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Another Reason Not to Fear Fish
As health-conscious consumers, we are rightfully concerned about mercury in our food supply. Long regarded as one of the most dangerous environmental toxins, mercury was a frequent cause of poisonings reported throughout the past century, most originating from industrial accidents. However, the incidents in Minamata and Niigata Japan that sickened over 3000 residents between 1956 and 1965 were unique in that the illness was caused by eating fish contaminated with methyl mercury. And, since mercury (unlike most other toxins) has no difficulty in crossing both the fetal membrane and blood-brain barrier, there is a logical concern that the risk of infants developing permanent cognitive impairment may be increased via transmission of mercury from the womb (of fish eating mothers) to the fetus (as witnessed in Minamata).
The infant cognition/mercury hypothesis has been the subject of several studies on coastal tribes and societies - most producing scant or no support to the widespread belief that learning disabilities in children are linked to elevated seafood consumption. Put into perspective, however, the people of Minamata were consuming fish that contained approximately 40 ppm of mercury, unquestionably the result of 27 tons of industrial mercury waste being dumped into Minamata Bay. By comparison, most fresh water and salt water fish (including farmed) contains a mere ¼ to ¾ ppm with a few predator fish (like shark) jumping up to 2-3 ppm. So it's conceivable that the lack of evidence (apart from tragic events like Minamata and Niigata) may simply be attributable to low, overall planetary consumption.
Low mercury consumption = No mercury poisoning is certainly a viable intuitive explanation of why incidents of mercury-induced illness are rare. However, the puzzle appears to be more complex. The FDA has set an "action level" of mercury at 1 ppm - 1/10th the level (i.e. 10 ppm) thought to cause disease in frequent fish consumers. Knowing, however, that there are coastal tribes who depend entirely on seafood as their main food source, and recognizing that fish harvested from their local waters contains 6-7 ppm just doesn't add up when screened against their near total lack of mercury toxicity, including neuropathology in children.
The biggest clue to solving the riddle may be found in the Faroe Islands where a higher incidence of suppressed cognition development among the fish-eating populace has been noted. Unlike other coastal societies, however, the Faroe residents consume high amounts of pilot whale - a fish containing only a scant amount of selenium. Other fish known to be low in selenium includes marlin, tarpon, and some shark. Fish most commonly consumed (tuna, salmon, sole, tilapia, etc.) are rich in selenium. Studies of people who consume lots of high-selenium fish and little or no selenium-deplete seafood (such as the residents of the Seychelles Islands) show no negative mercury-related health effects.
The possible positive effect of selenium in reducing mercury toxicity is well-supported in chemistry and physiology. Sometimes called a "mercury magnet", selenium has a particularly strong affinity for mercury and has the ability to sequester mercury molecules making them biologically inactive - a fact verified in animal studies. Low bioavailability means less methyl mercury to bind to key brain receptor sites in the developing fetus, ultimately reducing the risk of neuropathology. Although the compromising effect of selenium against mercury poisoning is still debated, it is highly likely, in my opinion, that the protective hypothesis is valid - especially when weighed against the high selenium/mercury ratios in common seafood. In tuna, for instance, there are approximately 19 molecules of selenium for every molecule of mercury (for a ratio of 19). Other fish having equally impressive ratios include salmon (S/M=15), halibut (S/M=10), cod (S/M=8), Pollock (S/M=8), snapper (S/M=6), grouper (S/M=5), swordfish (S/M=5), etc. As expected, the ratio for the pilot whale is low, i.e. 0.3. Thus, even under ideal conditions, at least 2 mercury molecules out of 3 would be biologically available to induce illness.
With a predominance of nutrition research yielding negative implications, it is like a breath of fresh air to discover a natural protective mechanism that protects us against an environment and food-borne illness. Fish has always been a good choice for avoiding heart disease with as little as one serving per week reducing the risk by over 30%. And now that the long shadow of potential mercury poisoning has been lifted, it looks even more inviting.
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