Question

How does vitamin D work?

Vitamin D comes in two important forms for human metabolism:1

• Vitamin D2: The form found in plants (also called ergocalciferol)
• Vitamin D3: The form found in animals (also called colecalciferol)

Vitamin D3 is produced in our skin in response to ultraviolet B (UVB) rays from the sun.1 Vitamin D2 is found in some plants in our diet and can be produced commercially for fortification of foods. Both forms of vitamin D are used for supplementation1, but studies indicate that vitamin D2 is much less potent and has a shorter duration of action than cholecalciferol.2,3,4

Vitamin D3 (cholecalciferol) is converted to 25-hydroxycolecalciferol [25(OH)D or calcidiol] in the liver.1 In the kidneys 25(OH)D is hydroxylated to 1,25-dihydroxyvitamin D [1,25 (OH)2D or calcitriol] - the active form of vitamin D which is released back into the blood stream.1

1,25(OH)2 D3 functions through a single vitamin D receptor.5 1,25(OH)2 D acts primarily on the duodenum, regulating how the body uses calcium and phosphorus.1,6 However, vitamin D receptors are present in many of the body's tissues5, including cells of the pancreas, immune system, macrophages, vascular endothelium, stomach, epidermis, colon, muscles and placenta.1

The characteristics of 1,25 (OH)2D are those of a hormone, and so vitamin D is considered a prohormone rather than a true vitamin.1

References 

  1. Thatcher T & Clarke B. Vitamin D insufficiency. Mayo Clinic Proceedings, 2011; 86(1):50-60. Available at: www.ncbi.nlm.nih.gov/pmc/articles/PMC3012634/
  2. Tjellesen L, Hummer L, Christiansen C, Rødbro P. Serum concentration of vitamin D metabolites during treatment with vitamin D2 and D3 in normal premenopausal women. Bone Miner. 1986 Oct;1(5):407-13.
  3. Trang HM, Cole DE, Rubin LA, Pierratos A, Siu S, Vieth R.  Evidence that vitamin D3 increases serum 25-hydroxyvitamin D more efficiently than does vitamin D2. Am J Clin Nutr. 1998 Oct;68(4):854-8.
  4. Armas LA1, Hollis BW, Heaney RP. Vitamin D2 is much less effective than vitamin D3 in humans. J Clin Endocrinol Metab. 2004 Nov;89(11):5387-91.
  5. DeLuca H. Overview of general physiologic features and functions of vitamin D. Am J Clin Nutr, 2004; 80:1689-96.
  6. Holick M. Vitamin D: a D-Lightful health perspective. Nutrition Reviews, 2008; 66(2):182-194.