fish are considered beneficial during pregnancy due to their content of other valuable nutrients. Cooking and frying fish can also help reduce mercury content.
New measures such as the Selenium Benefit Value (HBV-Se) and Benefit-Risk Value (BRV) have been introduced to assess the selenium-to-mercury balance in fish and seafood - a key factor for human health.

• Mercury is among the most dangerous environmental toxins
• Our selenium intake affects mercury’s toxicity

References:

A.O.S. Jorge et al. Protection of Selenium Against Methylmercury in the Human Body: A Comprehensive review of Biomolecular Interactions. Biology and Life Sciences Forum 2025

Michael Gochfeld, Joanna Burger. Mercury interactions with selenium and sulfur and the relevance of SE: HG molar ratio to fish consumption advice. Environ Sci Pollut res Int. 2021

Rosewell Timmerman, Stanley Omaye. Selenium´s Utility in Mercury Toxicity: A Mini-Review. Scientific Research. 2021

Nicholas V.C. Ralston, Laura J. Raymond. Mercury´s neurotoxicity is characterized by its disruption of selenium biochemistry. 2018