How Dangerous is Mercury in Fish? Do we Need to Limit Fish Intake?

Mitchell Ryan
May 16
7 min read

Updated: Sep 10

Fish is often lauded as a nutritional powerhouse, brimming with lean protein, omega-3 fatty acids, and essential vitamins and minerals. These nutrients are vital for heart health, brain function, and overall well-being. Yet, lurking beneath the surface of this healthy food is a potential concern: mercury contamination. The question of how worried we should be about mercury in our fish intake is a complex one, fraught with varying opinions and scientific findings. This post aims to delve into the science behind mercury in fish, assess the real risks, and provide practical strategies to minimize exposure while still enjoying the benefits of seafood, including the fascinating role of selenium.

Delicious and nutritious. But is mercury in fish a dangerous problem?

The Journey of Mercury to Our Plates:

Mercury is a naturally occurring element found in the Earth's crust. Human activities, such as burning fossil fuels and industrial processes, release mercury into the atmosphere, where it eventually settles into waterways and oceans. Once in aquatic environments, microorganisms transform inorganic mercury into methylmercury, a highly toxic organic compound that is easily absorbed by fish.

The concentration of methylmercury in fish increases as it moves up the food chain – a process called biomagnification. Small fish consume phytoplankton containing methylmercury, and then larger predatory fish eat these smaller fish, accumulating higher levels of the toxin in their tissues over their longer lifespans. This means that larger, longer-lived predatory fish tend to have the highest concentrations of mercury.

How Dangerous is Mercury in Fish?

Methylmercury is a neurotoxin, meaning it can harm the nervous system. The developing brains of fetuses, infants, and young children are particularly vulnerable to its effects. Exposure to high levels of methylmercury during these critical developmental stages can lead to:

Neurological problems: Including developmental delays, learning disabilities, attention deficits, and motor skill impairments.
Cognitive deficits: Affecting memory, language, and visual-spatial abilities.

In adults, high levels of mercury exposure can also have adverse health effects, including:

Nervous system damage: Causing tremors, muscle weakness, numbness, and memory problems.
Cardiovascular issues: Some studies suggest a link between high mercury levels and an increased risk of heart disease.
Kidney damage: High exposure can impair kidney function.

As a rule, larger fish will contain more mercury- something to keep in mind when selecting fish

Assessing the Risk: How Much is Too Much?

The level of concern about mercury in fish intake depends on several factors, including:

The type of fish consumed: As mentioned earlier, larger, predatory fish like shark (flake), swordfish, king mackerel, tilefish, and some types of tuna tend to have the highest mercury levels. Smaller, shorter-lived fish generally have lower levels.
The frequency and amount of fish consumed: Occasional consumption of high-mercury fish is less concerning than frequent, large servings.
Individual susceptibility: Fetuses, infants, young children, and women who are pregnant or breastfeeding are considered the most vulnerable populations.

Regulatory bodies worldwide, including Food Standards Australia New Zealand (FSANZ), and the U.S. Environmental Protection Agency (EPA) and Food and Drug Administration (FDA), have established guidelines and recommendations for safe fish consumption based on mercury levels. These guidelines typically advise limiting or avoiding high-mercury fish for vulnerable populations and recommend consuming a variety of lower-mercury options.

Navigating the Seafood Aisle

Choosing Lower-Mercury Options:

The good news is that you don't have to eliminate fish from your diet entirely to minimize mercury exposure. Many types of fish are relatively low in mercury and still provide valuable nutrients. Here are some examples of fish generally considered lower in mercury:

Salmon (wild and farmed): A rich source of omega-3 fatty acids.
Canned light tuna: Typically made from smaller tuna species like skipjack.
Shrimp, prawns, and bugs: Popular shellfish options.
Cod: A lean white fish.
Haddock: Similar to cod, another good source of lean protein.
Sardines: Small, oily fish packed with nutrients.
Herring: Another oily fish with omega-3s.
Anchovies: Small, flavorful fish often used in sauces and as toppings.
Oysters: A good source of zinc and other minerals.
Scallops: A mild-flavored shellfish.
Trout (freshwater): Often farmed and lower in mercury.
Whiting: A delicate white fish.

Fish to Consume in Moderation or Limit:

Certain fish tend to have higher mercury levels and should be consumed less frequently, especially by vulnerable populations. These include:

Albacore (white) tuna: Contains more mercury than canned light tuna.
Bluefish: A popular sport fish.
Grouper: A type of reef fish.
Mahi-mahi: A flavorful fish often found in warmer waters.
Spanish mackerel: Higher in mercury than chub mackerel or Atlantic mackerel.

Fish to Avoid or Severely Limit (Especially for Vulnerable Groups):

The following fish typically have the highest mercury levels and should be avoided or severely limited, particularly by pregnant women, breastfeeding mothers, and young children:

Shark (flake): A large, predatory fish.
Swordfish: Another large, long-lived predator.
King mackerel: A large, fast-growing mackerel.
Tilefish (from the Gulf of Mexico): Some varieties have very high mercury levels.
Marlin: A large billfish.
Orange roughy (deep-sea perch): A long-lived fish.
Ling: A type of cod-like fish.
Southern bluefin tuna: A large, migratory tuna species.

Practical Strategies to Minimize Mercury Intake:

Choose a variety of fish: Don't rely on just one or two types of fish. Incorporate a diverse selection, prioritizing lower-mercury options.
Be mindful of portion sizes: Follow the recommended serving sizes for fish. For adults, a typical serving is around 150 grams (5 ounces). For children, it's smaller, around 75 grams (2.5 ounces).
Pay attention to advisories: Heed local and national guidelines regarding fish consumption, especially if you are in a vulnerable group. In Australia, FSANZ provides specific recommendations for pregnant women, breastfeeding mothers, women planning pregnancy, and young children.
Limit consumption of high-mercury fish: Reduce how often you eat the fish listed in the "avoid or severely limit" category. If you do consume them, do so infrequently and in small portions.
Canned tuna considerations: Opt for canned light tuna over albacore tuna more often, as it generally contains lower mercury levels.
Consider the source: Be aware that mercury levels can vary depending on the species, size, age, and the environment where the fish lived.
Fish oil supplements: Fish oil supplements are generally not a significant source of mercury because mercury binds to the protein in the fish flesh, not the oil. However, it's always wise to choose reputable brands that test their products for contaminants.
Recreational fishing: If you catch your own fish, be aware of any local advisories regarding mercury contamination in specific waterways.

The Protective Role of Selenium: A Nutritional Counterbalance?

Interestingly, some nutrients found in fish may offer a degree of protection against mercury toxicity. One such nutrient is selenium, an essential trace mineral that acts as a potent antioxidant and plays a crucial role in various bodily functions, including thyroid hormone metabolism and immune function.

Research suggests that selenium can bind to mercury, forming a compound that may be less harmful and easier for the body to excrete. This interaction has led to the concept of the "selenium-to-mercury molar ratio" (Se:Hg ratio) in fish. Some scientists propose that the health risks associated with mercury in fish may be lower when the fish contains a higher molar ratio of selenium to mercury.

Brazil Nuts: A Selenium Powerhouse:

One of the richest natural dietary sources of selenium is the Brazil nut. Just one or two Brazil nuts can provide more than the recommended daily intake of selenium for adults. This has led to some intriguing suggestions about incorporating selenium-rich foods like Brazil nuts into the diet, particularly when consuming fish that may contain some mercury.

The theory is that the extra selenium from foods like Brazil nuts could potentially help to counteract the negative effects of mercury by binding to it and facilitating its removal from the body. While research in this area is ongoing and more human studies are needed to definitively establish the protective effects of dietary selenium against mercury from fish, the potential for a nutritional counterbalance is a fascinating area of study.

Important Considerations Regarding Selenium:

While selenium is essential and may offer some protection against mercury, it's crucial to note that:

Selenium itself can be toxic in high doses: The tolerable upper intake level for selenium in adults is 400 micrograms per day. Excessive consumption can lead to selenosis, with symptoms like gastrointestinal distress, hair loss, nail abnormalities, and neurological problems.
More is not always better: While incorporating selenium-rich foods like Brazil nuts in moderation can be beneficial for overall health, megadosing on selenium supplements in an attempt to "detoxify" from mercury is not recommended and could be harmful.
The Se:Hg ratio is complex: The interaction between selenium and mercury in the body is complex and influenced by various factors, including the forms of mercury and selenium, individual metabolism, and overall dietary status. Relying solely on the Se:Hg ratio as a guide for safe fish consumption without considering established guidelines is not advisable.

Fish and Chips can be high in mercury- check the fish that's being offered!

The Takeaway: Informed Choices, Not Avoidance

So, how dangerous is mercury in fish? The answer is that we should be informed and make conscious choices, rather than being overly worried and eliminating a valuable food group from our diet.

Fish offers significant nutritional benefits, and for most people, the risks associated with mercury can be effectively minimized by:

Prioritizing lower-mercury fish.
Consuming a variety of seafood.
Being mindful of portion sizes and consumption frequency.
Following established dietary guidelines, especially for vulnerable populations.

The potential role of selenium in mitigating mercury's effects is an interesting area of research, and including selenium-rich foods like Brazil nuts as part of a balanced diet can contribute to overall health. However, it should not be seen as a license to consume unlimited amounts of high-mercury fish.

Ultimately, a balanced approach that embraces the nutritional benefits of fish while making informed choices to minimize mercury exposure is the most sustainable and health-promoting strategy. By understanding the risks and implementing practical tips, we can continue to enjoy the bounty of the sea while safeguarding our long-term well-being.

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