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Measuring the amount of mercury present in the environment or food sources may provide an inadequate reflection of the potential for health risks if the protective effects of selenium are not also considered.Chicago Tribune: Swordfish showed the highest mercury levels in the Tribune tests, averaging 1.41 parts per million, well above the 1.0 limit at which regulators can confiscate fish. In FDA testing, swordfish has averaged 0.97 parts per million. (December 11, 2005) Response: Swordfish Guidelines vs. Confiscation
The Tribune here states the common as though rare, and the well-known as though concealed. That it to say, reporting that high-end predators such as swordfish carry higher levels of MeHg in tissue and internal organs is well-known, especially since the FDA, EPA, and other federal and state agencies have advisories for these fish. CT reported mercury levels from a small sample of 18 Chicago area swordfish, ranging from 0.47 ppm to 3.07 ppm, with an average of 1.41 ppm. These levels are neither unusual nor remarkable when compared to the 605 swordfish samples taken by FDA over the 1990-2002 periods. This larger sampling showed a range from 0.1 ppm to 3.22 ppm, with a mean average of 0.97 ppm. FDA's mean mercury concentration for swordfish agrees well with the mean of 0.95 ppm reported by the EPA. From the literature, Mendez et al. (2001) recently analyzed 192 specimens of Southwest Atlantic swordfish ranging from 10 to 412 kg by weight. They found total mercury values of between 0.04 and 2.21 ppm with a mean of 0.62 ± 0.35 ppm. They also reported levels for two groupings by fish body weight. Those weighing less than 100 kg had a mean of 0.53 ppm ± 0.02 ppm; those weighing more than 100kg had a mean of 0.94 ± 0.06 ppm. These findings emphasize the importance of size and weight for determining likely mercury tissue levels. CT’s sampling and analysis is highly inferior to that of the FDA (18 samples versus 605) or Mendez et al., promotes a statistically biased mean of 1.41 ppm and calls for confiscation of the swordfish. Once again, EPA’s RfD definition (daily consumption over a 70 years period before discernible concern) is missing, providing no context for evaluating the meaning of CT’s data; which practical meaning is that Chicago consumers would have to daily -- for a lifetime running – select and eat only swordfish above the mean before exceeding EPA’s RfD. The odds of this happening are likely near zero. On top of this, EPA’s ultra-conservative RfD provides a safety cushion of 10 fold and does not consider the anti-oxidant role of selenium (discussed below). The Canadian approach
From a public health perspective, a zero-mercury/zero-risk regime encouraging widespread banning of all fish with mercury content above the numerical threshold of 1.0 ppm would be counter-productive, even harmful. Instead, the Canadian approach seems more sensible: Health Canada has established a guideline level of 0.5 parts per million (ppm) for mercury in most commercial fish. This guideline is enforced by the Canadian Food Inspection Agency (CFIA). It was first set in the 1970's and, based on a recent re-evaluation, is still considered appropriate to ensure that the health of Canadians is protected from the toxic effects of methyl mercury. Health Canada continues with highly conservative guidelines: Therefore, these species [Note: not canned tuna] are exempted from the 0.5 ppm guideline and, in their case, another risk management strategy is followed, namely, issuance of advisories recommending appropriate restrictions on (amounts and frequency of) consumption. In this way, these species can continue to be enjoyed by consumers as part of an occasional meal. As noted in Sec. 3, the FDA/EPA joint fish advisory urged even more strict precaution for pregnant women, women of childbearing age and young infants and children to avoid fish species with high mercury contents, listing particularly swordfish, shark, king mackerel and tilefish (also called golden bass or golden snapper). Another key consideration is that in the marine consumption guidelines by Arctic rim nations and the State of Alaska, there is the persistent, fundamental concern that an unbalanced focus on mercury content risks an unhealthy disregard for benefits of essential daily nutrients and vitamins. These include high-quality protein, lysine, iodine, sulfur-containing amino acids, copper, calcium, zinc, iron, manganese, selenium, omega-3 polyunsaturated fatty acids, vitamin E (anti-oxidants) and more. The Selenium (Se) Factor
Some studies (Raymond and Ralston - 2004) have suggested that proportionally higher selenium levels in consumed ocean fish may counteract the toxicity of fish tissue MeHg. Swordfish for example, while higher in MeHg, is a superior source for omega-3 fatty acids and selenium. A recent survey of 11 commercial sea foods in New Jersey confirmed that selenium concentrations exceeded those of mercury; for some species the selenium to mercury ratio was as high as 23. An analysis of 39 types of commonly consumed fish and shellfish in Modena, Italy by Plessi et al. (2001) also confirmed "a large excess of selenium in relation to mercury". Both fish selenium and amino acids are well-documented for helping reduce toxicity by inhibiting entry/transport of MeHg into brain cells. This critical finding may suggest why Minamata-like poisonings have not been diagnosed from chronic MeHg-exposures through large daily intake of fish not directly contaminated with multiple chemicals. For example, Robinson et al. write: Per capita fish consumption in Seychelles is known to be high by world standards, and it is understood that Hg exposure results primarily from high rates of fish consumption. The ongoing Seychelles Child Development Study (SCDS) has initiated a nutritional study (Seychelles Child Development Nutrition Study) that focuses on mothers during pregnancy and offspring in early childhood…Results from the SCDS have shown that, despite considerable and continuous prenatal exposure to methyl mercury (MeHg) through the regular consumption of fish, there is no evidence suggesting that child development in the main cohort (tested at 6, 19, 29, 66 and 107 months) has been adversely affected. These consistent findings suggest that other constituents of the Seychellois diet might protect against the toxic effects of Hg. Experiments have shown that Se can reduce the neurological toxic response resulting from exposure to Hg. Also, among the Amazonian population of Rio Tapajós’s gold-mining region no typical symptoms of Minamata disease were detected at mean MeHg hair levels of 13.8 ppm or 21 ppm. Even an individual with the extreme exposure value of 303 ppm presented with no symptoms of Minamata-like illnesses. The scientific hypothesis that selenium plays a key role for naturally limiting the toxicity of methylmercury for these Amazonian fish eaters was recently clarified by Lima et al. (2005): Mercury is a major public health concern because of its widespread occurrence in the environment and its toxic effects on humans, mainly through fish ingestion. On the other hand, selenium is known by its antioxidant effect....Our study demonstrated that the presence of Hg in fish is positively correlated to the concentrations of Se [see Fig. 11-A]. As previously mentioned, the correlation between these metals was also reported in riverside communities, hair samples from the Amazon, which confirms the exposure via consumption of fish to be a significant source of human exposures. Thus, the high contents of Hg found in this work and the absence of official reports on mercury poisoning in the inhabitants of Amazon region suggest that Se can be acting as a detoxification agent. Fig. 11-A illustrates that as mercury content increases with weight for two freshwater species taken from rivers and artificial lakes in the Cachoeira do Piria Municipality of ParaState, the selenium content increases disproportionately more. The measurements by Lima et al. (2005) confirm that the selenium-to-mercury ratios are generally greater than one and range from 1.28 to 17 for a wide variety of carnivorous, omnivorous and herbivorous fish species. So, what are the key biochemical roles played by selenium and related selenium-based proteins and enzymes, and how does selenium interact with mercury for reducing toxic outcomes? First, selenium, as selenoprotein/seleno enzymes (including 2 of the 22 primary amino acids), is known as an important determinant for maintenance of human health regarding (a) immune function, (b) protection from viral infection, (c) reproduction, (d) mood, (e) thyroid function, (f) cardiovascular health, (g) oxidative-stress or inflammatory conditions, and (h) cancer prevention (Rayman 2000). About 35 Se-containing proteins are present in the cells of the human body. While there are other enzymes that assist the seleno-enzymes in detoxifying the free radicals, they are less active in brain tissues. Selno-enzymes are thus known to perform vital functions in brain tissues (Ralston 2003). The role of Se is further underlined by the fact that it is the only trace element to be specified in the genetic code – selenocysteine, now recognized as the 21st amino acid.  Secondly, regarding selenium’s interaction with MeHg, Raymond and Ralston (2004) offer the following observations: Measuring the amount of mercury present in the environment or food sources may provide an inadequate reflection of the potential for health risks if the protective effects of selenium are not also considered. Selenium's involvement is apparent throughout the mercury cycle, influencing its transport, biogeochemical exposure, bioavailability, toxicological consequences, and remediation. Likewise, numerous studies indicate that selenium, present in many foods (including fish), protects against mercury exposure. Studies have also shown mercury exposure reduces the activity of selenium dependent enzymes. While seemingly distinct, these concepts may actually be complementary perspectives of the mercury-selenium binding interaction. Owing to the extremely high affinity between mercury and selenium, selenium sequesters mercury and reduces its biological availability. It is obvious that the converse is also true; as a result of the high affinity complexes formed, mercury sequesters selenium. This is important because selenium is required for normal activity of numerous selenium dependent enzymes. Through diversion of selenium into formation of insoluble mercury-selenides, mercury may inhibit the formation of selenium dependent enzymes while supplemental selenium supports their continued synthesis. ... In summary, studying the pathology of mercury toxicity may require a more insightful question than simply, 'How much mercury is consumed?' The more appropriate question may be, 'Is a sufficient amount of free selenium available in the cell to create the necessary selenoenzymes or is it too much selenium lost by binding to mercury? Ralston also reports that moderate additions of Se to lakes in Sweden reduced Hg-concentrations in fish by 75%! 173 In earlier research Ganther et al. reported: The implications of this study are that the danger for man of Hg in tuna may be somewhat less than anticipated, and that total Hg contents in the diet or even in the blood may not be valid criteria [for deciding toxicity]….It thus appears that Hg and Se tend to be accumulated together in tuna. Selenium in tuna, far from being a hazard in itself, may lessen the danger to man of mercury in tuna. 174 This critical science (missing from the CT series and the activist literature) may, in part or whole, point to an evolution of bio-adaptability for persistent, natural exposure to mercury – environmental mercury present since the earth formed 4.5 billions years ago. Reporting on interactions between selenium and mercury, Raymond and Ralston (2004) speculated upon a novel explanation that may contribute insight as to why the Faroe Children Study (discussed in Sec. 1) reported detecting "adverse neuropsychological" problems in association with prenatal exposure to methylmercury, but the better-designed study conducted in the Seychelles Islands did not yield any consistent effects, despite years of careful research testing. Raymond and Ralston (2004) observe: “The diet consumed by the Faroe Islanders includes [Pilot] whale where the Seychelles Islanders' diet does not. Whale is known to contain PCBs as well as possibly other toxins not typically found in fish. [See the list of chemicals in Sec. 1] Additionally, the amount of MeHg in some types of whale meat analyzed has been reported to be exceedingly high. The concentration of mercury present in whale rises continually with and can exceed the selenium content. This is seen in high-end predator whales such as pilot whales rather than filter feeders such as bowhead whales. (see Fig. 11-B) Mercury concentrations in samples of pilot whale have been 5,000 times greater than the Japanese government's limit for mercury contamination of 0.4 ppm. In contrast to whales, methylmercury concentrations in fish rise with age, but as their mercury contents increase, so do their selenium concentrations. To our knowledge, there are no reports of mercury exceeding selenium concentrations in any ocean fish.” Conclusion  Through out, CT has failed to contextually inform readers of the rich variety of essential nutrients and vitamins contained in swordfish and other popular seafood species, including the important protective role played by selenium. The mercury-attenuating effect of selenium needs particular consideration. |
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