Effect of raw chitin from Hermetia illucens exuviae and lauric acid on growth performance and nutrient utilization, gut morphology and functionality, and gene expression in rainbow trout (Oncorhynchus mykiss) fed semi-purified diets

Abstract This study aimed to evaluate the effects of Hermetia illucens (black soldier fly) exuviae, as a chitin source (CHI), and lauric acid (LA) on the intestinal physiology of rainbow trout ( Oncorhynchus mykiss ). Four isoproteic and isolipidic semi-purified diets were tested: a control diet (CTRL), a diet with 1.5% CHI (C-CHI) and two diets with 1.5% CHI and LA at 0.1% and 0.2% (C-CHI-LAU1 and C-CHI-LAU2, respectively). A total of 144 juvenile rainbow trout (91.3 ± 2.9 g) were fed the experimental diets for 12 weeks (2% body weight daily feeding rate), after which liver and digestive tract samples were collected for analysis. Our findings showed that CHI and LA inclusion did not affect growth performance and biometric indices. The C-CHI diet reduced mucosal fold length and altered goblet cell mucin composition. Moreover, the C-CHI diet decreased PepT-1 signal intensity. None of the diets affected immunohistochemical alkaline phosphatase expressions. Brush border membrane and digestive enzyme activity as well as the chitinolytic enzyme activity were modulated in digestive tract-specific manner. The expression of fabp2 was downregulated by C-CHI and C-CHI-LAU1 diets, while slc2a1 was downregulated by C-CHI-LAU1. In conclusion, 1.5% CHI and up to 0.2% LA supplementation did not impair rainbow trout growth performance, but 1.5% CHI negatively affected intestinal morphology, mucin composition, and some intestinal health markers and nutrient transporters. However, the combination of CHI with LA in the diet for rainbow trout mitigated the adverse effects of CHI.