Maintaining an optimal selenium steady state: A nutritional solution
Selenium can be added to the diet in either inorganic or organic forms (Fig. 1). The advantage of using organic selenium (L-selenomethionine, L-SeMet) over inorganic sources (e.g. sodium selenite or selenate) is its ability to be incorporated directly, without conversion, into general body proteins as a methionine source. L-selenomethionine is the only selenium compound that has this ability. The incorporated selenium, in the form of L-selenomethionine, acts as a storage of selenium in the animal. This stored selenium ensures optimal supply, even during stressful periods.
If necessary, the stored selenium gets metabolised to selenide (H2Se) then to de novo selenocysteine (SeCys). This molecule will be incorporated, as the active site, in selenoproteins. Other selenium compounds, such as SeCys and sodium selenite, are not storable but will be metabolised to de novo SeCys. These compounds will be quickly excreted when intake is in excess. L-selenomethionine will only be metabolised to selenide when there is a need. This form is therefore less prone to excretion and toxicity reactions (Rayman, 2004).
Aquatic protein challenge: A case for L-selenomethionine
Traditionally, fishmeal is the preferred protein source in aquatic feeds. Due to limited availability, pressure on wild fish stocks and variable prices, there is an interest in alternative, sustainable protein sources. Plant meals, for example, are suitable alternatives in the growing global aquaculture industry. However, replacing marine ingredients in fish feed with plant sources changes the nutrient composition of the feed. Selenium concentration of fillets is reported to be highly impacted by high levels of substitution, reducing the added value of fish consumption (Lundebye et al. 2017; Betancor et al. 2016).
Although selenium levels are decreasing within the fish, the demand for selenium to protect against (oxidative) stress remains. Stressors (e.g. environmental, metabolic) are an important issue for the productivity and profitability of fish farms. These stressors may cause increased oxidative damage to lipids, proteins and DNA and increased mineral mobilisation from tissues and their subsequent excretion. High stress may therefore lead to increased mineral requirement. L-selenomethionine is established to be a highly available selenium source leading to higher selenium deposition compared to inorganic selenium sources (Figure 2). It can therefore counteract selenium depletion caused by plant based diets.