Mercury in canned tuna still a concern

New tests reinforce a need for some people to limit consumption

Canned tuna, Americans’ favorite fish, is the most common source of mercury in our diet. New tests of 42 samples from cans and pouches of tuna bought primarily in the New York metropolitan area and online confirm that white (albacore) tuna usually contains far more mercury than light tuna.

Children and women of childbearing age can easily consume more mercury than the Environmental Protection Agency considers advisable simply by eating one serving of canned white tuna or two servings of light tuna per week. A serving is about 2.5 ounces. Expect a 5-ounce can to contain about 4 ounces of tuna plus liquid.

The heavy metal accumulates in tuna and other fish in an especially toxic form, methylmercury, which comes from mercury released by coal-fired power plants and other industrial or natural sources, such as volcanoes.

Fortunately, it’s easy to choose lowermercury fish that are also rich in healthful omega-3 fatty acids. That’s especially important for women who are pregnant or might become pregnant, nursing mothers, and young children, because fetuses and youngsters seem to face the most risk from methylmercury’s neurotoxic effects.

Results from our tuna tests, conducted at an outside lab, underscore the longheld concern for those people. We found:

• Every sample contained measurable levels of mercury, ranging from 0.018 to 0.774 parts per million. The Food and Drug Administration can take legal action to pull products containing 1 ppm or more from the market. (It never has, according to an FDA spokesman.) The EPA compiles fish advisories when state and local governments have found high contaminant levels in certain locally caught fish.
• Samples of white tuna had 0.217 to 0.774 ppm of mercury and averaged 0.427 ppm. By eating 2.5 ounces of any of the tested samples, a woman of childbearing age would exceed the daily mercury intake that the EPA considers safe.
• Samples of light tuna had 0.018 to 0.176 ppm and averaged 0.071 ppm. At that average, a woman of childbearing age eating 2.5 ounces would get less than the EPA’s limit, but for about half the tested samples, eating 5 ounces would exceed the limit.

In 2006 we scrutinized the results of the FDA’s tests in 2002 to 2004 of mercury levels in hundreds of samples of canned tuna. The agency’s white-tuna samples averaged 0.353 ppm; light tuna, 0.118 ppm. But we found that as much as 6 percent of the FDA’s light-tuna samples had at least as much mercury as the average in white tuna—in some cases more than twice as much.

Given the uncertainties about the impact of occasional fetal exposure to such high levels, we urged the FDA to warn consumers about occasional spikes in mercury levels in canned light tuna. More than four years later, the FDA still hasn’t issued such a warning. When we asked why, an FDA spokesman indicated that the agency had already taken the spikes into account when formulating its mercury advice.

Bottom line

Canned tuna, especially white, tends to be high in mercury, and younger women and children should limit how much they eat. As a precaution, pregnant women should avoid tuna entirely. Our answers to the questions in Fish Q & A can help you get the nutritional benefits of fish and minimize exposure to mercury.