Fish is one of the most beneficial natural food products. If you are not yet aware of its health benefits, read here.
It is a source of high-quality protein, vitamins, minerals, and incredibly beneficial for health unsaturated fatty acids omega-3.
This combination of beneficial ingredients explains its popularity among those who care about their health, are losing weight, and athletes.
For the latter, it is relevant that fish contains not only high-quality protein with a complete amino acid profile and good digestibility but also omega-3, as scientific research confirms their important role in muscle recovery processes after training and their growth.
However, in the material about the benefits and harms of fish, we mentioned that in the modern world all types of fish and seafood are characterized by contamination with toxic chemicals, one of which is mercury.
Questions arise: how dangerous is the mercury content in fish for human health? Which fish accumulates the most mercury? Are there ways to remove mercury from fish and minimize harm when it enters the body?
We will discuss this.
Mercury in Fish: The Background of the Problem
The issue of mercury presence in fish and its potential harm to human health was not relevant until 1956, when a curious incident occurred in the Japanese town of Minamata, located by the sea 1.
In April 1956, a five-year-old girl with convulsions was admitted to the hospital in Minamata; she had difficulty walking.
In the following weeks, even more patients with similar symptoms were brought to the clinic. But the doctors could not determine the cause.
Scientists were invited to investigate the incident, and they soon discovered similar symptoms of muscle convulsions and impaired muscle function in cats living in the same area.
Also, seabirds were found on the coast that could not move on their feet and crawled on the ground.
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This prompted scientists to think that the root cause of the illness in children, cats, and birds is.. sea fish.
It was suggested that seawater is contaminated with waste from a chemical plant located on the shore, which was confirmed in 1962 when significant mercury levels were found in the waters near the factory.
Studies of people with characteristic symptoms of the disease, whose main food source was fish, showed a maximum mercury level in their blood of 705 mg/kg, while the average level among Japanese living far from the coastline was 4 mg/kg.
The excess is more than 170 times…
As a result of mercury poisoning, patients lost control over their muscle function. In severe cases, they experienced paralysis and fell into a coma.
More than 1780 people died after consuming fish contaminated with mercury…
Scientists first encountered the problem of mercury contamination in fish in the 1950s in Japan. At that time, about 1800 people died, many were hospitalized in a state of convulsions. Similar symptoms were observed in seabirds and local cats.
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Where does mercury in fish come from?
Mercury naturally exists in the environment, in soil and plants, and enters the atmosphere as a result of evaporation, volcanic eruptions, and forest fires.
However, the main source of mercury pollution is human industrial activity: factories and plants discharge it along with waste into rivers; a significant amount also enters the atmosphere when coal is burned at power plants.
Mercury has also been used in dentistry for over 100 years in the production of filling materials (amalgams).
From the atmosphere, mercury falls into rivers, lakes, and oceans with precipitation. According to some estimates, as a result of human industrial activity, the concentration of mercury in the waters of the world’s oceans has increased threefold (compared to the pre-industrial era) 3.
Most of this mercury is inorganic, meaning it is poorly absorbed by living organisms and is quickly expelled without accumulating in organs and tissues.
However, some of the mercury in water is converted by bacteria into an organic form – methylmercury, which is easily absorbed, not excreted, and accumulates in the body over time.
Methylmercury is initially absorbed by phytoplankton (algae), which serves as food for krill and shrimp, which in turn are eaten by small fish, and small fish are eaten by larger ones… and so on.
The longer the lifespan of the fish and the higher it is in the food chain, the more mercury accumulates in its body.
Predatory fish absorb all the mercury that has been accumulated throughout the life of smaller fish.
As a result, the concentration of mercury in the body of large predatory fish, which are at the top of the food chain, can exceed its concentration in seawater by tens of millions of times 1.
Mercury enters fish from seawater, the main source of pollution of which is industrial enterprises. It accumulates the most in large predatory fish species.
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Research confirms the relationship between fish consumption and increased mercury concentration in the body
Scientific studies confirm the relationship between the consumption of fish and seafood and the concentration of mercury in various tissues of the human body.
For example, interesting are the studies of the population in Sweden, where the lakes are known for high levels of mercury pollution.
The concentration of mercury in the body (hair) of the population in this region averages 0.9 µg per gram (across the region), while those Swedes who consume fish once a week have twice as much, i.e., 1.8 µg per gram 8.
The maximum concentration – 18.5 µg per gram – was recorded in a man who ate fish several times a week.
The two most popular species of freshwater predatory fish are perch and pike, with an average mercury content of 0.7 µg/g.
How to understand these numbers?
WHO (World Health Organization) prohibits the sale and therefore does not recommend consuming fish with mercury content exceeding 1.0 ppm (µg/g) per day 3.
That is, it is obvious that the perch and pike caught in Sweden accumulate almost the maximum amount of mercury.
At the same time, a clear pattern is visible between the concentration of mercury in fish and in the human body.
Similar data were obtained from studies of the population of Morocco from coastal regions of the Mediterranean Sea, who consume 3 to 5 servings of fish per week 9.
The waters of the Mediterranean Sea also have a very high degree of mercury pollution due to emissions from industrial enterprises.
Particular concern in this specific study is raised by the fact that scientists found high mercury levels in the blood of 50% of pregnant women, ranging from 3-7 µg/g. This is 3-7 times higher than the norm…
Why is this dangerous? Read on.
Mercury content in the human body depending on the region of fish and seafood habitation 3
What are the dangers of mercury?
The health consequences of mercury poisoning are diverse and depend on the amount of this metal accumulated in the body.
For example, the history of the Minamata incident in Japan shows that their spectrum ranges from neurological diseases to fatal outcomes 10,18.
Particularly dangerous is the effect of mercury on the fetus, which can lead to irreversible damage to the developing brain.
Children born during the Minamata incident had a higher incidence of mental retardation, seizures, cerebral palsy, blindness, and deafness.
If a child’s body is regularly exposed to small doses of mercury, the child may appear normal during the first months of life but later demonstrates low intelligence, abnormal muscle tone, and motor function, attention, and visual-spatial orientation disorders.
If an adult is exposed to mercury, the central nervous system is also affected, and the first symptoms may appear after several months 10.
Although fish is considered beneficial for cardiovascular health, research shows a significantly higher mortality risk among those who consume it regularly.
Elevated mercury levels in the body are associated with a higher risk of cardiovascular diseases 3,11,13.
This is one of the very likely causes of high mortality among the Finnish population, who regularly consume fish—a product considered healthy for the heart and blood vessels 11.
In one study of over 1800 men from Finland, it was shown that at a mercury concentration in the body of ~2 µg/g (which is reached by consuming about 30 grams of fish daily), the risk of heart attack increases by 2-3 times (!).
Some scientific data suggest that elevated mercury levels in the body suppress immune system function and increase the level of “bad” low-density cholesterol in the blood 10,12.
Additionally, adults whose mothers consumed significant amounts of mercury during pregnancy more often experience elevated blood pressure.
Summarizing the consequences of mercury poisoning for adults, we get the following list 2,3,10:
- paresthesia (tingling in the limbs due to nerve damage);
- ataxia (disruption of the coordination of different muscle movements);
- sensitivity disorders;
- muscle tremor;
- hearing impairment and complete loss;
- difficulties in walking;
- blurred vision and blindness;
- death.
The accumulation of mercury in the human body leads to the development of neurological diseases, suppresses the function of the cardiovascular and immune systems. Severe mercury poisoning can lead to death. The consequences of mercury exposure are especially dangerous for fetuses and children.
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The mercury content varies in different types of fish and seafood
Representatives of the Biodiversity Research Institute have systematized data on the degree of mercury pollution in the world’s oceans, as well as its content in the most popular types of fish and other seafood.
They also correlated this data with the volumes of catch in the world’s oceans to determine in which regions the population is most at risk from consuming fish and seafood.
The main conclusions of the scientists 3:
“The entire world’s ocean is polluted with mercury.
The degree of pollution is not uniform everywhere and is determined by the source of pollution in the area: atmospheric precipitation or emissions from industrial enterprises.
Certain types of fish and seafood that are widely used by humans as food accumulate mercury in amounts that exceed safe levels.
At the same time, some other types of fish and seafood contain relatively low levels of mercury that are safe for human health.
The concentration of mercury is significantly lower in those types of fish that are small in size and have a short lifespan. Their consumption is not associated with health risks.
However, in some regions, even in these “safe” types of fish, the mercury level in tissues may exceed safe levels.”
The entire world’s ocean is polluted with mercury. Today, there is no fish without mercury. Its content varies among different types of fish and seafood.
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Which fish accumulates the most mercury?
Very high mercury content is characteristic of large, long-lived, predatory fish.
Among them, the top places are occupied by marlin or sailfish, Pacific bluefin tuna (common tuna), swordfish, and king mackerel, which can weigh several hundred kilograms and reach several meters in length.
These types of fish are not recommended by scientists to be consumed more than once a month.
Large fish generally feed on smaller fish, which contain mercury in small amounts.
The longer a predator lives, the more “small fish pass through it,” and the higher the accumulation of toxins in its body.
Fish that accumulates mercury in large amounts
Recently, shark meat has been gaining popularity in some European countries and especially in Asian countries. It should be noted that the mercury content in it is one of the highest (see the table below).
Mercury is found in very large quantities in large, long-lived, predatory fish
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Which fish contains less mercury?
According to scientific data, there are no mercury-free fish today. It is present in some form in all seafood.
According to expert estimates, by 2050, the mercury content in the northern part of the Pacific Ocean will double, which means its concentration in the fish living there will significantly increase 3. Of course, this applies not only to the Pacific Ocean.
Nevertheless, the following rule will always be true: those types of fish and seafood that feed on algae (i.e., are “herbivorous”) live for a short time and, accordingly, are small in size, accumulate significantly less mercury.
For example, these include shrimp, cod, haddock, herring, sardines, and flounder.
Fish that accumulates mercury in relatively small amounts
No fish exists without mercury. The lowest concentration is found in small fish that feed on algae and live for a short time.
Mercury in Canned Tuna
Tuna is one of the five most common types of fish in the global seafood market.
There are several species of it.
Canned tuna is mainly made from yellowfin, skipjack, and albacore tuna, which are characterized by relatively low mercury content and are therefore relatively safe.
Common or Pacific bluefin tuna is among the fish with very high mercury content. It is usually not canned but used for direct preparation of various dishes.
Canned tuna is relatively safe in terms of mercury content. The most mercury is accumulated by common tuna.
Mercury Content in Fish
The mercury content in fish and other seafood is measured in units of ppm (parts per million), which is equivalent to the number of milligrams per 1 liter of water or the number of micrograms per 1 gram.
The table below provides data on the average mercury content in some popular types of fish and seafood (information from the FDA website 6):
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| Mercury Content in Fish and Seafood 6 | |
| Type of Fish and Seafood | Average Mercury Content, ppm |
| Shrimp | 0.009 |
| Oysters | 0.012 |
| Sardines | 0.013 |
| Tilapia | 0.013 |
| Canned Salmon | 0.014 |
| Anchovies | 0.016 |
| Fresh Frozen Salmon | 0.022 |
| Squid | 0.024 |
| Atlantic Mackerel | 0.05 |
| Mullets | 0.05 |
| Atlantic Pollock | 0.055 |
| Flounder | 0.056 |
| Crab | 0.065 |
| Freshwater Trout | 0.071 |
| Herring | 0.078 |
| Hake | 0.079 |
| Lobster | 0.093 |
| Pike | 0.095 |
| North American Lobster | 0.107 |
| Carp | 0.110 |
| Cod | 0.111 |
| Canned Tuna | 0.126 |
| Fresh Frozen Striped Tuna | 0.144 |
| Freshwater Perch | 0.150 |
| Spanish mackerel | 0.182 |
| Halibut | 0.241 |
| Tuna, canned, longfin | 0.350 |
| Tuna, fresh, frozen, yellowfin | 0.354 |
| Tuna, fresh, frozen, albacore | 0.358 |
| Marlin | 0.485 |
| Tuna, fresh, frozen, bigeye | 0.689 |
| King mackerel | 0.73 |
| Shark | 0.979 |
| Swordfish | 0.995 |
Recommendations for Fish Consumption to Minimize Mercury Harm
Pay attention to the color levels in the charts above. They simultaneously illustrate the concentration of mercury and how often and in what amounts the corresponding fish can be consumed.
The table below provides a breakdown. The meaning is very simple: the higher the concentration of mercury, the less of that fish should be in the diet.
| Fish Consumption Recommendations Based on Mercury Content | ||
| Mercury Concentration (ppm*) | Consumption Recommendations | |
| Green | <= 0.05 | Unlimited |
| Yellow | 0.05 – 0.11 | 2 times a week |
| Orange | 0.11 – 0.22 | 1 time a week |
| Pink | 0.22 – 0.95 | 1 time a month |
| Red | > 0.95 | do not consume |
* 1 ppm = 1 µg/g = 1 mg/kg = 1 mg/l = 1000 µg/l
According to official recommendations, the level of mercury in human blood should not exceed 5.0 µg per liter 7.
For this, its amount in the diet should not exceed 0.1 µg per kilogram of body weight per day or ~50 µg per week for a 70-kilogram person 18.
If we eat, for example, 100 grams of fish from the red category, we will receive a dose of mercury of ~95 µg, which is twice the weekly…
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Scientists recommend limiting the consumption of fish with high mercury content: the higher the concentration of mercury in it, the less of such fish should be in the diet.
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Are there ways to remove mercury from fish?
No. No cooking methods affect its content 14.
The only way to limit mercury intake into the body is to restrict the consumption of those fish and seafood that accumulate mercury in large quantities.
How to remove mercury from the body?
Synthetic drugs
There are several ways to remove mercury from the body.
In medicine, special synthetic chelating or complexing agents are used, which can react with inorganic and organic mercury, resulting in an increased concentration of mercury in urine and a decreased concentration in the body.
Among such substances are: DMPS (2,3-dimercapto-1-propane sulfonate, Dimaval, and Unithiol) and DMSA (meso 2,3-dimercaptosuccinic acid or succimer).
Both substances – DMPS and DMSA – are water-soluble oral medications, less toxic analogs of BAL (2,3-dimercapto-1-propanol, British Anti-Lewisite dimercaprol) – a fat-soluble drug that is administered via intramuscular injections.
BAL is characterized by a high rate of side effects (observed in ~55% of patients) and may also redistribute mercury concentration in the body, increasing it in the brain.
DMPS was developed in the Soviet Union in 1958 and gained popularity in Western countries in 1978. It is a well-researched drug approved for use in Germany and China.
It is widely used by representatives of alternative medicine to treat the consequences of mercury exposure from amalgams used in dentistry for filling teeth.
Selenium
An interesting property is possessed by the mineral selenium.
According to some scientific studies, selenium has the ability to bind with toxic substances.
In the case of mercury, it acts as a kind of magnet for it: by entering into a chemical reaction with mercury, it neutralizes its negative properties 15,16,17, forming chemical compounds (Hg-Se) that are excreted from the body.
The exact mechanism is still being studied by scientists; however, some of them recommend maintaining selenium levels in the body within normal ranges.
There are no methods to remove mercury from fish. Scientists talk about the possibility of neutralizing the harmful effects of mercury with the mineral selenium, and synthetic substances are widely used in medicine.
Pregnant women are at increased risk
As mentioned, fish is a unique source of protein, vitamins, and minerals, as well as omega-3 unsaturated fatty acids.
The latter play an important role in the development of children’s nervous systems. Their intake by women in sufficient amounts is critical during pregnancy and breastfeeding.
On the other hand, mercury, which contaminates all seafood, is a neurotoxic metal, especially dangerous for the developing nervous system of children…
What to do?
The FDA’s (Food and Drug Administration) position on this matter is as follows:
“Women and children are advised to include fish and shellfish in their diets, as they are a source of many beneficial nutrients.
Women of childbearing age, pregnant women, breastfeeding mothers, and young children should avoid certain types of fish with high mercury content, and consume those that do not accumulate mercury in dangerous amounts.” 4.
Relatively safe fish species for pregnant women 5:
- anchovies;
- Atlantic mackerel;
- cod;
- flounder;
- haddock;
- hake;
- herring;
- pollock;
- salmon;
- sardines;
- tilapia;
- trout;
- canned tuna (except for albacore tuna).
References
2 Shivam Kothari, et la. High mercury seafood consumption associated with fatigue at specialty medical clinics on Long Island, NY. Prev Med Rep. 2015; 2: 798–802.
3 David C Evers, et. al. Patterns of global seafood mercury concentrations and their relationship with human health. ResearchGate, Technical Report (PDF Available) · January 2012. https://www.researchgate.net/publication/282781270_Patterns_of_global_seafood_mercury_concentrations_and_their_relationship_with_human_health
4 https://www.fda.gov/Food/FoodborneIllnessContaminants/Metals/ucm351781.htm
5 https://www.fda.gov/downloads/Food/ResourcesForYou/Consumers/UCM536321.pdf
6 https://www.fda.gov/Food/FoodborneIllnessContaminants/Metals/ucm115644.htm
7 https://www.nap.edu/read/9899/chapter/1
8 Johnsson C., et al. Hair mercury levels versus freshwater fish consumption in household members of Swedish angling societies. Environ Res. 2004 Nov;96(3):257-63.
9 Elhamri H, et al. Hair mercury levels in relation to fish consumption in a community of the Moroccan Mediterranean coast. Food Addit Contam. 2007 Nov;24(11):1236-46.
10 https://www.nap.edu/read/9899/chapter/3#17
11 Salonen JT, et al. Intake of mercury from fish, lipid peroxidation, and the risk of myocardial infarction and coronary, cardiovascular, and any death in eastern Finnish men. Circulation. 1995 Feb 1;91(3):645-55.
12 Fillion M, et al. A preliminary study of mercury exposure and blood pressure in the Brazilian Amazon. Environ Health. 2006 Oct 10;5:29.
13 Guallar E, et al. Mercury, fish oils, and the risk of myocardial infarction. N Engl J Med. 2002 Nov 28;347(22):1747-54.
14 https://www.chicagotribune.com/news/chi-mercury-2-story-story.html
15 https://drjockers.com/selenium-detoxify-mercury/
16 Geir Bjørklund, et al. Molecular interaction between mercury and selenium in neurotoxicity. Coordination Chemistry Reviews, Volume 332, 1 February 2017, Pages 30-37
17 S Skerfving. Interaction between selenium and methylmercury. Environ Health Perspect. 1978 Aug; 25: 57–65.
18 Dariush Mozaffarian, MD, DrPH; Eric B. Rimm, ScD. Fish Intake, Contaminants, and Human Health. Evaluating the Risks and the Benefits. JAMA. 2006;296(15):1885-1899.
