Saponins are naturally occurring amphiphilic molecules and have been associated with many biological activities. The aim of the present study was to investigate whether soya saponins trigger the onset of soyabean-induced enteritis in Atlantic salmon (Salmo salar L.), and to examine if dietary soya saponins increase the epithelial permeability of the distal intestine in Atlantic salmon. Seven experimental diets containing different levels of soya saponins were fed to seawater-adapted Atlantic salmon for 53 d. The diets included a fishmeal-based control diet, two fishmeal-based diets with different levels of added soya saponins, one diet containing 25 % lupin kernel meal, two diets based on 25 % lupin kernel meal with different levels of added soya saponins, and one diet containing 25 % defatted soyabean meal. The effect on intestinal morphology, intestinal epithelial permeability and faecal DM content was examined. Fish fed 25 % defatted soyabean meal displayed severe enteritis, whereas fish fed 25 % lupin kernel meal had normal intestinal morphology. The combination of soya saponins and fishmeal did not induce morphological changes but fish fed soya saponins in combination with lupin kernel meal displayed significant enteritis. Increased epithelial permeability was observed in fish fed 25 % defatted soyabean meal and in fish fed soya saponin concentrate independent of the protein source in the feed. The study demonstrates that soya saponins, in combination with one or several unidentified components present in legumes, induce an inflammatory reaction in the distal intestine of Atlantic salmon. Soya saponins increase the intestinal epithelial permeability but do not, per se, induce enteritis.

Fish are believed not to synthesize vitamin D through UV exposure but to meet their requirements from dietary sources. The high vitamin D levels found in many fish species are thought to originate from UV‐induced synthesis in plankton, with vitamin D subsequently accumulating through the aquatic food chain. Atlantic salmon is a rich dietary source of vitamin D, but limited data exist on circulating vitamin D levels. To address this, plasma levels of the three main vitamin D metabolites were measured in Atlantic salmon from Norwegian fish farms using mass spectrometry. Here, we show that salmon reared in open sea pens have significantly higher vitamin D levels than those raised indoors. Monitoring an outdoor farm over 18 months revealed a distinct seasonal pattern between vitamin D levels and day length. In a follow‐up experiment, indoor‐raised fish were divided into two groups: one remained indoors, while the other was transferred outdoors for 52 days. Both groups received the same commercial feed, yet the outdoor group exhibited a fivefold increase in whole‐body vitamin D content. These surprising findings provide field evidence of endogenous vitamin D synthesis in fish induced by natural sunlight. This discovery could have important implications for aquaculture, emphasizing the risk of suboptimal vitamin D levels in farmed fish when sunlight exposure is limited.

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.