Study Title:

How Vitamin D Works in Your Body

Study Abstract

Context: Vitamin D receptors (VDR) are found in most tissues, not just those participating in the classic actions of vitamin D such as bone, gut, and kidney. These nonclassic tissues are, therefore, potential targets for the active metabolite of vitamin D, 1,25(OH)2D. Furthermore, many of these tissues also contain the enzyme CYP27B1 capable of producing 1,25(OH)2D from the circulating form of vitamin D, 25OHD. This review is intended to highlight the actions of 1,25(OH)2D in several of these tissues but starts with a review of vitamin D production, metabolism, and molecular mechanism.

Evidence acquisition: Medline was searched for articles describing actions of 1,25(OH)2D on parathyroid hormone and insulin secretion, immune responses, keratinocytes, and cancer. Evidence synthesis: Vitamin D production in the skin provides an efficient source of vitamin D. Subsequent metabolism to 1,25(OH)2D within nonrenal tissues differs from that in the kidney. Although VDR mediates the actions of 1,25(OH)2D, regulation of transcriptional activity is cell specific. 1,25(OH)2D inhibits PTH secretion but promotes insulin secretion, inhibits adaptive immunity but promotes innate immunity, and inhibits cell proliferation but stimulates their differentiation.

Conclusions: The nonclassic actions of vitamin D are cell specific and provide a number of potential new clinical applications for 1,25(OH)2D3 and its analogs. However, the use of vitamin D metabolites and analogs for these applications remains limited by the classic actions of vitamin D leading to hypercalcemia and hypercalcuria.

Study Information

Bikle D.
J Clin Endocrinol Metab.
2008 October
Professor of Medicine and Dermatology, Veterans Affairs Medical Center and University of California, San Francisco, CA.

Full Study