Vitamin D physiology

The most active form of vitamin D is 1,25-dihydrocholecalciferol (calcitriol). The mechanism of its action seems to be similar to that mediating the actions of the steroid hormones: that is, binding to an intranuclear receptor protein which in turn binds to regulatory sequences on chromosomal DNA. This receptor is known to be present in most nucleated cells in the body, and vitamin D has come to be regarded as a steroid hormone.

The most dominant function of vitamin D in its active form (calcitriol) is the elevation of plasma calcium and phosphate levels which are required for bone mineralization (2).

• In the gut, vitamin D induces calcium-binding protein which increases calcium and phosphate absorption
• In bone, vitamin D at low to moderate concentration causes mineralization of bone. This may be a direct effect or may be indirectly through the elevation in calcium and phosphate concentration. At supraphysiological concentration, there is bone resorption.
• In the kidney, calcitriol together with parathyroid hormone (PTH) stimulates the renal distal tubule reabsorption of calcium, ensuring retention of calcium by the kidney when needed

Reference

• (1) Royal Osteoporosis Society (2020). Vitamin D and Bone Health: A Practical Clinical Guideline for Patient Management.
• (2) Institute of Medicine (US) Committee to Review Dietary Reference Intakes for Vitamin D and Calcium; Ross AC, Taylor CL, Yaktine AL, et al., editors. Dietary Reference Intakes for Calcium and Vitamin D. Washington (DC): National Academies Press (US); 2011. 3, Overview of Vitamin D. Available from: https://www.ncbi.nlm.nih.gov/books/NBK56061/# (accessed 15 May 2022)