To ensure the sustainability of aquaculture, carnivorous fish such as rainbow trout must change their “menu” from a fish-based diet to one without fish ingredients. Feed manufacturers have typically replaced fish ingredients (oil and protein) with plant-based ingredients. However, there is still a major problem with the growth performance of fish fed with a 100% plant-based diet due to a decrease in feed intake and feed efficiency. In addition, in many fish species fed fishmeal and fish oil, the intake of digestible carbohydrates (CHO) can improve protein utilization for growth (protein sparing effect) and reduce nitrogen waste. The aim of this work was to study, for the first time, the incorporation of digestible carbohydrates (up to 20%) in a 100% plant-based diet, thus modifying the CHO/Proteins ratio in rainbow trout. We carried out 3 experiments to test the effect of the CHO/Proteins ratio on the intestinal microbiota, the trout metabolism either in the long term (after 12 weeks of feeding) or in the short term (3 weeks of feeding). Production of short-chain fatty acids (SCFAs), key molecules in the microbiota-host molecular dialogue was also analyzed. In addition, the cumulative effect of inulin supplementation and variations in CHO/Proteins ratio was studied to evaluate the effects on carbohydrates utilization and immune parameters.
The high carbohydrate/protein ratio had a significant effect on alpha and beta diversity and on the abundance of various lactic acid bacteria, as well as Bacillus, in both short-term, and long-term experiments, in the microbiota associated with digesta, while the microbiota associated with mucus was less affected. The change in microbial diversity could be linked to the differences observed in AGCC production which may play an important role in host metabolism. In terms of metabolism, the introduction of dietary carbohydrates did not impair trout growth performance, and was not associated with dysfunctional glucose homeostasis and intermediary metabolism. A decrease in several inflammatory markers was also observed with the high CHO/Protein ratio.
In contrast, incorporation of the prebiotic inulin had no positive effect on carbohydrate utilization.
In conclusion, the incorporation of 20% digestible carbohydrates in a 100% plant diet seems to be a promising way to optimize the use of new aquaculture feeds (without marine resources) in rainbow trout.
Acknowledgements: This work was carried out as part of a PhD funded by the Conseil Départemental des Landes (French local authorities).