[Metabolism and main effects of vitamin D]

Abstract

Vitamin D is not a vitamin stricto sensu as its main source does not come from diet. Vitamin D should rather be considered as a prohormone. To become fully active, vitamin D must be hydroxylated into 25(OH)D in the liver and then into 1,25(OH)2D (also called calcitriol) in the kidney, but also in many other tissues. The main classical effects of vitamin D concern bone and calcium/phosphorus metabolism. Many non-classical effects of vitamin D are suggested by the quasi-ubiquitous presence of the vitamin D receptor and by myriads of studies showing an association between vitamin D deficiency/insufficiency and an increased incidence or a poor prognostic of many diseases. The 25(OH)D serum concentration is the biological index that defines vitamin D status. There is currently no absolute consensus on the definition of vitamin D deficiency. Many experts consider that a 25(OH)D level less than 50 nmol/L corresponds to vitamin D deficiency whereas a concentration between 50 and 75 nmol/L corresponds to vitamin D insufficiency. These definitions are mostly based on the musculoskeletal effects of vitamin D.