Vitamin D binding protein greatly improves bioactivity but is not essential for orally administered vitamin D

Abstract

Abstract Vitamin D3 is a prohormone that is essential for calcium homeostasis. It is naturally produced in the skin by ultraviolet‐B (UVB) irradiation of 7‐dehydrocholesterol. In the absence of skin production, vitamin D3 can also be obtained from oral sources. However, the actual biological equivalence of naturally produced (i.e., UVB‐irradiation of skin) and oral vitamin D3 has not been determined. We previously identified a unique and specific transport mechanism for skin‐generated vitamin D3 which requires vitamin D binding protein (DBP); a mechanism that differs from absorption and transport of oral vitamin D3. In the following report, we examined the impact of this difference on the biological activity of vitamin D3. We report that UVB‐generated vitamin D3 is more potent at raising serum calcium compared to oral vitamin D3, with the total biological activity being twofold higher. By examining the excretion of radiolabeled vitamin D3 injected unbound or pre‐bound by DBP, we attributed the increased activity of skin‐generated vitamin D3 to a significant reduction in biliary excretion of DBP‐bound vitamin D relative to unbound vitamin D. Thus, removal of vitamin D3 from the skin by the natural DBP system markedly improves biological activity compared to that given orally.

Keywords

• ultraviolet light
• vitamin D
• vitamin D binding protein