Facts: This particular nugget of misinformation originated with the US FDA, although the tuna lobby has been more than happy to propagate and benefit from it.
The truth is, while canned light tuna has only one-third as much mercury as canned albacore tuna, it still has a well-above-average mercury content. The average mercury level in the US seafood supply as a whole, calculated by FDA (and confirmed by our own independent analysis) is 0.086 part per million. The average level in canned light tuna, 0.118 part per million, is 37 percent higher than the overall average. Given its very large market share and this elevated mercury content, canned light tuna is the biggest single source of methylmercury exposure in the American diet, accounting for 16 percent of the mercury in the seafood supply.
Further, not all canned light tuna is the same. The FDA average figure of 0.118 part per million is based on extensive sampling of major US brands of tuna—Bumblebee, Star-Kist and Chicken of the Sea. But FDA has not done much testing of “minor” tuna brands, including brands imported from South and Central America. Some of the latter brands have average mercury levels much higher than the average in US brands. Also, amounts of mercury in individual cans of even the major US brands vary, and some cans have much higher than average levels. Because of this variability and uncertainties about the timing of potentially harmful exposure during fetal development (see “The Reference Dose is for Lifetime Exposure“), several consumer and public-health organizations have advised pregnant women to avoid all tuna, including canned light. We think that’s sound advice.