Effects of Selenite and Selenate on the Growth, Nutrient Composition, Selenium Species, and In Vitro Digestibility of Mealworm Tenebrio molitor

This study systematically compared the growth performance, nutrient composition, accumulation and speciation of selenium (Se), and in vitro bioaccessibility in yellow mealworm (Tenebrio molitor L.) larvae, which were reared on substrates supplemented with selenite (Se4+) and selenate (Se6+) at concentrations of 0, 5, 10, and 20 mg/kg over 28 days. The results showed that high Se concentrations (≥10 mg/kg) significantly reduced larval biomass, with Se6+ having a slightly stronger inhibitory effect than Se4+. The mealworms effectively accumulated Se in a dose- and form-dependent manner. Peak total Se concentrations were observed on day 14, after which there was a decline, suggesting the presence of potential elimination mechanisms, such as moulting. The bioaccumulation factors (BAFs) were all below 1, indicating its limited enrichment capacity for both Se4+ and Se6+. Nutrient composition was altered, with both Se forms stimulating crude protein and polysaccharide synthesis while inhibiting fat accumulation. Mineral content (Mg, Fe, Zn) was also modulated, with differences observed between the Se4+ and Se6+ treatments. Notably, mealworms exhibited a remarkable ability to biotransform inorganic Se into organic forms, with organic Se proportions exceeding 79% in all treatments. Selenate was more efficiently bio-converted, yielding a higher proportion of organic Se. In vitro gastrointestinal digestion revealed significantly higher Se bioaccessibility from Se6+-treated mealworms (up to 85.12%) than from Se4+-treated ones (up to 60.67%). Analysis of the bioaccessible fraction by Se speciation identified SeCys2 as the dominant compound (>92% of the detected species), with much lower levels of SeMet. Trace amounts of unmetabolised Se6+ were only detected in the Se6+-exposed groups. These findings highlight T. molitor as an efficient bioreactor for producing bioaccessible, organically bound Se, primarily as SeCys2, with Se6+ being the more favourable precursor for generating a high-quality, bioavailable source of Se for potential use in feed or food.