Elevated Dietary Vitamin D Supplementation During Winter can Mimic Vitamin D Levels Induced by Summer Daylight Exposure in Atlantic Salmon

Vitamin D 3 (cholecalciferol) is an essential nutrient for vertebrate calcium homeostasis, skeletal integrity, and immune function. In aquaculture, declining use of fishmeal and fish oil has reduced dietary vitamin D 3 levels in salmon feeds, raising questions about tissue deposition and nutritional quality of farmed fish. This study aimed to investigate the metabolic fate of elevated dietary vitamin D 3 in farmed Atlantic salmon ( Salmo salar ) under commercial production conditions in southern Norway. Fish were fed either a control diet (~4000 IU/kg vitamin D 3 ) or a test diet with a 10‐fold higher inclusion (~40,000 IU/kg vitamin D 3 ) for 32 weeks from November to July 2025. Plasma, liver, and muscle samples were analyzed for vitamin D 3 and its metabolites [25(OH)D 3 and 1,25(OH) 2 D 3 ], alongside plasma biomarkers of health. Salmon tolerated the high dietary vitamin D 3 without observed adverse effects on growth and mortality. Elevated dietary inclusion significantly increased vitamin D 3 concentrations in plasma, liver, and muscle, with less pronounced effects on hydroxylated metabolites, compensating for lower levels during winter months. However, seasonal variation was evident in all tissues investigated. Overall, the results demonstrate that high dietary vitamin D 3 supplementation increases tissue concentrations in Atlantic salmon without compromising health, thereby improving the nutritional profile of farmed salmon fillets for human consumption.