Modulation of nutritional, antioxidant properties and bioaccumulation of bioactive molecules in Tenebrio molitor larvae via bioactive-rich by-product feeding strategies

• Olive and quinoa by-products modulate T. molitor defatted meal composition • Larvae accumulate bioactive compounds from olive oil by-products • Bioaccumulation efficiency is low but pentacyclic triterpenes enrichment is notable • DPPH antioxidant activity improves, while that of ABTS stays unchanged • DPPH inhibition is partially explained by total bioactive accumulated compounds Tenebrio molitor is gaining attention not only for its high nutritional content but also for its functional potential. This study evaluated the effects of including different levels of olive leaves (OL), olive pomace (OP), and quinoa husk (QH) in T. molitor larval diets, on the nutritional composition, amino acid profile, digestibility, phenolic content, antioxidant activity (DPPH and ABTS assays), as well as targeted bioactive compounds analysis in the insect meals. Insect meals had protein contents between 54–58%, with OP30 and QH15 showing slightly higher values than the control. Chitin content increased significantly with by-product inclusion, reaching 11.3% in OP50. The amino acid profile remained balanced in OP and QH diets, maintaining ∼51% essential amino acids, but high OL inclusion reduced the content of both total and essential amino acids. Protein digestibility ranged from 74–86%, and gastrointestinal hydrolysis remained consistent across diets. Phenolic content and antioxidant activity improved with OL and OP inclusion. Bioactive compounds from OL and OP were detected in insect meals, but, while diets were rich in phenolic compounds (oleuropein or hydroxytyrosol), the meals mainly accumulated pentacyclic triterpenes (oleanolic acid, maslinic acid and ursolic acid), that showed the best efficiency of bioaccumulation. Additionally, total bioactive content correlated strongly with the DPPH antioxidant activity of the meals. In conclusion, the inclusion of moderate levels of the tested by-products improves the functional value of T. molitor meals without impairing nutritional value. Moreover, when feeding bioactive-rich by-products, these strategies for value improvement of insects simultaneously align with circular economy practices.