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.