Vitamin D actions to regulate calcium and skeletal homeostasis

The endocrine action of plasma 1,25-dihydroxyvitamin D plays a key role in the regulation of plasma calcium and phosphate homeostasis with activities on the intestine, kidney and bone. A current, controversial question is whether vitamin D exerts direct actions on bone cells to regulate bone mineral homeostasis. Results from clinical, rodent model and in vitro studies on human bone cells provide an impressive body of data to support this proposal particularly at the level of serum 25-hydroxyvitamin D status. Each of the major bone cell types is capable of metabolising vitamin D to the active metabolite, 1,25-dihydroxyvitamin D. Thus under conditions when bone tissue synthesis of 1,25-dihydroxyvitamin D is optimal, vitamin D activity enhances bone mineral status. Dietary calcium and phosphate intakes are the critical environmental cues together with vitamin D status to determine whether 1,25-dihydroxyvitamin D exerts an anabolic or catabolic action on bone mineral status.

► Endocrine actions of 1,25-dihydroxyvitamin D are key in plasma calcium and phosphate homeostasis.
► Critical serum 25-hydroxyvitamin D level to maintain plasma 1,25-dihydroxyvitamin D is > 30 nmol/L.
► Bone metabolism of 25-hydroxyvitamin D maintains bone mineral homeostasis.
► Critical serum 25-hydroxyvitamin D level for bone metabolism is > 75 nmol/L.
► Dietary calcium intake modulates vitamin D metabolism at the kidney and bone.