Limited in vivo conversion of dietary D3 to 25-OH-D3 in rainbow trout and undetectable endogenous photo-production of D3 in indoor-raised fish

Vitamin D is essential for salmonids and is routinely supplemented in feeds as cholecalciferol (D 3 ). As an intermediate metabolite in the in vivo conversion of D 3 to active vitamin D (1,25-OH-D 3 ), directly adding calcifediol (25-OH-D 3 ) to feed may improve vitamin D status. An 84-day feeding trial was conducted in rainbow trout ( Oncorhynchus mykiss ) raised indoors, to compare dietary 25-OH-D 3 and D 3 at levels of 200, 400, or 800 μg/kg. Zootechnical performance and the profiles of vitamin D metabolites in plasma, muscle, and liver were assessed. D 3 and 25-OH-D 3 appeared in plasma, muscle, and liver in a dose-dependent manner. Both metabolites supported optimal zootechnical performance and maintained vitamin D status at all inclusion levels, as indicated by body weight, feed conversion, specific growth rate, and plasma 1,25-OH-D 3 concentrations. Plasma and muscle 25-OH-D 3 profiles showed that the conversion of dietary D 3 to 25-OH-D 3 in vivo was limited. Additionally, undetectable levels of D 3 in fish that did not receive this metabolite in their feed indicated the lack of any significant endogenous photo-production of D 3 . This study demonstrates that 25-OH-D 3 can support growth in salmonids. While adding D 3 to fish feeds increases respective levels in the body, the conversion of D 3 to 25-OH-D 3 is limited. Therefore, to raise in vivo levels of 25-OH-D 3 above this threshold, it needs to be direclty added to the feed. Since salmonids raised indoors do not produce detectable D 3 endogenously, they rely on dietary sources to meet their vitamin D requirements. • The in vivo conversion of dietary D 3 to 25-OH-D 3 is limited in trout. • Photo-production of endogenous D 3 is not detectable in indoor-raised trout. • Dietary 25-OH-D 3 can sustain normal growth in trout. • Dietary 25-OH-D 3 supplies both the circulation and muscle with 25-OH-D 3 .