SUSTAINABILITY FEED & ADDITIVE MAGAZINE October 2025 61 differentiating available selenium sources, it is important to note that only L-selenomethionine (L-SeMet) can be incorporated into animal protein and therefore can provide a safe deposit of selenium inside the body. L-SeMet is a methionine molecule, but instead of sulphur, the sulphur in the molecule is replaced by selenium. Thereby, L-SeMet can be incorporated as an amino acid in animal tissue. All other forms of selenium cannot be stored and can only be used for the synthesis of selenoproteins used in the animal metabolism. Due to the ability to create a selenium reserve in the body, L-SeMet is the most effective form of selenium, contributing to an optimal selenium status. Several commercial selenium products contain L-SeMet, like selenized yeast, OH-Selenomethionine (OH-SeMet) and pure L-selenomethionine (L-SeMet). However, the content and bioavailability of L-selenomethionine for these different products varies significantly and therefore it is important to carefully consider this, as to make sure you are using the most bioavailable, and therefore sustainable form of selenium. Selenized yeast products are produced by culturing yeast on a high-selenium media. This causes the selenium to be incorporated into the yeast, partly as L-SeMet. Not all selenium will be incorporated as L-SeMet, part will be incorporated in other forms such as selenocysteine. In Europe, selenized yeast products should contain at least 63% of their selenium as L-SeMet (EFSA) but in practice, these contents can sometimes be even lower. An important consideration on the L-SeMet in selenized yeast, is that this is incorporated into the yeast protein. Before L-SeMet can be absorbed, the yeast protein first needs to be digested and split up into peptides and amino acids. Digestibility of yeast protein is around 80%, and taking that into account, the average selenized yeast product will contain 63% SeMet * 80% digestibility = 50% digestible L-SeMet. The remaining selenium in the product is present in other forms, which cannot be used for creating selenium deposits in the animal. OH-SeMet on the other hand is a synthetically produced molecule which must be converted into L-selenomethionine before it can be used by the animal. This conversion takes place by two enzymatic steps in the liver and kidney. This conversion causes the product to ‘lose’ some of its efficacy (±20%) and results in a bioavailability of around 80%. Bioavailability of OH-SeMet is often explained by considering the relative utilization of different methionine isomers, where L-methionine is considered at 100% utilization and OH-methionine at 80% (in monogastric animals). Pure L-SeMet sources are also produced by chemical synthesis, but these products contain all selenium in the form of L-SeMet and are considered to have 100% bioavailable L-SeMet . Because of the 100% bioavailable L-SeMet content, these products are considered to allow for the most optimal selenium status of animals. The evidence for optimal efficacy of L-SeMet vs other organic selenium sources has been confirmed by Van Beirendonck et al. (2018) in an in vivo broiler trial. In total, 140-day-old chicks were divided over 7 treatments; Control: no added selenium (Se), T1: control + 0.20 ppm Se (sodium selenite), T2: control + 0.20 ppm Se (L-SeMet), 0.16 ppm Se (L-SeMet), 0.20 ppm Se (OH-SeMet), 0.20 ppm Se (selenized yeast; 45% L-SeMet), 0.20 ppm Se (selenized yeast; 29% L-SeMet). Treatment 2 and 3 have L-selenomethionine from Excential Selenium 4000; Orffa Additives BV.
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