Vitamin D – Structure and Occurrence

   Structure of Vitamin D

Many compounds have anti-rickets properties but only two of them are designated vitamin D. Vitamin D2, now called ergocalciferol is produced by irradiation of ergosterol which was first isolated from ergot; Vitamin D2 is thus of vegetable origin. Vitamin D3 which is now called cholecalciferol is of animal origin and can also be produced in the human skin by ultraviolet rays which convert provitamin D3 (7-dehydrocholesterol) to the active Vitamin D3 per square centimeter. Melanin and keratin interfere with this reaction; dark skinned populations are more liable to suffer from vitamin D deficiency.

The formation of ergocalciferol and cholecalciferol from their precursors is shown below:

The reaction, ergsterol àergocalciferol is not 100% complete. Other compounds are also formed, e.g. lumisterol and tachysterol both of which are devoid of vitamin D activity. Tachysterol on hydrogenation is converted to dihydrotachysterol or A.T. 10 D3 have same physiological actions in man. In nature they occur as esters.

   Occurrence of Vitamin D

Vitamin D is much less widespread than vitamin A. Cod and other fish liver oils are the best sources. Flesh of oily fish and egg yolk are also good sources; livers of mammals and milk are poor sources. Vitamin D content of milk and eggs greatly depends on the diet of cow and hens and on their exposure to sunlight. Human milk is poorer in vitamin D than cow’s milk. Plant foods and vegetable oils are very deficient in this vitamin and it can be said that vitamin D in the diet is only derived from animal sources. Foods containing cholesterol (e.g. milk) can also form cholecalciferol by irradiating them with U.V rays, although this can destroy some other dietary ingredients.

It is clear from above that most natural foods including those derived from animals are low in vitamin D content. Man should therefore try to obtain as much as sunlight as possible to activated provitamin D3 to vitamin D3. If sunlight is filtered through smoke and dust or through window glasses its effectiveness is reduced.