How do you avoid mercury in tuna?

Mercury and Fish: The Facts

The Truth About the Mercury in Seafood and Other Fish

Mercury in Tuna Fish

Americans now eat more shrimp than any other seafood item (see Top 10 list), but tuna is the most popular fish we consume. One out of every six of our seafood meals is tuna, and most of it is canned tuna. Of the two popular varieties, about three times as much canned light tuna (the less expensive variety) is consumed as canned albacore (or “white”) tuna. Fresh and frozen tuna steaks and tuna sushi are also popular choices, although they make up much smaller fractions of the market. Canned tuna, especially the canned light variety, is an inexpensive source of high-quality protein. Although sales have been declining as Americans have responded to a growing variety of seafood choices, canned tuna is still very popular. It’s a favorite sandwich ingredient for families with children, and a staple of the Federal school lunch program.

Unfortunately, as long-lived ocean predators, tuna accumulate methylmercury. Shrimp has the lowest methylmercury level among seafood items, with 0.012 part per million. In contrast, canned light tuna contains 10 times as much mercury, 0.118 part per million. Canned albacore tuna and fresh/frozen tuna steaks contain 0.353 and 0.384 part per million mercury, respectively, about three times as much as canned light has. Tuna sushi, often made from large bluefin tuna, which are older and therefore have higher mercury levels, contains about 1.0 part per million mercury, putting it on a par with swordfish and shark among the highest-mercury fish.

Because of its popularity and because all varieties of tuna have relatively high mercury levels, tuna is by far the largest source of methylmercury exposure in the American diet. In an analysis submitted to the FDA in April 2009, The Mercury Policy Project showed, using FDA’s own data, that tuna accounts for 37.4 percent of all the mercury in the US seafood supply. (See page 19 of the linked document.) By comparison, the four highest-mercury varieties combined, swordfish, shark, king mackerel and Gulf tilefish, account for just 6.5 percent of total mercury contributions. In other words, tuna is responsible for six times as much mercury exposure as the four very-high-mercury fish varieties the government advises pregnant women not to eat.

People who would like to minimize their methylmercury exposure therefore need to be aware of the central role played by tuna fish in that exposure. Achieving that awareness has not been as easy as it should be. Advertisements from the tuna industry have urged people, including pregnant women, to eat more tuna, and to ignore mercury concerns. And the government has been less than helpful. The 2004 EPA/FDA advisory urges women to limit their consumption of canned albacore tuna, but inaccurately lists canned light tuna as a “low-mercury” seafood choice, and recommends eating up to 12 ounces of it per week.

Fable: Canned light tuna is a “low-mercury” fish.

Facts: This particular nugget of misinformation originated with the US FDA, although the tuna lobby has been more than happy to propagate and benefit from it.

The truth is, while canned light tuna has only one-third as much mercury as canned albacore tuna, it still has a well-above-average mercury content. The average mercury level in the US seafood supply as a whole, calculated by FDA (and confirmed by our own independent analysis) is 0.086 part per million. The average level in canned light tuna, 0.118 part per million, is 37 percent higher than the overall average. Given its very large market share and this elevated mercury content, canned light tuna is the biggest single source of methylmercury exposure in the American diet, accounting for 16 percent of the mercury in the seafood supply.

Further, not all canned light tuna is the same. The FDA average figure of 0.118 part per million is based on extensive sampling of major US brands of tuna—Bumblebee, Star-Kist and Chicken of the Sea. But FDA has not done much testing of “minor” tuna brands, including brands imported from South and Central America. Some of the latter brands have average mercury levels much higher than the average in US brands. Also, amounts of mercury in individual cans of even the major US brands vary, and some cans have much higher than average levels. Because of this variability and uncertainties about the timing of potentially harmful exposure during fetal development (see “The Reference Dose is for Lifetime Exposure“), several consumer and public-health organizations have advised pregnant women to avoid all tuna, including canned light. We think that’s sound advice.

Why, then, would the FDA advise pregnant women to eat up to 12 ounces of canned light tuna per week? One reason is that FDA scientists simply were not very concerned about the risks of low-level methylmercury exposure. Also, when it was developing the 2004 advisory, FDA was in a difficult political position. Its draft Advisory, circulated for public and interested-party comments, urged women to limit their consumption of canned albacore tuna. The tuna industry had commented pointedly on the draft, stressing its concern that if the advisory said anything negative about mercury in tuna, it could have a substantial adverse impact on the billion-dollar annual market for canned tuna.

Consumer organizations, in comments on the draft Advisory, had urged FDA not merely to warn consumers about high-mercury fish, but also to identify low-mercury choices. FDA then had to determine what should be considered “low-mercury.” Where should it draw the line? As our table of mercury levels in fish shows, 0.050 parts per million might have been a sensible place to draw that line: Seven of the 11 top-selling fish and shellfish varieties and 15 types of fish and shellfish overall, have 0.050 part per million mercury or less. In all, 25 fish and shellfish categories (some of which, like crabs, clams and flatfish, contain many separate individual seafood varieties) have lower average mercury levels than light tuna. Why, then, would FDA choose to define “low-mercury” so it included canned light tuna, the largest source of methylmercury exposure in the diet?

At the FDA Food Advisory Committee meeting on December 10, 2003, FDA scientist Clark Carrington explained: “In order to keep the market share at a reasonable level, we felt we had to keep light tuna in the low-mercury group.” (See the transcript of the meeting, Carrington’s comments on pages 162-163.) So there you have it: FDA defined the so-called “low mercury” category specifically to include canned light tuna, not to help women minimize their exposure to methylmercury, and not because canned light tuna actually is low in mercury, but to protect the tuna industry’s market from the impacts it might have felt if FDA had explained truthfully that light tuna is Americans’ largest single source of methylmercury exposure.

Needless to say, we don’t believe the economic interests of the tuna industry should override the need for accurate public-health advisories about sources of mercury in the diet. Unless and until FDA corrects this error, however, the industry will benefit—and consumers will endure easily avoidable mercury exposure—because of the fable that light tuna is a “low-mercury” choice.

Mercury in Tuna: What Type Is Good and Bad?

Fish is rich in omega-3 fatty acids, protein, essential vitamins, and minerals—and it can be an excellent addition to a ketogenic diet.

Unfortunately, several species of fish contain very high levels of mercury, which can have numerous health consequences.

Tuna consumption is one of the most common sources of mercury exposure. Certain types of tuna can expose you to unsafe amounts of mercury, so it’s important to choose the right kinds to minimize your mercury intake.

What is mercury?

Mercury is a heavy metal that exists naturally in the environment. It’s a neurotoxin, so it can be very damaging to the brain and nervous system at high levels.

Mercury has increased in the environment over time since the Gold Rush in 1850. The combination of mercury in the environment paired with the use of mercury in modern medicine is exposing people to higher mercury concentrations than ever before.

High levels of mercury in the blood can lead to mercury poisoning or toxicity. Some common symptoms of mercury toxicity include:

• Tremors
• Headaches
• Nausea
• Poor coordination
• Peripheral neuropathy
• Skin rashes
• Poor memory
• Trouble speaking, hearing, and seeing
• Muscle weakness
• Twitches
• Insomnia
• Tachycardia
• Increased salivation
• High blood pressure
• Mood swings/anxiety
• Fatigue

80 percent of mercury accumulates in the kidneys, 10 to 15 percent in the liver, and 5 to 10 percent in the brain. In the brain, mercury concentrations are highest in and around the pituitary gland .

Why fish contain mercury

Most fish contain mercury because they’re exposed to mercury in their environment. Mercury is very volatile and has increased all over the environment and atmosphere.

Mercury settles in streams, rivers, oceans, and other water sources as methylmercury, the toxic form of mercury. Marine life consequently absorbs this methylmercury.

Methylmercury transfers upwards through the food chain, causing predatory fish like tuna, shark, swordfish, and other large fish to have higher mercury levels.

Check out this video to learn about the mercury found in tuna.

How much mercury is in tuna?

If you eat tuna, you want to avoid tuna with high mercury levels. Species of tuna like bigeye can have mercury levels of up to 1.816 ppm (parts per million).

Ahi tuna has around triple the amount of methylmercury found in canned light tuna and skipjack tuna. Albacore, canned white, yellowfin, and bigeye tuna have the highest mercury levels. This chart published by the FDA shows the mercury levels found in almost every species of fish.

Mercury is dose-dependent, so the risk of toxicity increases with higher levels of consumption.

A small child with a very low body weight shouldn’t consume tuna more than once a month. Babies in utero are very sensitive to heavy metals like mercury. Pregnant women should limit their tuna consumption.

The body can rid itself of excess mercury, but it takes time. Scientists are still determining the exact amount that causes toxicity and mercury poisoning.

Mercury may not affect you as much when you eat fish that’s higher in selenium. Selenium is a chelator and can counter mercury in the body.

If there’s too much mercury in your body, it can bind to the selenium enzymes, rendering them inactive. This can lead to symptoms of mercury toxicity.

Much like selenium, cilantro , zinc, and lipoic acid can also help rid the body of excess mercury.

What is the healthiest tuna?

If you want to eat more tuna, consuming tuna with less mercury is the healthiest choice. Skipjack tuna has very low levels of mercury. Canned light tuna—also known as chunk light tuna—has lower mercury concentrations than other types of canned tuna.

Try this keto-friendly tuna salad recipe with chunk light canned tuna. You can also find other keto-friendly foods on our keto food list .

Choosing where you get your fish can also lower your mercury exposure. For example, fish from the Mediterranean can contain three to five times more mercury due to industrialization, power plants, and volcanic activity in the area.

Consider adding other species of low-mercury fish to your diet. Mercury builds up in older, larger, longer-living fish. Fish like shark, grouper, swordfish, tuna, and other predatory fish have high concentrations of mercury compared to smaller species of fish.

Smaller fish and shellfish contain a fraction of the mercury found in larger fish. Other fish like canned sockeye salmon or canned pink Alaskan salmon have much lower mercury levels and can be used in the same way as canned tuna. Wild-caught sardines also contain lower levels of mercury compared to tuna fish.

Here are some low-mercury fish to add to your diet:

• Anchovies
• Catfish
• Clams
• Crabs
• Crawfish
• Flounder
• Haddock
• Herring
• Oysters
• Sardines
• Scallops
• Squid
• Trout
• Tilapia

Key takeaways

It’s almost impossible to avoid mercury, so the best thing you can do is focus on your intake. Limit or avoid consuming high-mercury fish like bigeye and yellowfin tuna, and add more low-mercury fish to your diet.

Consume fish that also has high levels of selenium to help counter mercury in the body. It’s possible to enjoy tuna and other fish without consuming as much mercury—you just have to choose the right types.

FAQ

1. How much mercury is in tuna?

Mercury levels in tuna can vary from 0.126 ppm to 1.816 ppm. Essentially, all species of fish and shellfish contain mercury because mercury settles in our water sources.

2. Is tuna actually high in mercury?

Yes. All tuna contains mercury in the form of methylmercury, and some types contain more than others. Bigeye tuna has the highest concentration of mercury.

3. How much tuna can I eat per week?

Species of tuna with lower mercury levels can be consumed 2 to 3 times per week. Children, nursing women, and pregnant women shouldn’t have tuna more than once per month.

4. How can I reduce mercury in tuna?

You can’t reduce the mercury in tuna, but you can choose fresh or canned tuna that has lower mercury concentrations.

5. Which type of tuna has the most mercury?

Chunk white and albacore tuna can contain double the amount of mercury found in skipjack and canned light tuna. Ahi tuna, which is typically yellowfin or bigeye tuna, can contain even more. Bigeye tuna contains the most mercury.

6. Which type of tuna has the least mercury?

Skipjack and canned light or chunk light tuna have the least amount of mercury. If you choose to eat tuna, try consuming these types. It’s still best to limit your consumption to two to three times per week.

Young children and pregnant women should limit their intake to once per month. You can also try adding other low-mercury fish to your diet.

7. Can I have canned tuna on keto?

Yes. Canned or tinned tuna can be a rich source of protein and healthy fat on a ketogenic diet. Be sure to choose canned light varieties or skipjack to limit mercury exposure.

8. Does canned tuna have more mercury?

Canned tuna does not have more mercury than fresh tuna. In fact, canned light varieties actually have less mercury than fresh ahi tuna.

9. How much mercury is toxic?

Blood mercury levels above 30 ng/mL have been associated with negative effects on the nervous system in both children and adults. Blood mercury levels of 100 ng/mL and above have been associated with mercury poisoning.

10. Why do fish contain mercury?

Mercury occurs naturally in the environment and has increased drastically due to industrialization and gold mining. Mercury settles in creeks, rivers, streams, and the ocean, so the fish absorb it in their environment.

Larger fish who live longer and eat other fish tend to absorb more mercury because it’s in their environment and food sources.

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Can Eating Too Much Tuna Give You Mercury Poisoning?

It’s the go-to «healthy» meal for uni students and gym bros alike. But how much tuna is too much? And how real is the risk of mercury toxicity?

Melbourne, AU
July 10, 2019, 12:05am
Image via Pixabay (edited)

I’m poor, and lazy, but also occasionally health conscious, so I eat quite a bit of canned tuna. Not what you’d call an extraordinary amount—maybe two to three cans a week—but enough for it to qualify as a dietary staple. Enough, at the very least, for me to stop and wonder from time to time: is this maybe too much? Can you overdose on tuna? Or, more specifically: what would it take for a person to overdose on tuna?

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Researchers at the University of California, Santa Cruz, recently put forward a similar query. In a study published in the Society of Environmental Toxicology and Chemistry journal, scientists surveyed a number of students about A: their tuna consumption, and B: their awareness of the health risks involved. Fifty-four percent of those students reported eating tuna about three times a week, while more than 99 percent reported low knowledge of the potential dangers associated with overconsumption—most notably, mercury poisoning.

It’s no secret that tuna, like most fish and shellfish, contains toxic, heavy metal mercury. The substance is often present in seawater in fairly small and benign doses, before getting absorbed by algae and entering the food chain. The amount of mercury in a single organism typically varies depending on the size of the animal, and increases as you go up the chain: a shrimp eats the algae, a tuna eats the shrimp, and that tuna develops a cumulative build-up of mercury in its blood through a process known as bioaccumulation. Since tuna are pretty big, they typically accumulate more mercury than smaller species of fish like salmon—and when we eat that tuna, we absorb that mercury. So, in theory, eating a certain amount of tunafish could lead to dangerous amounts of mercury in our blood.

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How much fish are we talking, though, before things start getting dicey? Food Standards Australia New Zealand (FSANZ) broadly suggests keeping one’s tuna consumption to between two and three 150 gram servings a week—which is roughly the size of a medium-sized tin. The specific amount is dependent on body weight, though. Larger people can usually handle more mercury, while smaller people—as well as pregnant people—should exercise a little more caution. The U.S. Environmental Protection Agency offers a formula that’s slightly more fine grain: recommending that an individual consume no more than 0.1 micrograms (a microgram equalling one millionth of a gram) of methyl mercury, per kilogram of body weight, per day.

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“Tinned tuna is a very low source of mercury, so people would have to be eating at least three cans a day for about six months before it really became a concern,” Melanie McGrice, an accredited practising dietitian, tells me over the phone. “Even pregnant women, who are one of the cohorts most at risk of mercury toxicity, can eat a small, 95-gram can on a daily basis throughout their pregnancy without concern of mercury toxicity.”

Over the course of her nearly-20 year career, Melanie claims to have seen “thousands of patients”—only two of whom have presented with mercury toxicity. “It is quite rare, and for both of those people the reasoning was that they were eating barely anything else other than fish and rice due to food allergies,” she says. “So they had a very, very limited diet.”

That’s somewhat reassuring. But it’s also worth noting that this “limited diet” of tuna and rice is a go-to meal for gym bros and college students who want to eat “healthy” without spending too much time or money. Sure, three cans a day for six months does seem like a “humongous” amount, as Melanie puts it. But it’s also not too hard to imagine a situation in which someone would be putting away that much fish on a regular basis—particularly if they’re cracking into the larger cans.

“Say somebody was trying to gain muscle bulk, and tuna’s obviously a rich source of protein, then that person may be eating six cans of tuna a day in an effort to increase muscle,” Melanie suggests. “That could be a problem if they were doing that for months on end. I would certainly think that a blood test to check their mercury levels would be warranted.”

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In the University of California study, seven percent of participants reported eating more than 20 tuna meals a week. Tests on some students’ hair furthered indicated mercury levels that were above what is considered «a level of concern».

There’s a range of symptoms and health complications that can come about as a result of high mercury levels. These might manifest as itching, burning, or even a sensation that small insects are crawling under one’s skin, as well as more visible symptoms like pink cheeks and swelling in certain parts of the body. In more serious cases, mercury poisoning can cause high blood pressure, low cognitive function, blindness, and lung and kidney dysfunction. For pregnant people, the dangers can be even more acute.

“Because your baby is obviously much smaller than an average person, they can develop mercury toxicity much more easily, and that can then actually lead to things like cosmetic problems or stillbirths,” says Melanie. “But if you’re an average person just living your life, having tuna as a bit of a staple in your diet, then I wouldn’t expect to see too many problems.”

So in short, yes: there is absolutely such a thing as too much tuna—but you’d have to be putting away several hundred grams a day, for a period of several months, before you started seeing any serious problems. The more relevant concern for people who insist on eating that much, Melanie suggests, is that they’d likely be forgoing other foods and vitamins in preference of canned fish.

“Tuna is quite a nutritious food to eat—but you still want to be having dairy, you still want to be having your whole grains, you still want to be having vegetables and fruits and healthy oils as well,” she stresses. “That said, if somebody has such severe food allergies that they’re really restricting their diet—or maybe they’ve just decided to put themselves on some sort of crazy diet for whatever reason—they could suffer from mercury toxicity.”

As with all things in life, then, moderation is key. Tuna’s a great source of iron, zinc, omega 3, and protein, sure—but it’s also a fairly reliable source of mercury. While the health benefits might ultimately outweigh the risks, it’s probably best to keep the habit to a single can a day.

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