Effects of a plant-based diet from first feeding on the intestinal expression of nutrient sensors in rainbow trout (Oncorhynchus mykiss)

To promote sustainable aquaculture practices, fishmeal and fish oil in aquafeeds are increasingly being replaced with more sustainable ingredients, primarily sourced from plants. However, for rainbow trout ( Oncorhynchus mykiss ), a carnivorous species, completely substituting marine-derived ingredients with plant-based ones results in reduced growth and significantly lower survival rates of offspring. These changes occur early in development and primarily relate to alterations in feed intake and feed efficiency. Despite this knowledge, the precise origins of these effects remain unexplored. In this study we investigated the abundance of nutrient sensors in the gastrointestinal tract (GIT) of rainbow trout fed from first feeding to 100 days with either a commercial-like diet (containing fishmeal and fish oil and a partial replacement by plant ingredients) or a completely plant-based diet. Firstly, our data revealed higher ingestion levels for trout fed with the commercial-like diet compared to the plant-based diets, regardless of the size of the feed, from their first meal up to 94 days. This difference in ingestion levels may explain the observed growth differences during this period. At genic level, results showed distinct temporal expression profiles for the mRNAs encoding the studied amino acid and fatty acid sensors in the rainbow trout GIT during the first months of life. For instance, mRNAs encoding certain transporters and receptors, such as the amino acid transporter Pept1 and the fatty acid transporter Fatp4, increased during the duration of the experiment. Conversely, others, like the amino acid transporter Lat4 and the fatty acid transporter Cd36, showed the opposite trend. Some mRNAs (e.g., ffar2a2 and ffar2a1b ) were undetectable at very early stages but became detectable by day 5 or 10 post-first feeding. Additionally, the temporal profiles of appetite-regulatory hormones mRNAs generally exhibited increasing levels until days 10 or 30 after the first meal, followed by a subsequent decline. In addition, our results demonstrated slight differences in the expression of nearly all nutrient sensors and appetite-regulatory hormones at early life stages when the trout were fed a plant-based diet. Overall, our study showed that feeding a plant-based diet from first feeding does not significantly alter the gastrointestinal expression of nutrient sensors during early life stages, likely excluding this aspect as responsible for the growth problems associated with vegetable diets. • Rainbow trout gastrointestinal mRNA levels of nutrient sensors showed varied patterns after initial feeding in early life. • The distinct expression timing of nutrient sensors may imply diverse functions for them during the physiological responses of the gastrointestinal tract to a first meal. • A plant-based initial diet does not significantly change nutrient sensor expression in rainbow trout alevins.