Vitamin D metabolites
Vitamin D has no biological activity. It reaches the circulation
largely via the lymphatics after a meal, or from synthesis in UVexposed
skin. Its fate is to be rapidly converted in the liver to 25-
hydroxyvitamin D [25(OH)D], or stored in adipose tissue. In turn,
the main role of 25(OH)D is to be converted into calcitriol by 1ahydroxylase
(Cyp27b1), an enzyme expressed in kidneys, placenta,
and diverse tissues. The half-life of serum 25(OH)D is 14e20 days,
providing a stable level of substrate for Cyp27b1 and a reliable
indicator of vitamin D body stores. In this review, calcitriol will be
used when discussing biological actions attributable to vitamin D,
25(OH)D will be pertinent to blood levels or estimates of sufficiency,
whereas vitamin D refers to the form that is ingested (diet
and supplements) or synthesized in skin.