We get asked a fair few questions on Vitamin D, some have received stern criticism highlighting my lack of understanding in this area (albeit from the same person).  I'm slowly learning the area and this is an educational area for me!

Vitamin D is a group of fat-soluble prohormones, the two major forms of which are vitamin D2 (or ergocalciferol) and vitamin D3 (or cholecalciferol).

From various e-mails the following seem key points:

• Vitamin D is made up (as highlighted above) of two main 'types' D2 and D3; D3 is more 'potent' than D2 - so recommendations for intake need to consider that. 
• Latitude is important.  The more Northern the person lives the less vitamin D is manufactured photochemically by the skin.  So a trial based in Australia is a different population to those in say the UK.
• The recommended daily allowance (RDA) is not the same as an adequate intake.  It appears that the RDA is based on the minimum needed to avoid rickets, as opposed to that needed for optimal health.
• It appears that vitamin D consumption via dietary means is - on the whole - insufficient, hence the need for some exposure to sunlight and/or dietary supplements.

A few good articles:

• Vitamin D Deficiency - The Once and Present Epidemic (editorial in a 2005 American Family Physician)
• Not enough vitamin D: Health consequences for Canadians (2007 review article in Canadian Family Physician)
• Circulating Vitamin D3 and 25-hydroxyvitamin D in Humans: An Important Tool to Define Adequate Nutritional Vitamin D Status (2007 review article in J Steroid Biochem Mol Biol.)
• The urgent need to recommend an intake of vitamin D that is effective (2007 editorial in American Journal of Clinical Nutrition)
• Vitamin D Deficiency: The cause of everything? (2007 webcast)