There is a huge amount of conflicting information in the media about consuming fish because of the risk of mercury toxicity versus the health benefits. Mercury is a toxic heavy metal which cycles through the atmosphere, water and soil in various forms. The oceans have the ability to act as a storage warehouse for mercury and recent studies show that an estimated 45,000 to 80,000 metric tons has been released into the oceans and that two thirds of this is estimated to be found in waters shallower than 1000 metres where many of the fish that we consume live.
Mercury contamination in the ocean is a serious issue that cannot be ignored. In this article we take a closer look at where the mercury comes from, how it gets into the food chain, the health benefits of eating fish versus potential mercury toxicity. We'll review the advisories from the government agencies and also take a look at the research and evidence so that we can make an informed decision.
The Source of Mercury in Oceans
Mercury is released in several ways which include both natural and anthropogenic processes which is defined as humans activities in exploiting and modifying the environment. Natural processes are mainly from volcanic activity and land emissions. When volcanoes erupt, they release mercury that is stored in the underground resevoirs. Mercury emissions from the land usually occur in the regions closer to the boundaries of tectonic plates where soils are rich in compounds that contain mercury. Natural weathering of the rocks and geothermal reactions can release this mercury. These natural phenomena account for a percentage of mercury emissions today, but anthropogenic emissions have increased mercury concentration in the environment by threefold. Mercury is spewed into the air from coal-burning power plants, steelworks and factories. That pollution can travel halfway around the world and then settle into lakes, rivers, and oceans. Other sources include cement production, consumer products waste and contaminated sites.
How Mercury Gets into the Food Chain
There are various ways that mercury finds its way into the oceans. The largest source of mercury in the ocean is deposited from the atmosphere. In addition, mercury enters the ocean via rivers, estuaries, sediments, and, hydrothermal vents. These sources also release organic mercury compounds such as methylmercury.
This type of mercury is known to be the most poisonous among the mercury compounds and is created when inorganic mercury circulating in the general environment is dissolved into freshwater and seawater. The mercury is then absorbed or ingested by small organisms and then it starts working its way up the food chain, becoming more concentrated with each step. That’s why larger, longer-living predators such as sharks and swordfish tend to have much higher levels of mercury than smaller fish such as sardines, sole, and trout. When you eat seafood containing methylmercury, more than 95 percent is absorbed, passing into your bloodstream. It can move throughout your body, where it can penetrate cells in any tissue or organ. Under certain conditions, methylmercury can negatively affect the central nervous system, particularly the developing brain of a fetus.
Health Benefits of Fish versus Mercury Risks
Fish has always been recognized as an excellent source of protein. In more recent years, cold-water fish have also been recognized as excellent sources of omega 3 fatty acids, including DHA and EPA. Risk of mercury contamination, however, has thrown some of these nutritional benefits into question, and the benefits-versus-risks of fish have become a matter of widespread debate. Do the nutritional benefits of fish, including their rich omega-3 fatty acid content, outweigh the risk of mercury exposure? Lets take a look at the research.
Based on my research, the nutritional benefits of fish certainly do outweigh the risk of mercury exposure, however, we need to take a few factors into account. The FDA and EPA issued recommendations about seafood in 2004, advising no more than 12 ounces of fish per week because of concerns about exposure to mercury. These guidelines were initially intended for women of childbearing age and pregnant women.
It seems that these recommendations have become an accepted fact among the mainstream media, the medical establishment, and the general public. But do these guidelines stand up to scrutiny? Are we robbing ourselves of a great source of protein and omega 3 fatty acids by restricting our fish intake based on these guidelines? Is it possible that pregnant women might not be getting enough omega 3 by following the FDA/EPA guidelines?
An FDA analysis of fish consumption data found that 50 percent of pregnant women surveyed actually ate fewer than 2 ounces a week, far less than the amount recommended. Because the nutritional benefits of eating fish are important for growth and development during pregnancy and early childhood, the agencies updated their advice by recommending a minimum level of fish consumption for these groups. The advice now recommends 2-3 servings of lower-mercury fish per week, or 8 to 12 ounces.(2)
In the 2017 EPA-FDA advice about eating fish, the agencies have concluded that the following people should eat more fish that is lower in mercury for important developmental and health benefits:
The advice recommends that women and children eat two to three servings (8-12 ounces for adults and children over age 10, smaller amounts for younger children) of a variety of fish and shellfish each week.(1) The question still remains, however, on whether we should we be eating more than 12 ounces per week to get the health benefits? Does this advice on a maximum limit of weekly fish consumption have any evidence to support it? What are the factors we need to take into account? Lets dig a little deeper into this.
Mercury and the Brain
Do you know the phrase ‘mad as a hatter’? One theory as to where this phrase came from is related to mercury poisoning experienced by hat-makers as a result of the long-term use of mercury products in the hat-making trade in 19th century England. Mercurial disease was common among hatters and included such symptoms as tremors, irritability, and mental instability.
Today, many studies show that high exposure to mercury induces changes in the central nervous system, potentially resulting in irritability, fatigue, behavioral changes, tremors, headaches, and cognitive loss. Mercury is harmful to the brain because of the toxic increase in reactive oxygen species (ROS). The brain uses a lot of oxygen, in fact, nearly 25% of the oxygen we breathe is used in the brain, but this process produces oxygen by-products through the process of oxidation that creates these reactive oxygen species (ROS) also known as free radicals. These free radicals damage the fats and proteins that make up the brain and research shows that this oxidative damage plays a role in the development and progression of neuro-degenerative diseases.(3)
The brain is very particular about what it lets in and therefore, we have something called the blood brain barrier to prevent undesirable compounds from getting through. Mercury, however, has found a loophole to get through this barrier and when it does, it can cause serious problems under certain conditions. Methylmercury can sneak across the blood brain barrier by binding to an amino acid called cysteine which can be found in foods such as meat, eggs and dairy. So it gets into the brain disguised as an amino acid. Fortunately, it can also be inhibited by other amino acids which means that a high protein diet can help to prevent the transport of methylmercury into the brain. Should we try to avoid cysteine in our diets? Absolutely not because cysteine plays an important role in the body and is a precursor to the potent antioxidant glutathione which we will discuss later in this article.
Mercury and Cardiovascular Disease (CVD)
For decades, the toxic effects of mercury were associated mainly with the central nervous system, however, methylmercury also produces profound cardio-toxicity. Research shows a correlation between mercury exposure and increased risk of hypertension, the progression of atherosclerosis and cardiovascular disease.(4)
The mechanism by which mercury produces toxic effects on the cardiovascular system is not fully understood, but it is believed to involve an increase in oxidative stress. Exposure to mercury increases the production of free radicals and a reduction in the activity of antioxidant enzymes, such as glutathione peroxidase. Research is also showing that mercury levels are predictors of the levels of oxidized low-density lipoprotein (LDL). Oxidized LDL particles are frequently found in atherosclerotic lesions and are associated with the development of atherosclerotic. Another mechanism by which mercury exerts toxic effects on the cardiovascular system is through the inactivation of the “paraoxonase”, an enzyme that slows the LDL oxidation process and that has an important anti-atherosclerotic action.(5)
The Role of Selenium
Selenium is an essential trace mineral that is important for many bodily processes, including cognitive function, a healthy immune system, and fertility in both men and women. It contributes to thyroid hormone metabolism and DNA synthesis, and as an antioxidant, it helps to protect against oxidative damage.
Dietary sources include brazil nuts, fish, brown rice, and eggs are also good source. The amount of selenium in food often depends on the selenium concentration of the soil and water where farmers grew or raised the food. Selenium is known to play an important role in mercury exposure because it prevents mercury toxicity. Research has shown that selenium consistently counteracts the adverse effects of mercury exposure.(6) Mercury is harmful because it deactivates selenium dependent enzymes called selenoenzymes. These are extremely important in the brain because they help to prevent oxidative damage. Mercury interferes with selenonzyme function by binding to selenium and when the level of mercury is high enough, it inhibits the function of selenonzymes preventing them from scavenging free radicals in the brain. Selenium is absolutely required for activity of these selenoenzymes. In fact, all forms of life that have nervous systems possess selenoenzymes to protect their brains from oxidative damage.
Research is showing that it is the ratio of selenium to mercury that matters, regardless of the absolute levels of mercury. When there is more selenium that mercury, the mercury cannot affect proteins or disrupt cell function. It is the total amount of both mercury and selenium consumed over time that is the important factor rather than the absolute levels of mercury within a given fish. Selenium to mercury ratios in excess of 1:1 are thought to counteract the adverse effects of mercury, protecting against mercury toxicity. Evaluation of the health risk posed by mercury exposure from seafood consumption requires consideration of selenium content as well.(7)
Therefore, as long as you are eating wholesome food rich in selenium or eating fish that contains more selenium than mercury, the amount of selenium in the body will always be in plentiful excess of mercury. That means that these essential selenoenzymes can get on with their job without being compromised by mercury. Fortunately, most fish that we consume has more selenium than mercury. However, there are exceptions. These are pilot whale, shark, tilefish, king mackerel and swordfish. Ocean fish generally contain far more selenium than mercury, and protect against rather than contribute to mercury toxicity. However, consumption of fresh water fish that have high mercury, but poor selenium content may pose far greater risks of toxicity than has previously been expected.
Unfortunately, the well-documented protective effect of selenium is largely ignored in both the medical community and the media when reporting on potential risks of fish consumption.
The Role of Glutathione
Glutathione is a peptide containing three important amino acids that have several important roles in the human body. It is a potent antioxidant that prevents damage to cells caused by reactive oxygen species. Researchers have suggested that cellular glutathione levels are a good predictor of life expectancy.
Glutathione has several roles in protecting the body from mercury toxicity. First, glutathione, specifically binding with methylmercury, forms a complex that prevents mercury from binding to cellular proteins and causing damage to both enzymes and tissue. Glutathione-mercury complexes also reduce intracellular damage by preventing mercury from entering tissue cells and becoming an intracellular toxin.(8)
Second, glutathione-mercury complexes have been found in the liver, kidney, and brain, and appear to be the primary form in which mercury is transported and eliminated from the body. Glutathione increases the antioxidant capacity of the cell, providing a defense against ROS and free radicals produced by mercury. As an antioxidant, glutathione appears to also protect against renal damage resulting from methylmercury toxicity. Glutathione works by facilitating the efflux or out-flow of mercury from the cells so that it can be excreted and also by the protective binding of mercury to prevent cellular damage.
Research published in 2006 showed that the depletion of glutathione increased methylmercury accumulation and increased oxidative stress caused by methylmercury. Conversely, supplementation with a glutathione precursor protects against methylmercury exposure in vitro. In vitro refers to the technique of performing a given procedure in a controlled environment outside of a living organism.(9)
Glutathione is poorly absorbed by the body. It seems the beneficial effect of glutathione can only be realized when the precursors are absorbed and the glutathione is manufactured in each and every cell. One such precursor is N-Acetyl-Cystine.(NAC) This is a modified version of the sulfur-containing amino acid cysteine. It replenishes intracellular levels of glutathione, helping to restore cells’ ability to fight damage from reactive oxygen species.(10) Whey protein also replenishes glutathione by boosting cysteine which helps rebuild glutathione levels.
Recommendations on Fish Consumption
The dietary safety limit for methylmercury set by the Environmental Protection Agency is 0.1 microgram per kilogram of body weight per day. Based on that, a blood level of 5.8 micrograms per litre of blood is what the agency considers a maximum acceptable level. But that guideline was set more than a decade ago. Some scientists and consumer safety advocates believe it should be changed because several studies published since then say adverse effects could occur at lower mercury blood levels.
However, given the latest research into the selenium and mercury ratio, perhaps the advice from the FDA and EPA is flawed. Who do we believe? How much fish can we eat without worrying about mercury toxicity? Based on the information and research available, we can make an informed decision for ourselves that makes sense. Lets take a closer look.
We are looking to eat fish that has more selenium that mercury and we want to avoid fish that is higher in mercury than it is in selenium. Generally speaking, most fish consumed by people have higher levels of selenium than mercury, making them perfectly safe to eat. Fish types with more selenium than mercury include the following but this is not an exhaustive list:
The fish that is considered unsafe due to having a greater amount of mercury than selenium include:
Omega 3 Fatty Acids
Omega 3 fatty acids are essential to good health and deficiencies are linked to chronic disease. These polyunsatured fatty acids (PUFAs) have many benefits including brain health, cardiovascular health, and they also have potent anti-inflammatory properties. Many people today are deficient in Omega 3 fats and consume far too much Omega 6 fats found in vegetable oils and processed foods.
Therefore, there is a severe imbalance between omega 3 and omega 6 which can lead to chronic disease. There are several Omega 3 fatty acids that we need to know about. DHA or Docosahexaenoic Acid is a long chain Omega 3 fatty acid and is important for brain and eye development and function as well as cardiovascular health. DHA is the most abundant omega 3 in the brain making up about 15 to 20 percent of your brain’s cerebral cortex. DHA is also abundant in retina making up 30 to 60 percent of the retina so it is easy to see why this is such an essential fatty acid. Low levels of DHA have been linked to memory loss, Alzheimer’s disease, depression and Bipolar disorder. EPA or Eicosapentaenoic Acid as a long chain Omega 3 fatty acid which is important for overall health. However, EPA is not stored in the body in any significant amount like DHA.
Fish is a fantastic source of omega 3 so it makes sense to include plenty of fish in our diet. The table below lists some of the seafood that are a good source of Omega-3 fatty acids and have more selenium than mercury, ranked by DHA/EPA levels.
Table 1. Good sources of omega 3 with a good selenium to mercury ratio.
What’s the Deal with Canned Tuna
According to a February 2007 Environmental Protection Agency’s (EPA) report, 39% of all mercury exposure from fish in the U.S. comes from tuna. Of this 39%, 18% comes from canned light tuna, 10% from canned albacore or white tuna, and 11% from fresh or frozen tuna.
There are two main kinds of canned tuna:
All canned white tuna is albacore. In the EPA update report, both Pacific and Atlantic albacore tuna (all forms, including canned and fresh) contained about triple the mercury content of both Pacific and Atlantic light tuna (yellowfin). The EPA reports that the mercury levels in albacore tuna are almost three times higher than yellowfish or skipjack. Canned light tuna is made up mostly of skipjack tuna.
From the perspective of absolute mercury levels in canned tuna, it is clear that light tuna (especially Pacific light tuna) — is better than albacore tuna (especially Atlantic albacore tuna) but should we be avoiding canned tuna based on this? If you use the selenium to mercury ratio as a basis to make this decision, then the answer is no. Tuna has a higher selenium to mercury level and is also rich in essential omega 3 fatty acids as can be seen in the Table 1 above. We might want to apply some moderation to canned tuna intake simply because it is taking up space in your diet and it is a canned product, whereas you could be eating fresh fish that also has a good selenium to mercury profile and a high level of omega 3 fatty acids. It is also creating more variety in your diet.
Sustainable Seafood Considerations
Bluefin tuna, have been so overfished that they can't reproduce fast enough to replace what's caught. If you care about sustainability, they should be on your do-not-eat list. How fish are caught also affects their sustainability. Longlining can be especially devastating because it involves one line that can have 3,000 baited hooks and stretch for up to 50 miles. The hooks dangle at a depth between 328 feet and 492 feet where the largest tuna—such as the threatened bluefin—tend to swim.
Many albacore are now caught by longlining too. These hooks also catch more than 80 kinds of non-targeted creatures, including endangered sea turtles so again, this is not an eco-friendly way to fish. A number of yellowfin populations are overfished now, so only pole-caught fish are considered a good choice for sustainability.
Should we be eating more than the 12 ounces per week recommended as a maximum by the FDA? The research suggests that we can indeed eat more than this because the net health benefits of consuming fish far outweigh the risks of mercury toxicity when we consume fish with a higher selenium to mercury ratio. These types of fish contain high levels of omega 3 fatty acids which are essential for good health and are a good source of protein. However, not everyone is a a fan of eating fish so these people my not be getting adequate amounts of Omega 3 fatty acids which are essential for health. If this is the case for you, it is best to take a high quality fish oil supplement that is derived from sustainable sources or include other foods in your diet that are rich in Omega 3.
3. Oxidative Stress in Neurodegenerative Diseases: From Molecular Mechanisms to Clinical Applications. Oxid Med Cell Longev. 2017; 2017: 2525967.
4. Fish, Mercury, Selenium and Cardiovascular Risk: Current Evidence and Unanswered Questions. Int J Environ Res Public Health. 2009 Jun; 6(6): 1894–1916.
5. Toxic Effects of Mercury on the Cardiovascular and Central Nervous Systems. J Biomed Biotechnol. 2012; 2012: 949048.
6. Selenium and Mercury Interactions with Emphasis on Fish Tissue. Environmental Bioindicators 4(4):318-334 · December 2009.
7. Selenium/mercury molar ratios in freshwater, marine, and commercial fish from the USA: variation, risk, and health management. Rev Environ Health. 2013;28(2-3):129-43.
8. Mercury toxicity and antioxidants: Part 1: role of glutathione and alpha-lipoic acid in the treatment of mercury toxicity. Altern Med Rev. 2002 Dec;7(6):456-71.
9. Glutathione modulation influences methyl mercury induced neurotoxicity in primary cell cultures of neurons and astrocytes. Neurotoxicology. 2006 Jul;27(4):492-500.
10. The Antioxidant Role of Glutathione and N-Acetyl-Cysteine Supplements and Exercise-Induced Oxidative Stress. J Int Soc Sports Nutr. 2005; 2(2): 38–44.
Sports & fitness nutritionist, researcher and author on a mission to improve the human condition. Focusing on evidence-based and outcome-based nutrition, training, mindset & environment