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Re: mercury toxic/ selenium protective

Posted by larryhoover on July 24, 2011, at 20:17:50

In reply to Re: mercury toxicity, posted by larryhoover on July 13, 2011, at 20:35:32

We were talking about the protective effects of selenium, with respect to mercury exposure from fish in our diets. Since I last posted here, I've done some further research, and I discovered that selenium supplementation is not only protective against mercury exposure, it is also restorative against past exposure. And, many fish with high mercury content actually have so much selenium in them, that they are of net benefit against other mercury exposures. Ive got links to three documents that were the main sources of those arguments.

In a nutshell, if selenium intake is high enough, then mercury exposure is of no consequence. Moreover, in selected fish species, the selenium content is high enough to not only make the mercury it contains of no consequence, there is sufficient selenium in excess so as to confer additional health benefits.

The first two have the same author, and are both powerpoint slide presentations.

The key slides are: #23) shows the effect of selenium supplementation on body weight when simultaneously exposed to mercury; #26) summary statements clearly express the relationship between mercury and selenium.

The key slides are: #10) the ratio of Hg:Se in various fish. All ratios below 1 are protective. #13) nice presentation of the fish with the greatest health benefit, based on the Hg:Se ratio. I would say that any fish with a score of 50 or better would be good picks to include in the diet.

This last reference is a paper, also in .pdf format. Youll note that the guy who did the above two presentations co-authored the paper, also. Ive pulled some selected quotes out of the text, for emphasis.

"Since then, numerous studies have shown selenium supplementation counteracts the negative impacts of exposure to mercury, particularly in regard to neurotoxicity, fetotoxicity, and developmental toxicity. The ability of selenium compounds to decrease the toxic action of mercury has been established in all investigated species of mammals, birds, and fish."

"Accordingly, if a toxin can enter the brain and disrupt selenoprotein synthesis, detrimental effects would be expected. Methyl mercury not only has the ability to cross the blood-brain barrier, but its exceptionally high affinity for selenium may enable it to specifically sequester the brains selenium and diminish selenoprotein synthesis. The affinity constant for selenocysteines selenium and mercury is ~10 ^-22, and the free selenides that form during each cycle of selenocysteine synthesis have an exceptionally high affinity constant for mercury: 10 ^-45 (29). Mercury selenide precipitates have extremely low solubility, ranging from 10 ^-58 to 10 ^-65;thus they are thought to be metabolically inert (30). It is reasonable then to assume that not only does selenium have an effect on mercurys bioavailability, but mercury may also have an effect on selenium bioavailability. Therefore, the understanding of the protective effect of selenium against mercury exposure may actually be backwards. Mercurys propensity for selenium sequestration in brain and endocrine tissues may inhibit formation of essential Se-dependent proteins (selenoproteins). Hence, seleniums protective effect against mercury toxicity may simply reflect the importance of maintaining sufficient free selenium to support normal selenium-dependent enzyme synthesis and activity (see figure)."

"...the upper limit of the estimated safe and adequate dietary intake is currently set at 200 microg per day."

So, I feel quite comfortable with the idea that most fish is of net benefit to health, notwithstanding the mercury it might contain. But, to be totally safe, a small supplemental amount of selenium might provide protection against past exposure, as well as future potential exposures.





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