Effects of dietary chitin on nutrient digestibility, cholesterol metabolism, digestive enzyme activity, and gut microbiome in rainbow trout

The growing interest in incorporating chitin-containing raw materials into aquafeeds necessitates a deeper understanding of chitin’s impact on fish performance and nutrient utilisation. This study investigated the effects of different dietary chitin inclusion levels on nutrient digestibility, cholesterol metabolism, digestive enzyme activity, and gut microbiome in juvenile rainbow trout (Oncorhynchus mykiss). Trout (∼ 70 g) were acclimated for two weeks to their experimental diets of either the control diet or a modified diet where 1.0 %, 2.5 %, or 5.0 % of wheat meal was replaced with commercial-grade chitin powder derived from shrimp shells, resulting in dietary chitin contents of 0.9 %, 2.1 %, or 4.5 % on a dry matter basis, respectively. Following this acclimation period, a two-week digestibility study was conducted during which triplicate groups continued to receive their respective diets. Feed conversion ratio gradually increased with higher dietary chitin inclusion levels, while apparent digestibility of dry matter, organic matter, protein, lipid, energy, ash, cholesterol, and all amino acids gradually decreased. The reduced digestibility of these nutrients may be attributed to chitin's physical binding properties, potentially interfering directly with the nutrients and/or indirectly with their respective digestive enzymes. While dietary chitin inclusion did not alter the overall gut microbiome composition, it promoted specific taxa known for chitin degradation activity, suggesting a microbial adaptation to chitin-containing diets. These findings highlight that while dietary chitin may enhance the presence of chitin-degrading microbes in the intestinal tract, these were unable to counteract chitin’s negative impact on overall nutrient digestibility observed within the studied period.