Bioavailability and bioactivity of vitamin D3 active compounds - Which potency should be used for 25-hydroxyvitamin D3?

Essential for estimating the dietary intake of vitamin D is to have valid data for the vitamin D content of the foods. In older food composition tables, data for vitamin D are mostly produced by biological assays, while newer tables are derived from specific chemical methods. Results derived from biological methods are expected to show the total activity of vitamin D compounds in the foods, while results derived from specific chemical methods have to be corrected for the relative bioavailability and bioactivity of each of the quantified vitamers in order to calculate the total vitamin D activity as the content of vitamin D3. Though no consensus has been established for the potency between vitamin D3 and 25-hydroxyvitamin D3, the proposed factor 5 is generally accepted. Few studies investigated the bioavailability and bioactivity factor for vitamin D metabolites. There is a lack of documentation for using factor 5 as the potency for 25OHD3 compared to vitamin D3. Based on studies made so far the result of 1.7 from the only study conducted according to a standardized biological method seems to be the best estimate for the potency. However, more research is needed using non-depleted animal or human as test system.