Aquafeed producers have the option to use organic (L-selenomethionine; Excential Selenium 4000) and inorganic Se sources in their diets. These chemical forms of Se affect the bioavailability in fish feeds differently (Figure 2, Berntssen et al., 2018). It is commonly accepted that organic Se shows greater availability and is more functional compared to inorganic Se (Prabhu et al., 2016).
To study the effect of replacing inorganic Se with organic L-selenomethionine in salmon, in vitro and in vivo trials were conducted by Prahbu et al. (2020), in which Orffa’s L-selenomethionine, Excential Selenium 4000 (Se4000), was compared to inorganic sodium selenite (SS) at various dietary levels.
Body composition, digestibility, retention and loss of Selenium
When Se contents are increased in the salmon diet, Se concentrations in the body are increased as well.
However, the increase in Se content in fish fillets is much higher when Se4000 is fed compared to SS. The slope ratio Se4000/SS for fish fillets — which is the amount of additional Se concentrated in certain parts of the body when Se4000 is fed — is 9.74 (Prahbu et al., 2020). This indicates that a dietary increase of Se4000 leads to 8.74 times more Se concentration compared to the same dietary increase of SS.
When looking at other specific Se concentrations in the body (Figure 3), it can be observed that the concentration of Se is higher at places in the body where it can be stored or used (the whole body and blood plasma) and lower in places where a lower Se concentration is desired (liver and kidney). Additionally, the amount of Se in the faeces is significantly lower when Se4000 is fed instead of SS (-0.73).