How to Know Which Tuna Is Most Toxic

Different species of tuna contain different levels of mercury, and some are better than others.

April 29, 2010

U.S.-caught albacore tuna is safest, but it's not in sushi.

The day it was discovered that tuna may contain large amounts of mercury, tuna salad sandwiches became social pariahs. Experts advised women and children to limit tuna consumption, as mercury can build up in the blood and even cause brain damage. In kids, high mercury levels can impair neurological development.

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But a study published in the journal Biological Letters finds that all tuna is not created equal. While most species contain dangerous levels of mercury, some have significantly lower amounts than others. Unfortunately, learning how to select the healthiest fish and avoid the unhealthiest can be difficult, thanks to strange, sometimes absent labeling on menus and packages.

THE DETAILS: The study authors worked with The New York Times to collect tuna samples from 54 sushi restaurants and 15 grocery stores in New York, New Jersey, and Colorado. In total, they collected 100 samples. Using DNA tests, they identified the exact species of tuna and, because mercury content is usually lower in tuna with a high fat content and vice versa, classified them as either lean (akami) or fatty (toro). Finally, each sample was tested for mercury content.

The highest levels of mercury were found in two species, bigeye and lean bluefin (bluefin akami), and the lowest levels were in yellowfin and fatty bluefin (bluefin toro). Every sample contained mercury at levels above those designated as safe by the Environmental Protection Agency (EPA). The researchers also found higher mercury levels than have been reported by the Food and Drug Administration (FDA), and noted that tuna sold in restaurants contained higher levels than tuna bought in grocery stores. Most likely this is because sushi-grade tuna is leaner and comes from larger fish, note the researchers. Finally, 77 percent of the tuna that came from grocery stores was yellowfin, which has relatively low mercury levels.

WHAT IT MEANS: Mercury-contaminated fish remains your primary source of exposure to this brain-damaging heavy metal, according to the EPA. And predatory fish like tuna, swordfish, and sharks contain the highest levels. Mercury does the most damage to women who are either breastfeeding, pregnant or of childbearing age, since the heavy metal can affect the developing brains of fetuses and infants. The EPA and Food and Drug Administration fish-consumption advisory stipulates that these women, and their children, should avoid entirely shark, swordfish, king mackerel, and tilefish, due to high mercury levels. The advisory also says to eat no more than two meals a week of fish or shellfish with lower levels of mercury, such as shrimp or wild Alaskan salmon. Tuna falls between those two categories, but it’s tough to know what you're eating when restaurants and grocery stores don't always advertise what species of tuna is on the menu.

To help you understand what you’re eating, here's a useful breakdown of tuna terminology, courtesy of the Monterey Bay Aquarium:

• Albacore. Albacore tuna is one of the healthiest fish you can eat, provided it's caught in the U.S. or British Columbia. This tuna is caught when it's younger and therefore has had less time to build up high levels of mercury, but albacore imported from other countries is caught when it's older and thus contains more mercury. So location is key. Canned albacore is always labeled "chunk white." You can buy American and Canadian albacore tuna from online sources including Heritage Foods USA, Pacific Fleet, MaryLu Seafoods, Wild Planet, and Wild Pacific Seafood.

• Chunk Light. Unlike "chunk white," "chunk light" tuna is a blend of different species and often includes meat from high-mercury bigeye tuna, along with less-contaminated yellowfin. It's best to avoid chunk light tuna. Instead, stock up on cans of American and Canadian albacore.

• Canned Light. "Chunk light" and "canned light" aren't same thing, either. The most common type of tuna sold as "canned light" is skipjack, which is sometimes referred to as yellowfin, and contains relatively low levels of mercury. However, according to the study, yellowfin samples still had enough mercury to make them unsafe for women and children. Again, best to avoid this type of tuna in favor of American and Canadian albacore.

• Market names. At a sushi restaurant or in the frozen fish section, tuna is often listed under market names that refer to cuts of meat, rather than species names. Thus, bigeye and yellowfin tuna can variously go by the names ahi, maguro, or toro. Bluefin tuna is often listed by its species name, but is also sold as kuromaguro, horse mackerel, atun de aleta azul, hon maguro, and toro. Along with its high mercury content, bluefin is heavily overfished and is listed as endangered. And the tests found that sushi tuna had the highest mercury levels, regardless of species.

Bottom line: If you're in a tuna-risk group, limit your tuna intake and stick to the safest type you can buy.

And when you do indulge in a can of tuna every now and then, here's how to make it taste amazing:

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