Vitamin D has multiple roles in the human body. In addition to its well established role in the regulation of calcium absorption and promoting bone growth, it is recognised for other health benefits including reducing risk of diseases such as type 1 diabetes and common cancers.
Two major forms of vitamin D are important in the human body, vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Vitamin D3 is the form synthesised in the skin in response to sunlight UVB exposure yet can also be obtained from animal based foods. Vitamin D2 on the other hand is the form synthesised by plants and is obtained from plant derived foodstuff. Both are metabolised in the liver to their respective 25OH vitamin D3 and D2 forms which are in turn converted to their active form in the kidneys.
The best indicator of vitamin D status is the serum concentration of 25OH vitamin D. For a correct diagnosis of vitamin D deficiency, insufficiency or intoxication, the assay must recognise both the D2 and D3 forms. As both forms have clinical relevance, the total concentration provides a sound basis for treatment.