Mercury poisoning: Something fishy about too much tuna?

Dear Alice,

I have been hearing more and more about how harmful eating big fish, such as tuna, can be because of high levels of methyl mercury in them. When searching for this on the internet, I found both spectrums of safe or not safe. I am worried because I eat 2-5 cans of tuna every single day, and besides having more protein than my body needs, am I at risk for mercury poisoning?

Dear Reader, 

Fish can be an important part of a balanced eating plan; it's loaded with high-quality protein and omega-3 fatty acids and it’s low in saturated fat and cholesterol. It’s also true that nearly all fish have at least trace amounts of methylmercury. The good news is that many of those fish commonly purchased in the United States, including several varieties of tuna, typically have lower levels of mercury and are safe to eat if consumed within the recommended serving. Various factors such as gender, age, and pregnancy status can influence the recommended amounts of tuna consumption per week. Amounts beyond this recommendation may put someone at risk for mercury poisoning if you're eating two to five cans per day. 

Albacore (white) tuna and light tuna are the two most common kinds of canned tuna. Due to its larger size, white tuna contains significantly more mercury—up to three times more—than light tuna. Depending on your age, the recommended serving of tuna per month varies. While children younger than six can safely eat one 3 oz portion of Albacore tuna a month, children between six and twelve years old can eat two 4.5 oz portions a month. It’s recommended that adult men eat no more than three 8 oz portions of Albacore a month whereas adult women eat no more than three 6 oz portions. Guidelines around light tuna are a bit more lenient due to its lower mercury content. That being said, adults can generally eat it once a week without any concerns. 

So how does mercury make its way into fish in the first place? Mercury found in fish can be the result of both natural and manmade sources. While volcanoes sometimes release mercury into the environment, presence of the element more largely comes from the waste of factories and other industrial settings. Mercury eventually travels to lakes, rivers, and oceans where microorganisms present in the water turn it into methylmercury–a highly poisonous form of mercury. Fish then absorb this chemical into their bodies from the water as mercury binds to the proteins in fish muscle tissue. This means that mercury may still be present in the fish when they are caught and consumed. 

Geographic location may also impact how much mercury fish take in from their surroundings. Other factors that impact mercury levels include what the fish eat, how long they tend to live, and where they are in the underwater food chain. Larger fish typically contain higher levels of mercury not only because they're heavier and have more surface area to absorb mercury, but also because they eat smaller mercury-containing fish, which increases their mercury content. Because of this, the Environmental Protection Agency (EPA) recommends staying away from shark, swordfish, king mackerel, and tilefish from the Gulf of Mexico, which tend to have higher levels of mercury. 

Most of the warnings about mercury poisoning highlight the danger for young children and pregnant women. This is primarily because exposure to mercury during development can cause neurological defects, including impairments in cognition, memory, attention, language, and fine motor skills. This can be especially important for pregnant people and those who are nursing to take note of because even if they show no signs or symptoms of mercury poisoning after eating fish, they can still transfer that mercury to infants through the placenta or via breastmilk. 

Mercury poisoning can also cause a number of issues in adults such as numbness in the fingers and toes, muscle weakness, and speech, hearing, and walking impairments. To date, research does not show an association between mercury exposure in humans and cancer, however, human studies are limited. If you find yourself experiencing any of these symptoms, it’s best to visit your health care provider as soon as possible. If you feel fine but are still concerned about the amount of mercury you are potentially consuming, consider swapping some of your weekly tuna for a variety of other low-mercury seafood options such as salmon, shrimp, pollock, catfish, cod, or tilapia. Canned salmon in particular might be a similarly priced replacement for canned tuna. 

The Washington State Department of Health created a healthy fish guide which informs consumers of the recommended weekly servings for safely eating fish. This resource also notes various health benefits for each of the fish. Lastly, if cost is of concern, there are many other options for non-meat or fish related protein that may better fit your budget. If eating meat is not an issue, you could try other non-fish animal sources of protein such as chopped canned chicken or lean deli meats as these can also be part of a well-rounded eating that don’t break the bank. 

Last updated Mar 31, 2023
Originally published Jan 12, 2007

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