Mercury poisoning: Something fishy about too much tuna?

Originally Published: January 12, 2007 - Last Updated / Reviewed On: January 25, 2008
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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 healthy diet; it's loaded with high-quality protein and omega-3 fatty acids and low in saturated fat and cholesterol. However, there has been much talk in the news over the past few years concerning high levels of methylmercury in larger fish, such as tuna, mackerel, and swordfish.

Mercury, a pollutant, is released into the environment from factories and other industrial settings. It travels to streams and oceans where bacteria present in the water turn it into methylmercury. Fish then absorb this chemical into their bodies from the water. Larger fish 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. The methylmercury from small fish can accumulate in the body of the larger fish, increasing the larger fish's mercury content. Because of this, the Environmental Protection Agency (EPA) recommends staying away from shark, swordfish, king mackerel, and tilefish (the four largest types of predator fish) since they're more likely to have dangerously high levels of mercury.

Albacore (white) tuna and skipjack (light) tuna are the two most common kinds of canned tuna. Due to its smaller size, light tuna contains significantly less mercury than white tuna. The EPA guidelines state that it's safe to eat up to twelve ounces of light tuna a week or six ounces of white tuna a week. Considering that the standard weight of a can of tuna is six ounces, you may be putting yourself at a risk for mercury poisoning if you're eating two to five cans per day.

Most of the warnings about mercury poisoning are targeted to young children and pregnant women because exposure to mercury during development can cause mental retardation or physical disabilities. Mercury poisoning in adults can cause numbness in fingers and toes, memory and vision loss, and tremors. If you find yourself experiencing any of these symptoms, you need to visit your health care provider as soon as possible. If you feel fine but are scared of prematurely swimming with the fishes, you might want to consider switching to other types of fish or seafood that don't contain as much mercury, such as salmon, sole, shrimp, herring, or haddock.

The National Resources Defense Council's Mercury Contamination in Fish - Consumer Guide to Mercury in Fish provides tools that can help make this transition proceed swimmingly. It contains a list that informs consumers of the frequency that a certain fish can be eaten safely, as well as a mercury calculator that generates a safe value for fish intake based on a person's weight and type of fish. You could also try substituting the tuna with non-fish sources of protein, such as chopped canned chicken, lean deli meats, or beans; these can also be part of a healthy diet.


January 25, 2008



I am going to graduate soon with my biology degree and have done much research on this subject. Alice is right. Just like the mercury builds up in the fish's body it also builds up...


I am going to graduate soon with my biology degree and have done much research on this subject. Alice is right. Just like the mercury builds up in the fish's body it also builds up over time. There is an accumulative effect. Right now you may not have any side effects, but if you continue eating tuna at the rate you are ten or twenty years from now you could be facing some serious health risks. Not to mention if you are a woman and plan on having children one day there could be some problems. Good luck on finding an alternative — maybe try chicken :)