Selenium is a key nutrient in animal nutrition and is crucial for optimal antioxidant status and immune function. Selenium in animal diets can be supplied via feedstuffs or via supplemented selenium. Selenium from feedstuffs is predominantly in the form of L-selenomethionine, which is the natural form of selenium in plant and animal protein. Supplemented selenium can be either in an organic or an inorganic form. Organic selenium has an important benefit compared to inorganic selenium due to the fact that selenomethionine is utilized by the body as an amino acid (in same way as methionine). Organic selenium allows Se reserves to build in tissues, mainly in muscles, in the form of selenomethionine which can be used in stress conditions to improve antioxidant defences (Surai, 2016). Se reserves in the body are important to ensure a good selenium and anti-oxidant status at all times. In addition to the benefit of selenium reserves, organic selenium in the form of L-selenomethionine is also able to provide efficient transfer of selenium to offspring via placenta, milk and eggs and to enrich animal products.
Insight in selenium metabolism
In the metabolism there is a crucial difference between selenomethionine and all other forms of selenium. Traditionally organic selenium was supplied via selenium enriched yeast. EU authorized selenized yeast products contain a minimum of 98% of selenium in an organic form, but not all of this organic selenium is selenomethionine. Part is in the form of selenocysteine or other (organic) intermediates. In the metabolism, all selenium compounds are recognized as a selenium supply and all can be used for selenoprotein (selenoenzymes) synthesis. Besides this general pathway, selenomethionine is the only organic selenium that can be built into body proteins instead of methionine. Via this specific pathway selenomethionine is able to build up selenium reserves in the body and is able to transfer selenium to the offspring via the placenta, milk or eggs. The selenium reserve in the body can be mobilized for later selenoprotein synthesis. An adequate selenium reserve ensures an optimal selenium and anti-oxidant status at all times, even in times of stress or low feed intake. However, dietary selenocysteine, similar to sodium selenite, is not effective in increasing selenium tissue concentration (Surai, 2016). Selenocysteine can be used in metabolism, but dietary selenocysteine is not used directly for selenoprotein synthesis, since it is first reduced to selenide from which new de novo selenocysteine is formed. Inorganic selenium is also reduced to selenide and then utilized for selenocysteine and selenoprotein synthesis.
Several producers offer selenium yeast products for animal nutrition. To evaluate the quality of selenized yeast products, it is important to check the selenomethionine level in these products. A survey was set up to check the selenomethionine content in commercially available selenized yeast products in the market. In total 32 samples were collected from different producers. The samples were analyzed for total selenium and for selenium in the form of selenomethionine. The survey was set up by Orffa in cooperation with the lab of CODA CERVA (Tervuren, Belgium). The applied method of analysis is specific for selenomethionine and is based on HPLC ICP MS after enzymatic extraction. The results are shown in graph 1.