Pharmacology and pharmacokinetics of vitamin D

This chapter describes the metabolic parameters of vitamin D and its metabolites according to the framework of pharmacokinetics. The pharmacokinetics of vitamin D is considerably more complex than a standard pharmacological agent, because of the required stepwise hydroxylation into the active form, 1,25(OH)2D, its endogenous, cutaneous synthesis, and presence of vitamin D and its metabolites in the circulation and tissues. Also, its binding to the vitamin D–binding protein (DBP) influences the properties of the vitamin D metabolites. These aspects are discussed throughout this chapter. The absorption, distribution, dose-response, metabolism, excretion, and toxicity of vitamin D and its metabolites are reviewed. In addition, we present the current knowledge of factors influencing 25-hydroxyvitamin D half-life, the influence of DBP on vitamin D metabolism, and the differences in pharmacokinetics between bound and free 25-hydroxyvitamin D.