Saturation kinetics of iodine accumulation in Tenebrio molitor (Coleoptera: Tenebrionidae): implications for feed biofortification.

Iodine deficiency remains a widespread nutritional concern, and biofortification of feed ingredients is a promising complement to salt iodization. Insects, such as the yellow mealworm, Tenebrio molitor Linnaeus (Coleoptera: Tenebrionidae), are gaining attention as sustainable protein sources, yet little is known about their capacity to accumulate iodine. This study quantified dose-dependent iodine accumulation and excretion, and assessed adverse effects, in T. molitor larvae reared on diets spanning more than 3 orders of magnitude of supplemented iodine; saturation kinetics here refers to dose-dependent rather than temporal behavior. Larval iodine increased with supplementation but followed a strongly non-linear trend, a pattern compatible with saturable, carrier-mediated uptake, although no direct mechanistic evidence was obtained. Within the range in which larvae remained viable, accumulation conformed to a saturation model, allowing the half-saturation constant and maximum attainable enrichment to be estimated; at higher doses, larvae did not survive and could not be depurated. Frass iodine rose far more steeply than larval iodine, indicating that surplus iodine was excreted rather than retained. Bioaccumulation efficiency declined progressively with dietary dose, exceeding unity only at low supplementation levels, where larvae concentrated iodine above dietary concentrations. Observational signs of toxicity emerged at elevated supplementation, including growth retardation, heterogeneous development, and increased mortality, culminating in complete population collapse. Practically, the enrichment achievable within the range tolerated by larvae already exceeds concentrations reported as toxic to some target species, indicating that safe inclusion levels will be set by the receiving animal rather than the insect's accumulation capacity.