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nightsurfer on 2009-08-28Why would vitamin D be prescribed when vitamin D3 is available over-the-counter?
Let's review the known differences between vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol):
--D3 is the human form; D2 is the non-human form found in plants.
--Dose for dose, D3 is more effective at raising blood levels of 25-hydroxy vitamin D than D2. It requires roughly twice to 250% of the dose of D2 to match that of D3 (Trang H et al 1998).
--D2 blood levels don't yield long-term sustained levels of 25-hydroxy vitamin D as does D3. When examined as a 28-day area under the curve (AUC--a superior measure of biologic exposure), D3 yields better than a 300% increased potency compared to D2. This means that it requires around 50,000 units D2 to match the effects of 15,000 units D3 (Armas LA et al 2004).
--D2 has lower binding affinity for vitamin D-binding protein, compared to D3
--Mitochondrial vitamin D 25-hydroxylase converts D3 to the 25-hydroxylated form five times more rapidly than D2.
--As we age, the ability to metabolize D2 is dramatically reduced, while D3 is not subject to this phenomenon