A novel black soldier fly protein hydrolysate improves the muscle growth of rainbow trout (Oncorhynchus mykiss) via the PI3K/AKT/mTOR pathway

The black soldier fly (BSF) has shown great potential in animal feed due to its abundant nutrients. However, high lipid content and chitin composition of BSF may have adverse effects on animal health. To address this issue, a novel BSF protein hydrolysate (BSFP) was developed through a series of processes. This study aimed to assess the growth performance and muscle development of rainbow trout fed with BSFP. Gradient levels of fishmeal (0%, 15%, 30%, 45%, 60%, and 75%) were replaced with BSFP to formulate six experimental diets, namely CD, BSFP15, BSFP30, BSFP45, BSFP60, and BSFP75, respectively. A total of 480 fish (initial weight of 106.58 ± 0.42 g) were randomly distributed to 6 groups with 4 replicates (20 fish in a replicate) and fed different diets for eight weeks. Results showed that dietary BSFP had no negative effects on the final body weight of rainbow trout (P = 0.106), with the optimal fishmeal replacement level exceeding 75%. No significant differences in survival (P = 0.564) or feed conversion ratio (P = 0.663) were observed among groups. Histological examination indicated that dietary BSFP resulted in significantly increased fiber density (P = 0.042). Furthermore, the springiness (P < 0.001) and chewiness (P = 0.036) of fish fillets were significantly increased with the addition of BSFP, while the adhesiveness was significantly decreased (P < 0.001). Molecular analysis showed that the relative expression levels of mrf4, myf5 and mtor genes were significantly upregulated (P < 0.05). Additionally, the p-PI3K level was significantly higher in the BSFP30 and BSFP75 groups than the CD group (P < 0.001), and the p-mTOR and p-AKT levels were increased linearly in the liver (P < 0.05). In muscle, the phosphorylation levels of mTOR, AKT, and PI3K were upregulated linearly and quadratically with BSFP addition (P < 0.05). Overall, BSFP shows promise as a sustainable protein source in rainbow trout diets, with potential benefits for fillet quality and muscle growth.