How does Vitamin D work?


  • Vitamin D3 is the only 'vitamin' which the body makes itself.  All other vitamins are acquired through dietary means.
  • Vitamin D3 is not technically a vitamin, it is a prohormone produced photochemically in the skin in response to sunlight (UVB)
  • Vitamin D3 has a molecular structure closely allied to that of steroid hormones (like cortisol and estradiol) 
  • Vitamin D3's important biological effects only occur as a consequence of its metabolism into a family of daughter metabolites
  • Pro-vitamin --->  Vitamin D3 ---> Circulating Vitamin D3 ---> Active Vitamin D3

UVB exposure converts pro-vitamin D (7-dehydrocholesterol) in the skin to pre-vitamin D3 which is then rapidly transformed into vitamin D3 (colecalciferol) by the skin's temperature and absorbed into the body.
Synthesis is reduced in individuals with dark skin as melanin absorbs UV radiation.

In the summer, a fair-skinned person in the UK would need 15 to 30 minutes on face and forearms at midday 2 or 3 times per week2,3without sunscreen to produce enough vitamin D to last over the winter. Skin synthesis is also reduced in older people.4


In northern latitudes, the amount of UVB present in the winter is insufficient to produce vitamin D3 and so the body relies on vitamin D stored in body fat, or from food and supplements


  1. Vitamin D council.  Available at
  3. Rhodes et al  2010;  J. Invest. Dermatology: 130, 1411 - 1418
  4. Holick MF, Matsuoka LY, Wortsman J. Age, vitamin D, and solar ultraviolet. Lancet. 1989;ii:1104--5.Robert P. Heaney, Barriers to Optimizing Vitamin D3 Intake for the ElderlyJ. Nutr. 136: 1123-1125, 2006.