HOW SELENIUM PROTECTS AGAINST MERCURY

Authored by: Shadae Zamyad, Nutritionist

For many people, the mention of mercury in seafood raises immediate concerns about potential health risks. Mercury, a naturally occurring element, can accumulate in the environment and find its way into the aquatic food chain. Industrial processes, such as coal-fired power plants and mining operations, have contributed to increased mercury levels in our atmosphere and land, which finds its way to our oceans and waterways. As a result, fish and other seafood can absorb and accumulate mercury in their tissues, leading to the worry that consuming these foods might pose a threat to our well-being.

Food policy and public health organizations have recognized the need to provide guidance on safe seafood consumption, particularly for vulnerable populations such as pregnant women and young children. The U.S. Food and Drug Administration (FDA) and the Environmental Protection Agency (EPA) have issued recommendations advising vulnerable consumers to choose fish species with lower mercury levels and to limit their intake of high mercury such as king mackerel, shark, and swordfish. These guidelines aim to help people make informed choices about seafood consumption while still reaping the numerous health benefits that fish provide.

However, it's crucial to approach the topic of mercury in seafood with a balanced perspective, considering the most current scientific research. While the presence of mercury in fish is a valid concern, it's equally important to understand the role of another essential nutrient found in seafood: Selenium. Selenium, which is a trace mineral, has been shown to have a protective effect against mercury toxicity, offering us a natural defense mechanism that can help mitigate potential health risks.

We'll explore scientific findings that shed light on how these two elements interact within the body and why the presence of selenium in fish can help alleviate concerns about mercury exposure. By understanding the bigger picture of seafood nutrition, we can make more informed decisions about incorporating fish into our diets, appreciating its role as an excellent source of protein, Omega-3 fatty acids, and other essential nutrients!

Mercury in Seafood: Should You Be Concerned?

Mercury poisoning from seafood consumption is a topic that often sparks unnecessary alarm, yet the reality for most Americans is far less threatening than sensationalist headlines suggest. While mercury is a legitimate environmental contaminant, the typical American diet and seafood consumption patterns make severe mercury toxicity extraordinarily rare. This is largely because the most commonly consumed seafood in the U.S. like salmon, shrimp, tilapia, pollock, and canned tuna, are all relatively low in mercury. Even better, these popular choices are high in selenium, which helps protect against mercury toxicity. The real health risk for most Americans isn't eating too much fish - it's eating too little and missing out on the crucial nutrients that seafood provides. And that is a truly widespread issue - according to the USDA over 90% of Americans currently don’t consume enough fish to meet even the minimum recommendation of 2 portions (12oz) of seafood per week.

Mercury blood test vial

The key to understanding mercury exposure lies in understanding both the source and quantity of consumption. The Environmental Protection Agency (EPA) and Food and Drug Administration (FDA) have established comprehensive guidelines that help consumers make informed choices. Most commercially available fish in the United States such as salmon, tilapia, and cod contain relatively low mercury levels. Even species with higher mercury content, like certain types of tuna, can be safely consumed in moderation. For the average American adult, the risk of mercury poisoning is so minimal that health experts recommend increasing seafood consumption for its nutritional benefits. The Omega-3 fatty acids, lean protein, and essential nutrients found in fish far outweigh the potential risks of trace mercury exposure. Pregnant women and young children are advised to be slightly more cautious, choosing low-mercury fish and limiting consumption of high-mercury species like shark, swordfish, and king mackerel.

Why some fish have higher mercury levels than others comes down to three key factors: their diet, lifespan, and fat content. Mercury builds up through the food chain in a process called bioaccumulation. Small fish absorb small amounts of mercury from their environment, then larger predatory fish eat many smaller fish over their lifetime, accumulating all that mercury in their own tissues. This is why top predators like shark and swordfish typically have higher mercury levels than fish that feed lower on the food chain, like sardines or herring. Lifespan also plays a crucial role. Species that live longer, like yellowfin and bigeye tuna have more time to accumulate mercury compared to shorter-lived species like salmon or sole. Finally, since mercury is stored in fatty tissue, species with higher fat content tend to accumulate more mercury over time. This explains why the fatty blubber of marine mammals contains particularly high mercury levels, while lean fish like Pacific Dover Sole or cod maintain relatively low mercury levels despite their size.

 

Understanding Selenium's Protective Mechanism

How Selenium Protects Against Mercury Poisoning

Relatively recent research carried out over the last decade has shown that selenium acts as a biochemical shield against mercury's toxic effects through a fascinating molecular interaction that has implications for human health. At the cellular level, selenium forms strong chemical bonds with mercury, creating stable complexes that prevent the heavy metal from causing oxidative damage to our cells. Dr. Nicholas Ralston, a leading researcher in mercury toxicology, explains this mechanism as a "molecular neutralization" process (Ralston et al., 2016). In practical terms, this means that selenium effectively traps mercury, rendering it less harmful to our body's delicate cellular structures. Dr Ralston’s groundbreaking study revealed that selenium contains specific enzymes that can actively counteract mercury's potential cellular damage (Ralston et al., 2016). These enzymes function like microscopic bodyguards, protecting our cells from mercury's destructive potential.

For the average person, this translates to some reassuring dietary insights. Many common foods naturally rich in selenium can help mitigate mercury exposure. There are Brazil nuts, of course, which are extraordinarily high in selenium, with just one nut providing more than the daily recommended intake. But be careful not to eat too many Brazil nuts (we all know how easy it is to eat handfuls and handfuls of nuts), as selenium consumed in such high doses can be harmful too. Seafood, on the other hand, provides selenium in just the right dosage. Species like Dover Sole, tuna, salmon, and halibut provide fantastic selenium levels to buffer against any potential mercury risks (Peterson et al.,2021). 

 

The Answer is in the Selenium : Mercury Ratio

Our understanding is now that it’s not the mercury levels in fish per se, but really the ratio of selenium to mercury that we need to look at. Think of selenium and mercury in fish like a balance scale - when there's more selenium than mercury, the potential toxicity is neutralized. The selenium helps shield your body from mercury's unwanted effects, while also supporting immune system function, support reproductive health, and helps maintain brain and cognitive function. Reassuringly, research from the University of North Dakota has shown that in many fish species, selenium levels exceed mercury concentrations, creating a natural protective mechanism (Ralston et al.,2016). In fact, most fish come with this in-built solution to mercury toxicity. Nature is fascinating that way.

Ocean fish are generally your safest bet. They tend to have more stable and healthy levels of selenium compared to freshwater fish, much like how a well-balanced ecosystem works. Most popular fish at your grocery store, like, salmon, cod, trout and haddock, as well as West Coast local species like Dover Sole, Petrale Sole, Rockfish and Lingcod have this protective selenium advantage. These fish are like hitting the nutritional jackpot. They're packed with selenium, high-quality protein, and those important Omega-3 fats that support brain and heart health, while keeping mercury levels low. Think of these as your "green light" fish that you can eat regularly without worry. Even fish that sometimes get a bad rap for mercury, like bluefin tuna, often have enough selenium to help protect you. However, it's still smart to mix up your fish choices rather than eating the same type every day. Imagine it like varying your exercise routine. You get better overall benefits when you don't do the same thing all the time!

Finally, we need to caveat, that it's not just about consuming lots of selenium. The body's ability to utilize selenium depends on various factors, including overall diet, individual metabolism, and nutritional intake. A comprehensive study in the Journal of Trace Elements in Medicine and Biology demonstrated that selenium's protective effects are most potent when selenium was consumed as part of a balanced diet, rather than through isolated supplements (Vinceti et al., 2020).

 

The Risk vs Benefits of Eating Seafood 

The scientific evidence supports the benefits of seafood consumption, with numerous large-scale studies and meta-analyses demonstrating that the health advantages far outweigh any potential mercury concerns. A landmark analysis in JAMA analyzed data from over 250,000 people, found that just two servings of fish per week could reduce the risk of death from heart disease by 36% (Mozaffarian et al.,2006). The FAO/WHO Expert Committee review examined data from over 20 countries and made a meaningful discovery. Countries where people eat more fish, like Japan and Mediterranean countries, consistently showed better cardiovascular health, cognitive function, and lower rates of depression (WHO et al.,2015). 

What makes these findings particularly compelling is that these benefits persist even in populations consuming fish species with higher mercury levels. That is largely due to the protective effects of selenium and the presence of Omega-3 fatty acids (Berry et al., 2008). A landmark Seychelles Child Development Study published in Neurotoxicology followed children for over 30 years in a population that eats about 10 times more fish than Americans do, found that even in populations consuming large amounts of fish, the presence of selenium and other nutrients provided protection against potential mercury effects. In fact, they saw consistent benefits in child development despite, or really because of, their high fish consumption (Shamlaye et al., 95).

parent with child cooking fish

The health benefits of regular fish consumption for our brains and neurological function are remarkable and wide-ranging. Omega-3 fatty acids, particularly EPA and DHA found in fish, are crucial for brain development, with studies showing improved cognitive function in both children and adults. Research published in Nutrients found that people who regularly ate fish had better overall health outcomes. Even just adding anchovies to their pizza or having a fish soup occasionally resulted in better brain function (Mesa et al.,2021). Another interesting study published in the American Journal of Nutrition discovered that regular fish eaters had a brain that was functionally about three years younger, on average, than those who rarely ate fish, translating to better memory for daily tasks and sharper cognitive skills (Zhang et al.,2015). 

Lastly, during pregnancy, Omega-3s are vital for fetal brain and eye development, which translates to measurably higher IQ scores and better development throughout childhood into adulthood. A study published in the American Journal of Nutrition showed that mothers who ate fish 2-3 times per week during pregnancy had babies with better early development scores, including improved attention spans and language skills (Oken et al., 2008). 

 

Eat Seafood with Confidence

As we look to the future of nutrition and public health, the message becomes clear: seafood isn't just safe to eat; it's essential for optimal health. By making informed choices about the types and variety of fish we consume, we're not just following dietary guidelines; we're tapping into an ancient wisdom embedded in marine life itself. The goal isn't to merely avoid risks but to actively embrace the tremendous benefits that regular seafood consumption provides. This understanding empowers us to make confident, informed decisions about our diet. Whether you're preparing meals for your family, planning for pregnancy, or investing in your long-term health, the science stands firm. Including a variety of seafood in your weekly diet is one of the most powerful steps you can take toward optimal health and well-being. 

Here are some practical ways to feel confident about your fish consumption:

How Much Seafood Should We Eat?

  • Focus on variety: Eat different types of fish throughout the week

  • Choose well-studied options: Fish such as Dover sole, salmon, sardines, and trout are excellent choices

  • Remember the selenium shield: Most ocean fish have more selenium than mercury, providing natural protection

  • Trust the research: Studies involving hundreds of thousands of people show the benefits of seafood far outweigh its risks

  • Follow recommended servings: Aim for 2-3 servings of fish per week

The bottom line? The extensive research shows that the real risk isn't from eating fish; it's from eating too little fish and missing out on their incredible health benefits. Regular fish consumption is linked to better heart health, improved brain function, and stronger early development in children. By understanding these facts, you can enjoy your seafood with confidence!

 
 
 

CONTINUE EXPLORING:

Common Questions

  • Some fish species — like king mackerel, shark, and swordfish — contain elevated levels of mercury, a naturally occurring element that becomes concentrated through environmental pollution. Mercury released into air and water accumulates in oceans and lakes, where it works its way up the food chain. As a rule of thumb, animals higher in the food chain tend to have higher mercury levels. That’s because the small amounts of mercury contained in smaller filter-feeding species get absorbed and concentrated by the larger predatory fish, leading to higher mercury concentrations in their tissues. Moreover, longer-lived animals tend to have higher mercury levels than shorter-lived species as they have more time to accumulate mercury in their tissues. And lastly, since mercury is stored in fat tissue, fattier species tend to have more mercury than lean species. (That’s why native people who eat whale or seal blubber often have elevated mercury levels.)

    The FDA and EPA advise limiting the consumption of these high-mercury fish, especially for pregnant women and young children. But it is not advised to avoid seafood altogether, because of the benefits of the many essential nutrients contained in fish, especially Omega-3s. Wild-caught ocean fish like salmon, cod, trout, and haddock—and local West Coast options like Dover Sole, Petrale Sole, or Lingcod — are low in mercury and high in health benefits. These species offer a nutritional bonus: they’re rich in high-quality protein, heart and brain-healthy Omega-3s, and selenium (which may help counter mercury’s effects). To understand more about how selenium protects against mercury poisoning, watch this short video by neuroscientist Dr Hibbeln. 

  • Fish that are high in mercury include shark, swordfish, king mackerel, bigeye tuna, and marlin. These larger species, which sit higher on the food chain and have longer lifespans, accumulate mercury over time through a process called bioaccumulation. Small fish absorb trace amounts of mercury from their environment, and when larger predatory fish consume many of these smaller fish, the mercury builds up in their own tissues. When we eat these fish, the mercury can bind to selenium in our bodies. If there’s more mercury than selenium available, it can cause a selenium deficiency, which can disrupt cellular functions and lead to toxicity.

    Fish with much lower mercury levels—and higher concentrations of selenium and Omega-3s—are recommended, especially for pregnant women and children. These include salmon, cod, trout, and haddock, as well as West Coast local species like Dover Sole, Petrale Sole, Rockfish, and Lingcod. These species tend to have higher selenium concentration along with high-quality protein and essential Omega-3 fatty acids that support brain and heart health.

  • While fish is a great source of protein and Omega-3 fatty acids, it also contains mercury - a valid concern, as mercury poisoning can lead to neurological disfunction. But it's equally important to understand the role of selenium, a trace mineral contained in great concentrations in most commonly consumed seafood. Selenium has been shown to have a protective effect against mercury toxicity, offering us a natural defense mechanism that can help mitigate potential health risks.  Selenium can bind with mercury, reducing its potential to cause harm in the body. 

    Some fish used in sushi like tuna, swordfish, and mackerel have higher mercury levels, and should be avoided by pregnant women and children. For safer sushi options choose wild-caught seafood with naturally higher selenium-to-mercury ratios such as salmon, shrimp, or West-coast species like Dover Sole and Chilipepper Rockfish. These allow you to enjoy the benefits of seafood while minimizing mercury exposure. 

 

References

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Beijer K, Jernelöv A. Ecological aspects of mercury--selenium interactions in the marine environment. Environ Health Perspect. 1978 Aug;25:43-5. doi: 10.1289/ehp.782543. PMID: 569051; PMCID: PMC1637184.

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Mesa, M. D., Gil, F., Olmedo, P., & Gil, A. (2021). Nutritional Importance of Selected Fresh Fishes, Shrimps and Mollusks to Meet Compliance with Nutritional Guidelines of n-3 LC-PUFA Intake in Spain. Nutrients, 13(2), 465. https://doi.org/10.3390/nu13020465

Mozaffarian D, Rimm EB. Fish Intake, Contaminants, and Human Health: Evaluating the Risks and the Benefits. JAMA. 2006;296(15):1885–1899. doi:10.1001/jama.296.15.1885

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Raymond LJ, Ralston NVC. Mercury: selenium interactions and health implications. Neurotoxicology. 2020 Dec;81:294-299. doi: 10.1016/j.neuro.2020.09.020. Epub 2020 Oct 14. PMID: 35587137.

Shamlaye CF, Marsh DO, Myers GJ, Cox C, Davidson PW, Choisy O, Cernichiari E, Choi A, Tanner MA, Clarkson TW. The Seychelles child development study on neurodevelopmental outcomes in children following in utero exposure to methylmercury from a maternal fish diet: background and demographics. Neurotoxicology. 1995 Winter;16(4):597-612. PMID: 8714866.

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