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Introduction
Selenium (Se) is an essential micro-element for both animals and humans. As an integral component of several enzymes, including glutathione peroxidase (GSH-Px), type I iodothyronine deiodinase, and thioredoxin reductase, Se plays crucial roles in various metabolic functions such as preventing oxidative stress, regulating thyroid hormones, maintaining cellular redox balance, and supporting immunocompetence [1, 2–3]. Furthermore, Se contributes to the detoxification of heavy metals and xenobiotics [4], highlighting its importance in metabolic health. Given these essential functions, Se deficiency is a significant global health concern, affecting nearly one billion individuals worldwide [5]. In China, approximately 72% of the county’s population is Se-deficient [6], with some regions reporting daily Se intake as low as 10 μg [7], far below the Chinese Dietary Reference Intakes recommendation of 50–60 μg/d for adults. This deficiency has been linked to the endemic occurrence of Keshan disease and Kaschin-Beck disease [8]. Similarly, in poultry production, Se deficiency in laying hens can lead to decreased productivity in laying hens, resulting in significant economic losses [9].
Se fortification of food products is widely regarded as an effective strategy to mitigate Se deficiency [2]. In the poultry industry, there has been a growing emphasis on incorporating elevated doses of Se into chicken feed, aiming to enhance the Se content in carcass meat and eggs [10, 11, 12, 13–14]. Traditionally, sodium selenite has been the most common Se supplement in poultry feed, but organic Se forms are also less toxic and environmentally more damaging compared to inorganic forms, making them a preferred choice for sustainable poultry production [15]. Among the organic sources of Se, selenomethionine (Se-Met) and selenium-enriched yeast (SeY) have been approved as Se feed additives in animal nutrition [16, 17–18]. Also, Se-Met and SeY have gained prominence due to their superior bioavailability and enhanced antioxidant properties [3, 14]. For instance, SeY is primarily composed of Se-Met, which can be incorporated directly and non-specifically into proteins (e.g., carcass meat, eggs, and milk) instead of methionine [19], making it more valuable in the food chain.
The European Food Safety Authority [20] reported that the inclusion of SeY at 0.5 mg Se/kg in layer diets significantly increased (~50%) the Se content of eggs compared to sodium selenite supplementation at the same dose, and no adverse effects...