Herbivorous Fish

I found an interesting study (Lino et al., 2018) that looked at the concentrations of both mercury and selenium in different species of fish in Brazil.  They found that carnivorous fish had the highest concentration of mercury and also the lowest concentration of selenium, while fish lower on the food chain were higher in selenium and lower in mercury.  The carnivorous fish accumulate higher levels of mercury because they have eaten so many fish themselves. 

The authors note that selenium intake has a protective effect against mercury exposure, so eating fish that are lower on the food chain should be recommended.  The selenium actually binds to the mercury, forming selenide-methylmercury, which is not toxic like mercury on its own.  They concluded the levels of selenium were not too high for human consumption in any of the fish, and if only herbivorous fish were eaten, it would be safe to consume 12-18 meals per week. 

Gochfeld et al. (2021) contend that total mercury exposure should still be evaluated and kept within recommendations, even if adequate selenium accompanies it.  They mention genetic variability in the body’s ability to eliminate mercury.  They would like to see more research to determine how much selenium is necessary for protection from mercury, and they think that levels in some species may not be enough.  They also caution on the toxicity of selenium itself, and that even if you were able to calculate the ideal amount of selenium and supplement, it wouldn’t be able to prevent all the toxic effects of mercury.  It seems that avoiding overconsumption of mercury containing fish is prudent, even if selenium status is adequate, but that selenium does offer some protection from mercury, enough to justify the inclusion of moderate amounts of low mercury fish in the diet for its well recognized health benefits.

References:

Gochfeld, M., & Burger, J. (2021). Mercury interactions with selenium and sulfur and the relevance of the Se:Hg molar ratio to fish consumption advice. Environmental Science and Pollution Research, 28(15), 18407–18420. 

Lino, A. S., Kasper, D., Guida, Y. S., Thomaz, J. R., & Malm, O. (2018). Mercury and selenium in fishes from the Tapajós River in the Brazilian Amazon: An evaluation of human exposure. Journal of Trace Elements in Medicine and Biology : Organ of the Society for Minerals and Trace Elements (GMS), 48, 196–201.