Fish is one of the healthiest foods you can eat.
That's because it's a great source of protein, micronutrients and healthy fats.
However, some types of fish may contain high levels of mercury, which is toxic.
In fact, exposure to mercury has been linked to serious health problems.
So what should I do? Does it mean you need to avoid all the fish? This article explains what you need to know.
Why is mercury a problem?
Mercury is a toxic heavy metal found naturally in air, water and soil.
It is released into the environment in several ways, including through industrial jobs such as burning coal or natural events such as volcanoes.
There are three main forms: elemental (metallic), inorganic and organic (1).
People can be exposed to mercury in various ways, such as breathing mercury vapors during mining and industrial work.
You can also be exposed by eating fish and seafood. This is because fish and shellfish are exposed to low concentrations of mercury due to water contamination.
Over time, you can concentrate on their bodies. This is usually in the organic form, which is known as methylmercury.
This is a highly toxic form that can cause serious health problems when it reaches certain levels in the body.
Bottom line: Mercury is a heavy metal of natural origin. It can accumulate in the bodies of fish in the form of methylmercury, which is highly toxic.
Some fish are too high in mercury
Many types of fish contain mercury.
One study found that 25% of the fish in 291 streams in the USA. UU They contained more than the recommended limit (2).
Another study found that one third of the fish caught off the coast of New Jersey had mercury levels above 0.5 parts per million, a level that could cause health problems for people who eat this fish regularly (3).
In general, the larger and longer-lasting fish tend to contain more mercury (4).
These include shark, swordfish, fresh tuna, marlin, king mackerel, Gulf of Mexico cod, northern pike and more (5).
Larger fish tend to eat many smaller fish, which contain small amounts of mercury. This is not easily removed from their bodies, so the levels accumulate over time. This process is known as bioaccumulation (6).
Mercury levels in fish are measured as parts per million (ppm). Below are the average levels in different types of fish and shellfish, from highest to lowest (5):
- Swordfish: 0.995 ppm.
- Shark: 0.979 ppm.
- King Mackerel: 0.730 ppm.
- Bigeye tuna 0.689 ppm.
- Needle: 0.485 ppm.
- Canned tuna: 0.128 ppm.
- Cod: 0.111 ppm.
- American lobster 0.107 ppm.
- White fish: 0.089 ppm.
- Herring: 0.084 ppm.
- Hake: 0.079 ppm.
- Trout: 0.071 ppm.
- Crab: 0.065 ppm.
- Haddock: 0.055 ppm.
- Whiting: 0.051 ppm.
- Atlantic Mackerel: 0.050 ppm.
- River crab: 0.035 ppm.
- Pollock 0.031 ppm.
- Catfish: 0.025 ppm.
- Squid: 0.023 ppm.
- Salmon: 0.022 ppm.
- Anchovies: 0.017 ppm.
- Sardines 0.013 ppm.
- Oysters: 0.012 ppm.
- Scallops: 0.003 ppm.
- Shrimp: 0.001 ppm.
Bottom line: Different types of fish contain varying amounts of mercury. Larger, longer-lasting fish usually contain higher amounts.
How mercury accumulates in fish and humans
Eating fish and shellfish is a major source of exposure to mercury in humans and animals. Exposure, even in small amounts, can cause serious health problems (7, 8).
In the same way that mercury can accumulate in fish, it can also accumulate in humans. This has led to mercury poisoning in extreme cases.
Interestingly, seawater contains only small concentrations of methylmercury.
However, marine plants like algae absorb it. Then the fish eat the algae, absorbing and retaining the mercury. Larger fish then accumulate higher levels when eating smaller fish (9, 10).
In fact, larger predatory fish can contain concentrations of mercury up to ten times higher than the fish they consume. This process is called biomagnification (11).
For human beings, the US government agencies UU They recommend maintaining mercury levels in the blood below 5.0 mcg per liter (12).
A study of 89 people conducted in the USA UU He found that mercury levels ranged between 2.0 and 89.5 mcg per liter, and 89% had levels higher than the maximum limit (13).
Another study in Sweden found that among 143 people, about half had hair mercury levels above the recommended limit (14).
In addition, these studies found that consumption of higher amounts of fish was related to higher levels of mercury.
In addition, many studies have shown that people who regularly eat larger fish, such as pike and perch, had higher levels of mercury in their bodies (14, 15).
Bottom line: Eating large amounts of fish has been linked to higher levels of mercury in the body, especially when eating larger fish.
Negative effects for health
Mercury is toxic and exposure to it can cause serious health problems (16).
In both humans and animals, higher levels of mercury are associated with neurological problems.
A study of 129 Brazilian adults found that higher levels of mercury in hair were associated with a decrease in fine motor skills, dexterity, memory and attention (17).
Recent studies have also linked exposure to heavy metals, such as mercury, to diseases such as Alzheimer's, Parkinson's, autism, depression and anxiety (18).
However, more studies are needed to confirm this link.
In addition, exposure to mercury has been linked to high blood pressure, an increased risk of heart attacks and higher LDL ("bad") cholesterol (19, 20, 21, 22, 23).
A study of 1,800 men found that those with the highest levels of mercury in their hair were twice as likely to die from heart-related problems as men with a lower mercury level in their hair (24).
However, it is likely that the nutritional benefits obtained from fish outweigh the risks of exposure to mercury (25).
Bottom line: Higher levels of mercury can damage neurological function and heart health. However, the health benefits of eating fish can overcome these risks.
Some people are at higher risk and should be extremely cautious
Mercury in fish does not affect everyone in the same way. Therefore, certain people should be very careful when it comes to eating fish.
This includes women who can become pregnant, pregnant women, nursing mothers and young children.
Fetuses and children are more vulnerable to the toxicity of mercury, and mercury can easily be passed on to the fetus of a pregnant mother or the baby of a breastfeeding mother.
An animal study found that exposure to even low doses of methylmercury during the first 10 days of conception affected brain function in adult mice (26).
Another study found that children exposed to mercury in the uterus had difficulties with attention, memory, language, and motor function (27, 28).
In addition, some studies suggest that certain ethnic groups, including Native Americans, Asians, or Pacific Islanders, may have an increased risk of exposure to mercury. This is likely because fish is a large part of their usual diets (29).
Bottom line: Pregnant women, breastfeeding mothers, young children, and people who consume large amounts of fish regularly have an increased risk of problems related to exposure to mercury.
How to eat fish safely
In general, you should not be afraid to eat fish.
The health benefits of eating fish are powerful, and fish is a major source of omega-3 fatty acids.
In fact, it is generally recommended that most people eat at least 2 servings of fish per week.
However, the FDA advises people at high risk of mercury toxicity (women who can become pregnant, pregnant, nursing mothers and young babies) to take into account the following recommendations (30):
- Eat 2-3 servings (227-340 grams) of a variety of fish each week.
- Choose low-mercury fish and seafood, such as salmon, shrimp, cod and sardines.
- Avoid fish with higher mercury content, such as fin fish from the Gulf of Mexico, shark, swordfish and king mackerel.
- When eating freshly caught fish, consult the fish tips for those particular streams.
Following these tips will help you maximize the benefits of eating fish and minimize the risks of exposure to mercury.
More on fish:
- 11 health benefits based on evidence of eating fish
- Wild salmon or hatchery: can some fish be bad for you?
- Sushi: Healthy or not healthy?
Reference: https: //www.healthline.com/nutrition/mercury-content-of-fish