Selenium’s potential as a mercury antagonist and the problem with mercury in fish
Selenium supports a number of different selenium-dependent proteins that are important for our energy turnover, metabolism, immune defense, fertility, and antioxidant protection. Selenium also has a special affinity for mercury and is therefore able to bind to this heavy metal and counteract its harmful impact on the brain, the nervous system, and other tissues. Once selenium is bound to mercury, however, the different selenoproteins are no longer able to use it. We are all exposed to a certain amount of mercury and that may result in a borderline deficiency of selenium. The problem is that other factors weigh in such as selenium-depleted crops because of the lack of selenium in the European farmland. What is important to realize is that mercury toxicity is insidious and certain fish such as predatory fish and whales in the upper part of the food chain contain large concentrations of mercury. However, therapeutic doses of selenium can prevent the toxic effect of the heavy metal, according to a new review article published in Scientific Research.
The accumulation of mercury in the food chain
Mercury (Hg) forms a number of different organic and inorganic compounds that are toxic to humans. Fossil fuels, incineration plants, gold extraction and other manmade activities have increased mercury concentrations in the atmosphere by 300 percent since the eighteen-hundreds. When the element form of selenium is oxidized, inorganic and water-soluble forms are formed and they spread to the surrounding environment by way of rainfall or aqueous soil. Afterwards, bacteria are able to methylate these selenium compounds, thereby forming methyl mercury, a particularly harmful, organic form that is highly toxic and accumulates through the food chain.
The concentration of mercury in foods such as fish may therefore become a health problem to many people and animals, and there are even other sources of this dangerous heavy metal. Mercury can easily pass through the blood-brain-barrier and accumulate in the brain. Also, mercury can accumulate in the thyroid gland, the liver, the kidneys, and in other tissues that have relatively high concentrations of selenium – simply because of the special relationship between selenium and mercury. We will talk more about that later.
Fetuses are particularly vulnerable because of the rapid cell division and because selenium is of vital importance to brain development and the nervous system.
Mercury is also a free radical source. Free radicals are aggressive molecules that attack our cells and cause oxidative stress.
Mercury poisoning is often insidious
Acute mercury toxicity causes stomach pain that is accompanied by vomiting, diarrhea, circulatory shock, and impaired kidney function. There is even evidence to suggest that chronic mercury toxicity is a much more serious health threat that is caused by long-term mercury exposure from amalgam fillings, fish consumption etc. Also, there is the risk that a possible diagnosis is not associated with the actual cause. Mercury exposure, especially if you are selenium-deficient, may result in a host of different symptoms (as shown below). Nonetheless, people are rarely tested for chronic mercury toxicity.
Typical signs of mercury poisoning (where other factors have been excluded)
Lower IQ (especially if the expecting mother was poisoned with mercury during her pregnancy)
Death – in the case of exposure to lethal doses
Research has also demonstrated that mercury can affect the development of other diseases such as sclerosis, fibromyalgia, cardiovascular disease, dementia, Alzheimer’s disease, autism, pelvic diseases, breast cancer, and rheumatism.
Selenium is an essential trace element that supports around 25 different selenium-containing proteins that are important for our energy turnover, immune defense, metabolism, and fertility. Selenium also supports some very powerful antioxidants (GPX) that protect the cells against oxidative stress caused by free radicals. Multiple studies show that selenium deficiency can increase the risk of infections, thyroid disorders, cardiovascular disease, fertility problems, and cancer.
European farmland generally contains very little selenium. An estimated 20 percent of Danes get less selenium than the recommended amount of 55 micrograms per day. This reference intake level is probably too low to begin with, especially for people who have too much mercury in their system.
Interactions between mercury and selenium
The toxic damage caused by mercury is primarily a result of the disrupted selenium metabolism. It turns out that chemical resemblance between mercury, selenium, and sulfur can cause serious damage at a cellular level because certain sulfur-containing molecules (thiols) have the ability to transport mercury into the cells. Here, mercury can disrupt or terminate many of the selenoproteins’ vital functions. In a sense, you can compare mercury to a young cuckoo bird that settles inside the cells.
The body’s selenium stores are a biochemical target for mercury, and it is estimated that mercury’s affinity for selenium is one million times greater than mercury’s affinity for sulfur.
For that reason, scientists believe that the symptoms of mercury poisoning occur as a result of exposed cells and tissues lacking selenium that is needed to support important enzyme processes and serve as a protective antioxidant.
On the other hand, selenium forms a chemical bond with mercury by forming mercury selenide that neutralizes mercury’s harmful impact. Selenium has a strong affinity for mercury but once it is bound to mercury it is no longer available to the many selenium-dependent proteins that are important for enzyme functions, metabolic processes, and protection against oxidative stress.
In other words, when you are exposed to mercury in toxic amounts it increases your need for selenium.
The problem with mercury in shellfish, fish, and whales’
Fish and shellfish contain protein, selenium, zinc, omega-3 fatty acids, and other useful nutrients. As mentioned before, mercury tends to accumulate through the food chain. It is therefore safer to eat plaice, tonguefish, flounder, cod, pollock, herring, anchovies, and salmon that are in the lower part of the food chain and have a more favorable selenium-mercury ratio. Also, small tuna contains far less mercury than large tuna. Most maritime food sources contain a higher amount of selenium than mercury – expect from whale, tarpon, marlin, and certain sharks. A study of pilot whales, which is consumed a lot in the Faroe Islands, showed that it has a negative impact because of the large mercury concentrations. It is all a matter of the balance between selenium and mercury in fish and other types of seafood. Still, according to the new review article, there is more to the story. Even a 1:1 ratio between selenium and mercury can cause toxicity if selenium and the mercury in the fish or fish consumer fail to form a bond and prevent damage.
The majority of fish are assumed to be safe for consumption because their selenium content is adequately high. However, one should realize that fish at the top of the food chain also accumulate other environmental toxins, and many mussels and fish from the Baltic Sea exceed the EU’s threshold levels.
The Danish Veterinary and Food Administration recommends for children younger than 14 and women trying to conceive or those who breastfeed not to consume fresh cuts of large predatory fish such as tuna. Also, they should avoid eating canned white tuna and albacore tuna. Children under the age of three years should avoid tuna, altogether, as their brains are still developing. The new dietary guidelines are based on studies that show that one in four children risks brain damage because of accumulated mercury.
Mercury detoxification and compensation for reduced selenium intake
Selenium supplements can increase mercury detoxification and compensate for the lack of selenium in our diets. It is best to take selenium yeast with many different selenium species because this emulates the natural selenium variety in a diet with many different selenium sources.
It is considered safe to take up to 300 micrograms of selenium per day. Higher doses, for instance in connection with mercury detoxification, should only be ingested in consultation with a physician
Important selenium-containing compounds
|Deiodinase type 1-3
|GPX 1-6 (glutathione peroxidase)
|Regulation of cytokines and inflammatory response in cells
|Antioxidant and selenium transportation in the body
|Selenoprotein R and N1
|Antioxidants with several functions
|Large concentrations in the brain. The functions are not properly known
|Supports cell structure and proteins
|Antioxidants, mitochondria, energy turnover, and metabllism
|Repairs oxidative damage
|Even minor selenium deficiency may result in suboptimal selenoprotein performance
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
Aparna P. Shreenath; Jennifer Dooley. Selenium, Deficiency. NCBI October 27, 2018
Hilten T Mistry et al. Selenium in reproductive health. Journal of Obstetrics and Gynaecologi. 2011
Malene Outzen et al. Selenium status and risk of prostate cancer in a Danish population. British Journal of Nutrition 2016
New Links between selenium and cancer prevention. HRB. December 2017
Clark LC et al: Effects of Selenium Supplementation for Cancer Prevention in Patients with Carcinoma of the Skin. JAMA: 1997.
Craig Weatherby. Mercury-Fighting Mineral in Fish Overlooked in Heated Debate. Vital Choice Wild Seafood and Organics 2006
DR-dokumentar: De ufødte børn 03-11-2014
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