Salmon can produce vitamin D from sunlight – new study provides exciting new insights into vitamin D metabolism in salmon!

In a recently published study, we show for the first time that Atlantic salmon can synthesize vitamin D3 endogenously when exposed to sunlight. This surprising finding changes our understanding of vitamin D metabolism in salmon and has important implications for modern aquaculture.

First, we discovered that salmon shortly after being released into open-air cages in the sea had significantly higher levels of circulating vitamin D in plasma compared to fish produced in indoor facilities. To investigate whether the differences could be explained by environmental factors such as salinity, we compared fish kept indoors in full salinity with fish swimming outdoors in freshwater. The result was clear: regardless of salinity, the fish that had access to natural light had significantly higher vitamin D levels than the indoor fish.

To see how this developed over time, we followed an outdoor facility in Rogaland over 18 months and measured plasma vitamin D levels throughout the different seasons. We found a clear and consistent seasonal profile: circulating vitamin D followed the amount of daylight throughout the year, with high levels in summer and low in winter.

Finally, we tested the effect of sunlight directly in a controlled experiment. A group of salmon was divided into two and half were kept indoors, while the other half of the group was moved outdoors with access to sunlight. Both groups were fed the same commercial feed. After 52 days, fish that had been exposed to sunlight had five times higher vitamin D levels in their bodies than the indoor fish.

Overall, the results show that sunlight is currently the most important source of vitamin D in salmon in outdoor facilities. The study also shows that salmon in winter and salmon kept indoors are at risk of having sub-optimal levels of vitamin D.

Link to the full article

From human nutrition we know that vitamin D is not only important for the absorption of calcium and phosphorus, but is also essential for a well-functioning immune system. Vitamin D affects, among other things, the tight-junction proteins that hold epithelial cells together, and thus helps maintain the barrier function in the skin and gills. Intact barriers are crucial for the health of salmon in the first weeks after release.

Another interesting implication is that wild salmon migrating to the ocean in late spring might be expected to achieve high vitamin D levels after swimming through shallow rivers with high sunlight exposure. This contrasts with farmed smolt coming from indoor facilities, which we show in this study can have very low levels.

The study was made possible by new LC-MS/MS methods developed at FishLab, which provide precise and quantitative determination of various vitamin D metabolites in fish. Today, FishLab offers analyses of vitamin D3, 25(OH)D3 and 1,25(OH)₂D3 in plasma, as well as vitamin D3 and 25(OH)D3 in tissue samples.