呢編係外國既文章:
Vitamin D3 synthesis, as most are aware, occurs in the skin with exposure to UVB and, as most are probably unaware, is a two-step process that also requires IR (infrared or heat). The first step converts a starting compound, 7-dehydrocholesterol, to preVitamin D3 utilizing the energy input of UVB in a rather fast reaction. The second step, the conversion of preVitamin D3 to Vitamin D3 is a slower reaction which relies on IR to provide the energy. This is much of the reason that reptiles bask.
Vit D3 synthesis: 7DHC → preVit D3 → Vit D3
Once the intake of the nutrients has occurred, the next step is the conversion of these nutrients into compounds which are usable by the organism. In the case of calcium, it is absorbed in the small intestine and transported by the circulatory systemunder the control of calcitriol. Calcitriol is the active Vitamin D in calcium regulation/utilization and is the product of a two-step Vitamin D3 metabolism process.
Vit D3 metabolism: Vit D3 → calcidiol → calcitriol
Vitamin D3 probably does not remain in high concentrations in the circulatory system but an intermediate metabolite (the step-one result), calcidiol, is likely used as a storage mechanism. Alternate metabolic conversions occur to prevent the overabundance of preVitamin D3 and Vitamin D3, some of which are reversible thereby extending the potential availability of the functional components. In most healthy organisms, this Vitamin D3 supply pool provides several days to several weeks of available source components.