Vitamin D is another fat-soluble vitamin, and was identified as necessary to prevent rickets and other diseases affecting bones. Vitamin D is called the "sunshine vitamin", because it is produced in the body by the reaction of solar UV irradiation with cholesterol.
This reaction occurs just under the skin. However, many people do not get enough sunlight, and Vitamin D is now routinely added to milk to ensure an adequate amount together with the calcium in the milk.
The Halachik issues relating to Vitamin D, however, are quite interesting. Two commercially available forms of this vitamin are available, Vitamin D2 and Vitamin D3. Vitamin D2 is produced by irradiating a chemical called ergosterol, converting it to ergocalciferol (Vitamin D2).
Ergosterol is produced by a fungus, so named because it was first isolated from a fungus growing on rye (ergot). Today, a strain of yeast that produces a far higher yield of ergosterol is grown in large fermentors and although it requires a Kosher certification, as do all fermentation products, it poses no innate Kashrus concern. Passover certification would depend on the Passover status of the yeast.
Vitamin D3 is produced by irradiating 7-dehydrocholesterol, a product derived from the cholesterol found in the skin, which is thereby converted into cholecalciferol (Vitamin D3). It is the source for this cholesterol that poses the potential Kashrus concern. Sheep's wool has been used as a clothing material since the dawn of time (see Sota 11a, where according to one opinion the clothing supplied to Adam and Chava was wool).
While on the sheep, however, wool is dirty and oily, and wool processors wash raw wool with various chemicals to remove this grease. From this washing process hey obtain wool grease, which is processed into a lubricant called lanolin. When lanolin is further processed and irradiated it is converted into Vitamin D3. The question is whether lanolin itself is Kosher, since it is an excretion from a live animal. For many years, some authorities recommended avoided using Vitamin D3 under the mistaken belief that the lanolin was somehow animal fat that was exuded from the flesh into the wool, a misunderstanding of the term wool "fat".
Were this to be the case, it would have been forbidden as Basar Min Hachai (flesh from a living animal). Upon further clarification, however, it was determined that lanolin was actually a secretion of the skin, and this concern became moot. Its status as a secretion of a living animal, however, does leave room for discussion. It may be argued that it is still subject to the rule of Yoze, something that is produced by a forbidden animal (in this case, one that is not properly slaughtered) remains prohibited. Much has been written on this point, with Halachik authorities arguing on both sides.
Some have argued that since, during processing, the lanolin becomes inedible it is no longer a subject of concern since the prohibition is only one of Yoze and not an inherent prohibition. In addition, some have argued more to the point - that wool is a permitted item per se and items derived from it pose no Halachik concern. On the other hand, some have argued that lanolin should be treated as any other prohibited excretion, and they thus avoid using Vitamin D3.