Dietary Intake of 25-Hydroxy Vitamin D3 from First Feeding Improves Feed Efficiency and Modulates Hepatic Metabolic Transcriptome in Rainbow Trout.

This study was conducted to determine whether early dietary supplementation with an intermediate vitamin D metabolite could improve metabolic efficiency and liver function in rainbow trout. Over a 20-week feeding trial from first feeding, rainbow trout were fed either a control diet containing vitamin D3 alone or a diet additionally supplemented with 80 µg/kg 25-OH-D3. Plasma analysis confirmed that 25-OH-D3 was efficiently absorbed, with detectable levels of 25-OH-D3 and its epimer, indicating enhanced vitamin D status. While growth was similar between the experimental groups, fish receiving 25-OH-D3 exhibited a significantly improved feed efficiency, indicating a better nutrient utilization. Whole-body composition analysis revealed higher protein and ash content and reduced lipid levels, consistent with transcriptomic evidence of modulated cholesterol and bile acid metabolism. Transcriptome profiling revealed 1301 differentially expressed genes in the liver, with strong modulation of pathways related to vitamin D metabolism, lipid handling, bile acid recycling, and ferroptosis. Additional signatures suggested improved nutrient partitioning and metabolic efficiency, potentially mediated by transcriptional and post-transcriptional mechanisms, including mRNA splicing. Consistent with its role on genome regulation via the vitamin D receptor, the data support the role of an active vitamin D system as a key potent modulator of efficient hepatic metabolism and function supporting improved feed utilization and long-term health outcomes in aquaculture.