New studies have shown that over a third of mercury in the US diet comes from tuna, and that not all tuna are equal in mercury content.

A study by Edward Groth in the journal Environmental Research analysed the US consumption of seafood and the mercury levels of each species to reveal that 37.4% of the US’s mercury consumption is from tuna.

Another study, by the American Museum of Natural History, revealed that not all tuna species have the same mercury content, and that not all parts of the fish have equal mercury content.

“We found that mercury levels are linked to specific species,” says Jacob Lowenstein, a graduate student affiliated with the Museum. “So far, the U.S. does not require restaurants and merchants to clarify what species they are selling or trading, but species names and clearer labeling would allow consumers to exercise greater control over the level of mercury they imbibe.”

The Museum’s study found that all the species of tuna tested – bigeye, yellowfin and bluefin – had mercury levels exceeding or approaching levels permissible by Canada, the E.U., Japan, the U.S., and the World Health Organization.

The study found that bigeye tuna and the dark, lean meat of bluefin tuna have the highest levels of mercury, followed by fatty bluefin (toro, the most expensive tuna) and then by yellowfin.

Mercury is ‘bioaccumulative’ – levels of the metal accumulate over an organism’s lifetime, and will be passed on to whatever eats that organism. Younger or smaller fish tend to have lower mercury levels than larger fish.

Mercury has no minimum safe consumption level, and prenatal exposure to methylmercury (the compound of mercury found in fish) has been implicated in deficits in cognitive development and neurological function.

“Because of the clear nutritional benefits of maternal fish consumption, women of childbearing age should eat fish, and probably should eat more than they do now, on average. But emerging evidence that no level of methylmercury exposure may be free of appreciable risk to the developing brain reinforces the urgency of advising women in this age group to choose low-mercury fish,” says Groth.

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