Selenium is an important trace mineral that is incorporated into a number of selenoproteins. Important selenoproteins include glutathione peroxidase, thioredoxin reductase, iodothyronine deiodinase and selenoprotein-P. Selenium is of interest because evidence suggests that a deficiency of selenium can increase the likelihood of certain types of cancer. For example, areas of China that have low selenium soil content have very high incidences of cancer and supplementation with selenium reverses this effect. However, selenium in the diet is no use if it is not absorbed by the gut. Selenium (and other nutrients) are defined as bioavailable if they are absorbed and utilised in normal physiological functions in the body. While the absorption and excretion of selenium can be measured, the selenium used in physiological processes is harder to quantify, and there are still large caps in our understanding of selenium metabolism.
The major species of selenium in plants are selenate (from the soil), selenomethionine (biosynthesised), selenocysteine (biosynthesised), selenium containing proteins (where selenomethionine and selenocysteine have been incorporated into proteins in place of methionine and cysteine, respectively), dimethylselenide (volatilised by non-accumulators), diselenide (volatilised by accumulators), as well as selenium-methyl-selenocysteine and γ-glutamyl-selenium-methyl-selonocysteine (detoxification products in the selenium accumulating Brassica and Allium families). Brassica and Allium vegetables accumulate selenium in these non-protein seleno-amino acids to prevent excess amounts of selenomethionine or selenocysteine causing toxicity in their proteins. Brazil nut trees (Bertholletia excelsa) are also selenium accumulating plants that differ from the Brassica and Allium plants because they accumulate selenomethionine. When livestock animals are fed inorganic selenium they tend to form selenocysteine, but when they are fed organic selenium from plant material they incorporate selenomethionine into their own proteins.
The absorption of selenium is generally high when compared to other minerals, and is ≥ 80 % from food. Inorganic selenate is absorbed at near 100% whereas inorganic selenite is less well absorbed at around 50 %. Absorption of selenium has been found to be lower in sea food compared to other animal sources in a number of studies, possibly because mercury present in seafood forms a selenium-mercury complex that makes absorption less likely. Absorption of dietary selenium is also blocked by a number of factors including guar gum and sulphur in the diet. Generally organic forms of selenium are better absorbed and retained that non-organic selenium, although supplementation with all forms of selenium raises levels of the enzyme glutathione peroxidase. Selenomethionine uses the same transport system as methionine and supplemental selenium yeast containing 66 % selenomethionine was shown to have absorption of 75 to 90 %.
Selenomethionine and selenocysteine seem to show better bioavailability because they are incorporated into tissue proteins where they act as a store of selenium. This selenium is released during normal body turnover. Because brazil nuts contain selenomethionine they are a very bioavailable source of selenium. Mushrooms also contain selenomethionine, but the bioavailability of mushroom selenium may be species specific, because studies have found differing bioavailabilities. Evidence suggests that organic forms of selenium are retained for longer in the body, perhaps up to 2.5 times as long as non-organic sources such as selenite. This may result from the slow turnover of stored selenomethionine and selenocysteine in the proteins of the skeletal muscle, erythrocytes and albumin when the selenium is stored in place of methionine or cysteine. Foods and organic supplements can therefore maintain selenium enzymes during times of depletion for longer that non-organic sources.
RdB
