Selenium is an important trace mineral in human nutrition. Selenium is perhaps best know as a cofactor for the enzyme glutathione peroxidase, an important component of the cellular reductive system. The glutathione reducing system (which includes glutathione, glutathione reductase, glutathione peroxidase and NADPH) is required for the conversion of damaging hydrogen peroxide to water. Low selenium intakes are associated with a decrease in the activity of glutathione peroxidase, with a resultant increase in free radical damage in cells and tissues. As activity of the glutathione antioxidant system decreases the risk of disease and mortality increases significantly. Dietary sources of selenium are therefore important to ensure optimal function of this important cellular antioxidant system. Brazil nuts are a well known source of selenium, but others exist. Tuna is also a rich source of the trace mineral, with studies suggesting that a tin of tuna (roughly 150 to 200 grams) many contain around 12.5 micrograms of selenium.
If a tin of tuna contains roughly 12.5 micrograms of selenium, it would appear that tuna is a reasonable source of selenium. Certainly consuming tuna regularly would contribute significantly to the selenium intake of most individuals. Of course there is nothing special about tinned tuna that means it accumulates selenium, and so fresh tuna will have a similar selenium content (about 150 parts per billion). However, while the selenium content of tuna is good, tuna is also a rich source of methylmercury. This results from the polluted waters that tuna inhabits. Bioaccumulation of mercury (and other pollutants) occurs as food passes up the food chain, such that large predatory fish such as tuna attain dangerous levels of certain pollutants such as mercury. Current recommendations are to limit consumption of large predatory fish and instead consume fish from further down the food chain (such as mackerel). Therefore sources of selenium such as brazil nuts and supplements are a preferable source of this mineral.
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