PARTNER FEATURE ▶▶▶
Toxic and unstable? Choose your selenium source carefully
The type of selenium and the form in which it is supplemented plays a crucial role in its safety, bioavailability and efficacy. A well-considered choice of selenium source is essential.
BY RICHARD MURPHY PHD, EUROPEAN BIOSCIENCE CENTRE, ALLTECH IRELAND S
100 90 80 70 60 50 40 30 20 10 0
Yeast A 22 Yeast B Yeast C ▶ ALL ABOUT FEED | Volume 30, No. 5, 2022
elenium supplements are available in several forms; inorganic mineral salts, such as sodium selenite or selenate; organic forms, such as selenium-enriched yeast in which selenoamino acid analogues such as
selenomethionine predominate; or chemically synthesised selenoamino acids and selenoamino acid analogues pro- duced synthetically. The distribution and accumulation of se- lenium and selenium form in animal tissues depends greatly on the type of selenium supplement, and the form in which selenium is presented plays a crucial role in its bioavailabil- ity and efficacy. Organic forms of selenium are the optimal nutritional source, and upon uptake within the cell, they get transformed into common seleno-intermediates for further use and/or excretion.
High SeMet content does not mean high bioavailablity There is a misconception within the feed industry regarding
Figure 1 – Selenium associated yeast fractions (adapted from Encinar et al, 2003)
Residual selenium following extraction
Yeast intracellular selenium (detergent extract)
Yeast intracellular selenium (protease digest)
Selenium associated with yeast cell wall
Water soluble Selenium
the total SeMet content of selenium products with the belief that “more is better.” Such arguments have no scientific basis and there is, indeed, no evidence that increasing the level of SeMet will equate to a better product. While the level of SeMet may vary among products, it is also to be anticipated that the bioaccessibility and availability of the SeMet that is released by the digestive processes in the GI tract will also differ. Critically, what is becoming increasingly clear from a research perspective is that the form in which selenium is presented will influence the stability and thus the cellular reactivity of the molecule. Less stable forms of selenium have enhanced toxicological properties, while those of more stable preparations are far less toxic. In one of the first studies, which demonstrated the unique- ness of individual selenium products, the authors examined three different commercial preparations of selenium- enriched yeast, subjecting each to a series of sequential extractions followed by various enzymic digestions designed to liberate selenocompounds which are associated with var- ious polysaccharide and protein fractions. These compounds were subsequently separated and speciated by SEC-ICP MS, and the recoveries in the various fractions from each yeast product compared (Figure 1). The results outlined in Figure 1 monitor the fractionation of the selenocompounds in yeast by using different extraction techniques.
All selenium yeasts are not the same Although there is a very common perception that all seleni- um yeast preparations are the same, this is not the case, and it is clear that the compartmentalisation of selenium within yeast is totally different between preparations. Just as there are differences among yeast strains at a genetic level, there are fundamental differences in the way yeasts distribute selenium within the cell. As subcellular deposition of selenium within selenium-en- riched yeast preparations is so widely different, it is rea- sonable to expect that these preparations will also differ in parameters such as shelf life, bioavailability and, indeed, toxicology. Rather than viewing these products in the same
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