Sashimi news: Mercury in tuna, marlin-- the threat may not exist

There are high levels of mercury in many popular eating fish, but the kneejerk assumption of human health hazard may not be appropriate.

It's just not that simple.

(Image: Bluefin tuna, Thunnus thynnus. Credit: NOAA's Fisheries Collection)

In fact, tunas and other fishes may also contain selenium, a substance that provides mercury protection for humans—binding up and detoxifying the mercury.

The issue has come to a head recently with a new publication on mercury, a publication that ignores the selenium protective issues.

The cited paper focuses strictly on mercury levels and the potential health impacts of those levels of mercury, without looking at other compounds in the fish that may reduce or eliminate the threat.

In fact, there's a body of evidence that tunas, in particular, contain a fair amount of selenium—and that tunas with high levels of mercury may accumulate even higher levels of protective selenium.

A 1972 study found that Japanese quail that were fed high levels of mercury in a corn and soy diet did not live as long as quail fed a diet with tuna in it. Rat studies showed that selenium decreased the toxicity of methylmercury in rats. The study concludes: “Selenium in tuna, far from being a hazard in itself, may lessen the danger to man of mercury in tuna.”

A 2007 study by John Kaneko and Richolas Ralston, on fish caught near Hawai'i, is even more clear: “Protective effects of selenium against mercury toxicity have been demonstrated in all animal models evaluated.” http://www.atuna.com/reports/selenium.pdf

It goes on to say that when studying dietary risks “considering mercury content alone is inadequate.”

Let's be clear. There is real seafood mercury threat in certain situations. Residents of Japan had significant health impacts on eating seafood after an industrial spill at Minamata Bay in Japan 50 years ago. And whale meat appears to accumulate mercury without some of the protective levels of selenium. Also, shoreline fish exposed to mercury pollution from the land can be implicated. And freshwater fish may not have consistent levels of selenium.

But most deep ocean fish appear safe in this regard.

In the Kaneko-Ralston study, the researchers cut up dozens of ocean-caught tuna of various species, swordfish, mahimahi, marlin, spearfish, opah and others. Among them, only the mako sharks sampled did not appear to have sufficiently high levels of selenium to be protective.

“Because of the significantly negative selenium health benefit value of mako shark, consumption of this fish during pregnancy would not be advisable,” the authors wrote.

Kaneko, a veterinarian, fisheries researcher and consultant, said the issue can be very complex, but “the evidence to met is pretty convincing” that most deepwater species are perfectly safe to eat from a mercury perspective.

A number of papers on seafood safety is available here.

© Jan TenBruggencate 2010