Only a limited number of foods naturally contain vitamin D such as fish, meat and offal, and eggs, and milk and dairy products. However, all these foods in addition contain the metabolite 25-hydroxyvitamin D (25OHD). From the few systematic studies which have been performed the food contents of 25OHD in animal foods are usually low but vary. Contents are typically very low in milk and fish (<0.1 µg/100 g), somewhat higher in meat and offal (0.2–0.4 µg/100 g) and up to 1 µg/100 g in egg yolk. It has been demonstrated that 25OHD is absorbed better and faster from the diet than native vitamin D and has metabolic effects of its own in regulating cell growth and calcium metabolism. Thus, the biological activity of 25OHD is greater than that of native vitamin D. However, there is as yet no consensus on the conversion factor that should be used for 25OHD to calculate vitamin D activity. Depending on the testing system used the factor varies from 1.5 to 5. If food contents and the higher potency of 25OHD are not included in dietary intake surveys, true vitamin D intake will be underestimated.
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