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b'It must be recognized that there is not a single vitamin D compound but that there is a family of compounds that exhibit vitamin D activity. The most important of the vitamin D compounds are vitamins D2 and D3, and the more important of these is vitamin D3 or cholecalciferol. The basis for this statement is that vitamin D3 is the vitamin produced in the skin by ultraviolet irradiation. Vitamin D2, however, is produced by ultraviolet irradiation of the plant sterol ergosterol. https://www.bocsci.com/vitamin-d-and-impurities-rlist-604.html'