Introduction
Vitamin D is essential for salmonids , yet its metabolism and physiological roles in these species remain incompletely understood. To exert its biological effects following absorption, dietary cholecalciferol (D3 ) must be metabolized in vivo to calcifediol (25-OH-D3), and subsequently to its active form, calcitriol (1,25-OH-D3). S upplementing with 25-OH-D3 can be effectively improve vitamin D status in fish by partially bypassing this metabolic process (Rider et al., 2024 & 2023). This study investigated the effects of oral intake of these plasma metabolites on vitamin D metabolism and calcium (Ca) and phosphorus (P) homeostasis by analysing their kinetics.
Methods
Single 22 µg oral doses of D3 or 25-OH-D3 , or a 2.54 µg dose of calcitriol-glycosides (1,25-OH-D3-gly) were administered to rainbow trout fed a practical diet containing 2,817 IU/kg D3 . The kinetics of circulating vitamin D metabolites, calcium (Ca), and phosphorus (P) were monitored in plasma over six days by sampling at 6, 24, 48, 72 and 168 hours post-gavage. A 42-hour feed withdrawal period was associated with the administration of the single oral doses.
Results
In fish receiving the baseline diet and prior to the gavage, neither D3 nor 25-OH-D3 were detectable in plasma, while 1,25-OH-D 3 was the most abundant vitamin D metabolite. A single dose of D3 produced a plasma D3 peak at 48 hours, whereas a single dose of 25-OH-D3 resulted in a smaller 25-OH-D3 peak at 72 hours. The D3 peak did not alter baseline 25-OH-D3 levels. A single dose of 1,25-OH-D3 -gly did not influence baseline 1,25-OH-D3 levels. N one of the single vitamin D doses significantly altered plasma levels of active vitamin D, Ca, or P.
Across all groups, p lasma 1,25-OH-D3 was significantly modulated over time. Plasma 1,25-OH-D3 levels s ignificantly declined reaching a minimum 24 hours after gavage and were not restored by any of the vitamin D treatments, see Figure 1. Circulating Ca, and especially P, also declined 24-hours after the drop in plasma 1,25-OH-D3.
Conclusions
These findings demonstrated that even at 1.7 times the NRC minimum of 1,600 IU/kg D3, neither D3 nor 25-OH-D3 were detectable in plasma . Although both D3 and 25-OH-D3 were rapidly absorbed, they were also rapidly cleared. The observed kinetics also indicated that short-term feed deprivation significantly reduced active vitamin D levels , and this decline was not mitigated by the single doses of any of the vitamin D sources . The subsequent reductions in plasma Ca and P suggest a temporal link with declining plasma 1,25-OH-D3 levels. This is the first study to connect circulating active vitamin D to feed intake in fish, pointing to a potential role of this vitamin in energy metabolism.
References
Rider, S., Verlhac-Trichet, V., Constant, D., Chenal, E., Etheve, S., Riond, B., Schmidt-Posthaus, H., & Schoop, R. (2023). Calcifediol is a safe and effective metabolite for raising vitamin D status and improving growth and feed conversion in rainbow trout. Aquaculture, 568, 739285.
Rider, S., Yamashita, E., Chenal, E., Cabo-Valcarce, P., Kuschel, F., Orellana, P., Ruiz, J., Hernández, A., & Dantagnan, P. (2024). Dietary calcifediol reduces mesenteric adiposity to the benefit of carcass growth independently of circulating vitamin D hormone in juvenile Atlantic salmon. Aquaculture, 585, 740687.