revised final - Agency for Toxic Substances and Disease Registry ...

MERCURY 168
2. HEALTH EFFECTS
The reduced bioavailability of the plant food diet compared with the animal-based diet (fish meal) may be
due to the presence of indigestible fibrous materials present in plants. Another factor that might affect
absorption is the form of mercury ( 203Hg and methylmercury in the corn and fish meal diets, respectively).
The experiments by Yannai and Sachs (1993) are different from other absorption experiments because
mercury was intrinsic to the fish, grain, or silage, while in other studies mercury is simply mixed with the
experimental diet, usually as mercury salts. In the Iraqi epidemic, methylmercury fungicide was applied
extrinsically to wheat that was made into bread. However, no studies were located that measured the
absorption of methylmercury when mixed with grain. It is also not known whether the putative
component(s) of grain affecting bioavailability are the same in corn and wheat.
Interaction with selenium in diet. The co-administration of methylmercury and selenium is known to
depress methylmercury toxicity (Cuvin-Aralar and Furness 1991; Imura and Naganuma 1991).
Furthermore, the level of selenium in human hair has been found to negatively correlate with the level of
mercury in brain tissue (Suzuki et al. 1993). Methylmercury forms a bismethylmercury selenide complex.
Selenium in foods (especially fish) may also complex with methylmercury and, therefore, may potentially
reduce the bioavailability of methylmercury. The available data indicate that neither methylmercury uptake
nor bioavailability is affected by its presence in fish. Experimental studies on the metabolism of methylmercury
in humans following oral ingestion using methylmercury bound to fish muscle protein have shown
that absorption is almost complete (95% absorbed) (Miettinen 1973). Animal studies also support this
absorption value. Data on cats given fish homogenates indicate absorptions of $90% of methylmercury
added to the homogenate, of methylmercury accumulated by fish in vivo, or from methylmercury proteinate
(Berglund et al. 1971). Using blood and tissue levels as evidence of absorption, Charbonneau et al. (1976)
concluded that there was no difference in the biological availability of methylmercury administered to adult
cats (0.003, 0.0084, 0.020, 0.046, 0.074, or 0.176 mg Hg/kg/day 7 days a week for 2 years) either as pure
methylmercuric chloride in corn oil added to a diet containing uncontaminated fish or as methylmercurycontaminated
fish. In the 2 highest dose groups (0.074 and 0.176 mg Hg), at 100 weeks of exposure no
significant differences were seen in total mercury concentrations in blood between groups receiving the
dose as methylmercuric chloride or as contaminated fish at the same dose level. In addition, monthly blood
levels were comparable for all dose groups. No significant differences were seen at 100 weeks in total
mercury concentrations in the nervous system tissue or other tissues (renal cortex, renal medulla, liver,
spleen, adrenal, bladder, atria, ventricle, ovaries, testes, muscle) between the 2 highest dose groups
receiving the dose as methylmercuric chloride or as contaminated fish at the same dose level.