Selenium is an essential trace element and plays a crucial role in maintaining an optimal health status. Selenium sources can be categorised as organic or inorganic. Here we delve deeper into organic selenium and, more specifically, selenomethionine.
Selenium is a key nutrient in animal nutrition and is crucial for an optimal antioxidant status and immune function. Selenium in animal diets can be supplied via raw materials or via supplemented selenium. Selenium from feedstuffs is predominantly present in the form of L-selenomethionine, which is the natural form of selenium in plant protein and animal tissue. The supplemented selenium can be either in an organic or an inorganic form. Organic selenium has an important benefit compared with inorganic selenium due to the fact that selenomethionine, the effective organic part of organic selenium, is utilised by the body as an amino acid (in same way as methionine) and is built into animal protein tissue. Via this metabolism, organic selenium in the form of L-selenomethionine is able to build up selenium reserves in the body, which ensures a good selenium and antioxidant status at all times and secures the efficient transfer of selenium to offspring via the placenta, milk and eggs.
Selenomethionine in metabolism
In the metabolism there is a difference between selenomethionine and all other forms of selenium. Traditionally, organic selenium was supplied via selenium-enriched yeast. Selenium yeast consists of up to 98% organic selenium, but not all of this organic selenium is in the form of selenomethionine. Currently, EU legislation mentions that 63% of the selenium in selenium yeast should be in the form of selenomethionine; more recent authorisations refer to 70%. The remaining part is in the form of selenocysteine, other (organic) intermediates or inorganic selenium. The difference between selenomethionine and all other forms of selenium in the metabolism, as far as actual knowledge is concerned, is explained in Figure 1.
All selenium compounds are recognised as a selenium supply in the metabolism and all can be used for selenoprotein (selenoenzymes) synthesis. Besides this general pathway, selenomethionine 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 mobilised for later selenoprotein synthesis. An adequate selenium reserve ensures an optimal selenium and antioxidant status at all times, even in times of stress or low feed intake. However, all other forms of selenium in selenised yeast, such as selenocysteine and other organic selenium intermediates, are reduced to hydrogen selenide and then utilised for de novo selenocys-teine and selenoprotein biosynthesis. Dietary selenocysteine is not used directly for selenoprotein synthesis, but is first reduced to selenide, from which new de novo selenocysteine is formed. Inorganic selenium is also reduced to selenide and then is utilised for selenocysteine and selenoprotein synthesis. Thus, the remaining part of selenium in selenized yeast (selenocysteine and (organic) selenium intermediates) follows the same pathway as inorganic selenium and is considered to be not more effective than inorganic selenium in the metabolism. L-selenomethionine is considered as metabolically effective organic selenium.
Level of selenomethionine in selenised yeast
Several producers offer selenium yeast products for animal nutrition. To evaluate the quality of selenised yeast products, it is important to check the selenomethionine level in these products. A market survey was set up to check the selenomethionine content in commercially available selenised yeast products in the market. In total, 28 samples were collected from seven different producers. The samples were analysed for total selenium and for selenium in the form of selenomethionine. The survey was set up by Orffa in co-operation with the lab of CODA CERVA (Tervuren, Belgium). The analysis method applied is specific for se-lenomethionine and is based on HPLC ICP MS after enzymatic extraction. The results are shown in Figure 2.
From this market survey, it can be concluded that there is a major variation in the level of Se in the form of selenomethionine between different commercial samples of selenised yeast. The overview shows differences between producers, as well as variations between batches of the same producer. This variation can also be found in scientific literature and has been confirmed in other commercial reviews. The level of selenomethionine is an important quality parameter. Nutritionists are inceasingly aware of this and request guarantees from their suppliers.
New generation of organic selenium
Recently a new type of organic selenium was introduced into the animal nutrition market. This product tackles the problem of variable concentration. The product* consist of L-selenomethionine and is considered 100% digestible. All selenium in the product is digestible and metabolically effective organic selenium. Several animal trials show the benefit of this new generation of organic selenium. For example, Figure 3 shows the results of a trial in laying hens in which different sources and dosages of selenium were evaluated.
Laying hens that received selenised yeast in the diet were able to deposit more selenium in the eggs than laying hens that received inorganic selenium. Moreover, the group that received a diet supplemented with L-Selenomethionine* had the highest selenium in the eggs, which was significantly higher than the selenized yeast. This trial demonstrates the higher selenium transfer of the new generation of organic selenium compared with selenised yeast. Similar to these results in laying hens, a second study in dairy cows showed the same trends (Vandaele ea, 2014), whereby L-Selenomethionine resulted in a higher transfer of selenium versus seleniumyeast and inorganic selenium resulted in the lowest response.
Stability in feed and premixes
In order to be efficient in the animal, a feed component should be stable throughout the feed production process. This means both in feed production and in premix production. The stability of L-SeMet has been investigated extensively and it has been concluded that the product is stable in pure form, mixed in a vitamin/mineral premix and in complete feed. It has to be noted that, for the analysis of SeMet in a high concentrated vitamin/mineral premix, a specific analytical protocol for the determination of SeMet should be applied. Not applying the correct protocol may lead to misinterpreting the results. If the correct protocol is used, the SeMet in premixes can be determined correctly. Figure 4 shows the results of a stability test of L-selenomethionine in a vitamin/mineral premix and shows that the product is stable during storage of the premix.
With the introduction of a new form of organic selenium, new opportunities arise for animal nutritionists. The new generation of organic selenium shows no variation in concentration and guarantees all its selenium in a digestible and highly effective organic form. It allows a more standardised amount of active ingredient (selenomethionine) to be delivered via the diets to the animals.
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