Temporal dynamics of hepatic mRNA levels of carbohydrate and lipid metabolism-related genes in rainbow trout: Impact of a plant-based diet from first feeding.

Early-life nutrition plays a critical role in shaping metabolic trajectories in fish, particularly in carnivorous species like rainbow trout (Oncorhynchus mykiss), which displays limited capacity to utilize plant-based diets. This study investigated the temporal changes in mRNA abundance of key hepatic genes involved in carbohydrate and lipid metabolism from first feeding to 100 days post-first meal in rainbow trout fry fed either a commercial-like or fully plant-based diet. mRNA abundance was quantified by RT-qPCR at six time points, revealing marked ontogenetic shifts in metabolic gene profiles regardless of diet. Except for gck (encoding glucokinase), glycolytic and gluconeogenic enzymes generally showed declining expression over time, while genes involved in lipogenesis, notably fasn (encoding fatty acid synthase) and acly (encoding ATP citrate synthase), increased progressively-suggesting a developmental shift toward lipogenesis and energy storage pathways. Despite similar patterns observed under both dietary conditions, fish fed the plant-based diet exhibited attenuated temporal transcriptional changes in some genes. These patterns were consistent with previously reported growth deficits in the same fish, potentially reflecting impaired metabolic acclimation. Our findings highlight that while the liver undergoes dynamic metabolic maturation during early development, plant-based diets may hinder this process, particularly in lipid handling. This work provides a molecular basis for improving feed formulations and supports the need for strategies that enhance metabolic flexibility during critical early stages in aquaculture species.