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

## TL;DR

This study shows that mealworms can convert toxic inorganic selenium into a bioavailable organic form, making them a potential source of healthy selenium-rich food.

## Contribution

The study reveals mealworms' efficient biotransformation of inorganic selenium into organic forms, particularly selenocystine, with high bioaccessibility.

## Key findings

- Mealworms convert inorganic selenium into organic forms, with selenocystine as the main product.
- Selenate (Se6+) is more efficiently transformed and yields higher bioaccessible selenium than selenite (Se4+).
- High selenium concentrations inhibit mealworm growth and fat accumulation but boost protein and polysaccharide synthesis.

## Abstract

The yellow mealworm is frequently regarded as a promising, sustainable, high-quality protein-containing foodstuff that can be consumed by humans. Selenium has been demonstrated to be an essential micronutrient for humans and animals, as it plays a crucial role in promoting physical health. Nevertheless, it is important to note that inorganic selenium is toxic to organisms at elevated concentrations. Furthermore, there is a paucity of information regarding the changes in nutritional components, selenium enrichment, and the biotransformation efficiency of the worms. The present study employed two types of inorganic selenium (selenite and selenate) to address the aforementioned issues, with the purpose of selenium-enriched mealworm cultivation and utilisation. Inhibited larval biomass was observed in the presence of elevated concentrations of inorganic selenium, with both Se forms stimulating crude protein and polysaccharide synthesis. A dose-dependent Se accumulation pattern and limited bioaccumulation capacity were identified in mealworms. However, a remarkable capacity for Se biotransformation was also observed, yielding a higher proportion of organic Se in mealworms, and selenocystine was identified as the predominant compound. The in vitro gastrointestinal digestion test demonstrated a high bioaccessibility of mealworm-derived Se.

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.