Bioactive Vitamin D3 Metabolites Improve Fillet Retention Without Affecting Growth

Efficient nutrient transfer from feed to fillet remains a key goal in sustainable aquaculture, particularly for micronutrients like vitamin D3 that carry both regulatory importance and consumer health value. Farmed salmonids are among the top contributors of vitamin D in human diets, yet the levels in fillet vary widely depending on feed formulation and production conditions.

This study evaluated whether supplementing rainbow trout diets with synthetic vitamin D3 alone or combined with its bioactive metabolites could enhance vitamin D3 deposition in muscle tissue under freshwater recirculating aquaculture system (RAS) conditions. The trial ran for 61 days at BioMar’s ATC Hirtshals facility using 12 diets that varied by inclusion level and source of vitamin D3.Ten cholecalciferol-based diets ranged from 8,100 to 77,400 IU/kg, and two additional diets included a combination of cholecalciferol (40,000 IU/kg) and either an ‘intermediate’ or ‘active’ Vit D metabolite at levels recommended by suppliers. All feeds were formulated with identical base composition and produced as 4.5 mm pellets.

Results 

Growth Performance 

No significant differences were observed in growth rate, feed intake, or feed conversion ratio across all treatments. Fish fed both synthetic vitamin D3 and combinations with its metabolite forms grew equally well, confirming that high-level supplementation and metabolite use are safe under commercial RAS conditions. 

Fillet and Whole-Body Vitamin D3 Deposition 

Vitamin D3 content in both fillet and whole body increased proportionally with dietary supplementation. Regression analyses confirmed a strong positive correlation between dietary cholecalciferol and vitamin D3 deposition (p < 0.001). The combination of cholecalciferol with ‘intermediate’ or ‘active’ Vit D metabolites also resulted in elevated fillet vitamin D3 levels, with the ‘active’ metabolite achieving higher deposition than synthetic vitamin D3, likely due to an apparent sparing effect in fat stores. ­

Pigmentation and Fillet Quality 

Fillet redness, assessed by MINOLTA a* values, increased linearly with higher vitamin D3 supplementation levels. Neither the ‘intermediate’ or ‘active’ metabolites negatively impacted pigmentation, and all treatments remained within the commercially acceptable range for fillet appearance. 

Transfer Efficiency 

The data suggests that the efficiency of vitamin D3 transfer from feed to fillet can vary depending on the source and dose. While synthetic D3 followed expected deposition trends, a combination with the ‘active’ metabolite showed potential to enhance retention efficiency without increasing dietary load, likely by bypassing hepatic metabolism and supporting direct peripheral deposition. 

This trial confirms that both synthetic and bioactive vitamin D3 sources can be used to safely enrich rainbow trout fillets without compromising growth or product quality. The use of the ‘active’ Vit D metabolite mayoffer a cost-effective strategy to improve vitamin D3 retention in edible tissue. These findings support the development of feed solutions tailored for nutritional fortification in premium aquaculture products.