Exploring the effects of dietary inulin in rainbow trout fed a high-starch, 100% plant-based diet

Abstract

Abstract Background High dietary carbohydrates can spare protein in rainbow trout but may affect growth and health. Inulin, a prebiotic, could have nutritional and metabolic effects, along with anti-inflammatory properties in teleosts, improving growth and welfare. We tested this hypothesis in rainbow trout by feeding them a 100% plant-based diet, which is a viable alternative to fishmeal and fish oil in aquaculture feeds. In a two-factorial design, we examined the impact of inulin (2%) as well as the variation in the CHO/plant protein ratio on rainbow trout. We assessed the influence of these factors on zootechnical parameters, plasmatic metabolites, gut microbiota, production of Short-Chain Fatty Acid and lactic acid, as well as the expression of free-fatty acid receptors genes in the mid-intestine, intermediary liver metabolism, and immune markers. Results The use of 2% inulin did not change significantly the fish intestinal microbiota, while interestingly, the high CHO/Protein ratio group shows modification of intestinal microbiota and in particular the beta diversity, with 21 bacterial genera affected, including Ralstonia, Bacillus, and 11 lactic-acid producing bacteria. There were higher levels of butyric, and valeric acid in groups fed with high CHO/protein diet but not with inulin. The high CHO/Protein group shows a decrease in the expression of pro-inflammatory cytokines (il1b, il8, tnfa) in liver and a lower expression of the genes coding for tight-junction proteins in mid-intestine (tjp1a, tjp3). However, the 2% inulin did not modify the expression of plasma immune markers. Finally, inulin induced a negative effect on rainbow trout growth performance irrespective of the dietary carbohydrates. Conclusions with a 100% plant-based diet, inclusion of high levels of carbohydrates could be a promising way for fish nutrition in aquaculture through a protein sparing effect whereas the supplementation of inulin in combination with such alternative diets needs further investigations.