Mercury is a poison. Certain fish have high levels of mercury. To avoid being poisoned, then, you should avoid eating fish.
That’s the oversimplified syllogism that has guided many consumers over the last few years, ever since alarms were raised about the harmful health consequences that arise from exposure to seafood with high levels of mercury.
People attending the recent meeting of the Western Pacific Fishery Management Council were given a contrary view by Nicholas Ralston, a research professor at the University of North Dakota who has studied the relationship between mercury and selenium, an essential trace element that is also found in ocean fish.
Cynics might easily dismiss Ralston’s arguments as meshing a little too nicely with the interests of his hosts — people whose livelihood depends in large measure on a robust market for seafood.
But hear him out.
Three years ago, when the U.S. Food and Drug Administration advised young children and pregnant women against a diet high in tuna, swordfish, and other large ocean fish, Ralston says, it was acting on the basis of a study that showed children born to women in the Faroe Islands, where pilot-whale meat accounts for a large portion of the diet, had “subtle adverse effects” of mercury poisoning as indicated on developmental tests.
Pilot whales, however, have much higher levels of mercury in their meant than do ocean fish — and, what is even more important from his perspective, he said, the whales have relatively little offsetting selenium. A similar study of children in the Seychelles islands, where the diet is heavy in ocean fish (but not marine mammals), found no harmful effects, he said. What’s more, the study found a positive correlation between diets high in ocean fish and test performance by the children in the study.
More recently, Ralston said, in late February, a study published by the Lancet, a respected British medical journal, strengthened the association between a diet high in ocean fish and neurological function. The article, by a team led by Joseph Hibbeln, a research wither the National Institute of Alcohol Abuse and Alcoholism, found that children born to women who had limited seafood in their diet to roughly the levels advised by the U.S. Food and Drug Administration performed more poorly on verbal IQ tests than did children whose mothers ha consumed higher levels of seafood during their pregnancy.
“The exciting thing,” Ralston said, “is that the study … of over 8,000 women found taht maternal seafood intake during pregnancy of less than two meals per week was associated with greater risk of their children being in the lowest quartile for verbal IQ and suboptimal outcomes in prosocial behavior, fine motor, communication, and social development scores.”
Unlike the Faroe Islands or Seychelles study, the foods to which the mothers were exposed were “identical to foods Americans are exposed to,” he said. “And not only is this a bigger study, it’s also a study that has none of the aspects that critics can point to and say, ‘yeah, but…’ This study has true IQ measurements. In the Faroe study, the IQ measurements were only approximated.”
“There are many concerns about mercury toxicity and a definite need to control mercury exposure,” Ralston said. “But there is conflicting evidence for the toxicity of trace amounts of mercury from diets including fish.” He noted that Omega-3 fatty acids are essential for heart function and brain development in infants and children, and help brain function in adults as well. And ocean fish are one of the best sources of such fatty acides. The diet of the average Japanese includes about 170 pounds of fish a year, but no widespread brain damage can be related to that, he said.
Partnering with Selenium
So how can the human body tolerate even relatively high levels of mercury?
The key, Ralston says, lies in selenium, which binds so tightly to mercury that it effectively locks up mercury, making it unavailable to be assimilated in the human body.
“Problems occur when the mercury-selenium ratio is high” and mercury locks up all the body’s selenium, he said. “But when the selenium-mercury ratio is high, mercury exposure is OK.” In fact, so long as the molar ratio favors selenium (that is, there are more selenium atoms than mercury atoms), the mercury levels of the sort found in even the largest ocean fish do not pose a problem, he says.
“Ocean fish consumption does not appear likely to contribute to mercury toxicity, but would instead be expected to be effective in reversing it,” he said.
Mercury at Home
Immediately following Ralston’s presentation, John Kaneko of PacMar, Inc., gave the council the results of his analysis of the mercury-selenium levels in fish that make up most of the local pelagic catch.
Kaneko’s study, financed in part by the National Oceanic and Atmospheric Administration, looked at mercury and selenium in four species of tuna, swordfish, three species of marlin, mahimahi, opah (moonfish), wahoo (also known as ono, a type of mackerel), monchong, and escolar.
All but one of the species surveyed had good selenium-mercury molar rations, he said. Of the local fish tested, only mako shark had higher levels of mercury than selenium.
— Patricia Tummons
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