The origin and metabolism of vitamin D in rainbow trout

An explanation for the origin and the high concentration of vitamin D (cholecalciferol) in some species of fish is still not apparent. Because fish may live in deep water and may, thus, not be exposed to solar ultraviolet (UV) light, it is commonly assumed that vitamin D found in their livers and adipose tissue has been derived from a food chain, originating in zooplankton exposed to UV light at the water surface. To investigate the metabolism and possible origin of vitamin D in fish, rainbow trout were reared from eggs, in the absence of light, and were fed a vitamin D-free diet. When small quantities of radioactively-labelled vitamin D were injected or fed to these trout, much of the radioactivity was found as excreted metabolites in bile. Hence, even when they are vitamin D deficient, trout vigorously catabolise and excrete exogenous vitamin D. The main vitamin D metabolite found in plasma of non-deficient trout was 1,25-dihydroxycholecalciferol [1,25(OH)2D3]. This was produced in the liver by an enzyme process that was strongly stimulated in vitamin D deficiency. When vitamin D was fed for several weeks to vitamin D-deficient trout, plasma 1,25(OH)2D3 levels rose to 180 pg/ml and the fish became hypercacemic. When vitamin D-deficient fish were inadvertently exposed to 60 W incandescent light for 24 h, they became moribund and died. It was subsequently found that vitamin D-deficient trout can produce vitamin D in skin when exposed to blue light at wavelengths between 380 and 480 nm. It is concluded that trout, like terrestrial vertebrates, produce 1,25(OH)2D3 as the functional form of vitamin D and that this has an effect on calcium homeostasis. Furthermore, vitamin D is formed in the skin of these fish by the photochemical action of visible light on 7-dehydrocholesterol. Elucidation of the physicochemical mechanism of this process requires further research.