From what I understand, the best ways to get Vitamin D are from food sources or the sun. Vitamin D is synthesized from cholesterol found in foods like wild salmon. Vitamin D is also synthesized by the body when the sun's UVB rays hit bare skin.
Why is the recommended supplement is called D3? Well, it turns out that the name Vitamin D is a little imprecise. Vitamin D is actually a group of steroid molecules:
- There's bioactive D called calcitriol,
- The plant form or D2 called ergosterol,
- and D3 or the kind we synthesize in our skin called cholecalciferol.
- The first product of the sun and skin synthesis is called cholecalciferol.
- Cholcalciferol travels to the liver where it is metabolized into calcidiol or 25(OH)D. Calcidiol is the storage form of Vitamin D and this is what is checked in Vitamin D blood tests.
- Calcidiol can then take one of two pathways. However, the primary pathway is to the kidneys where it is converted to a steroid called calcitriol. This controls blood calcium levels and is critical to our survival. That's why it's the primary pathway.
Unfortunately, it looks like many of us may not get enough Vitamin D from the sun or from foods. That's where the need for supplementation comes in. But how much? There's a lot of variation on this right now. The University of California at San Diego has an interesting discussion about current recommendations for Vitamin D levels. Just click on the title post to view the video.
National Institutes of Health, Office of Dietary Supplements - http://ods.od.nih.gov/factsheets/vitamind.asp
The Vitamin D Council - http://www.vitamindcouncil.org/vitamin-d-healthcare-professionals/
American Journal of Clinical Nutrition - http://www.ajcn.org/cgi/content/abstract/85/6/1586
Colorado State Endocrine Index - http://www.vivo.colostate.edu/hbooks/pathphys/endocrine/otherendo/vitamind.html