It is necessary for normal calcium and phosphorus metabolism and consequently for healthy bone and teeth development.
It is colourless, odourless, crystalline alcohol possessing steroid like structure, that means, it is related to sterol chemically. It is soluble in fats and fat-like solvents and is insoluble in water.
A decrease in potency is noted when it is exposed to increased temperatures, sunlight or oxygen. It is formed by the action of ultraviolet rays on sterols (steroids with 8 to 10 carbon atoms in the side chain at position 17 and an alcoholic hydroxyl group at position 3 are closed as sterols) (Steenbock and Hess 1924).
The ultraviolet rays cause rearrangement of atoms in the sterol molecule those results in the formation of calciferol or vitamin D.
Cholesterol (C27H45OH) and ergosterol (C28h43OH) are important sterols which on irradiation give rise to vitamin D and as such are called provitamin D.
Besides these other provitamins of vitamin D have also been noticed such as 22, 23 dehydrocholesterol, 7 dehydrocholesterol, etc.
The 7 dehydro- cholesterol is a natural provitamin which is always found in mammals. It changes to vitamin D, under the action of sunlight or ultraviolet rays.
Vitamin D (or Di) occurs in two forms D, and D3 Vitamin D2 and D3 are closely related. In 1932, Windans by the ultra-violet irradiation of ergosterol, isolated the first pure active substance, vitamin D2.
Ergosterol is a substance found only in lower plants such as yeast, and fungi (e.g., ergot, from which it is named).
Now vitamin D, has been named as ergocalciferol, of vegetable origin. Vitamin D3 was obtained later by the irradiation of 7 dehydrocholesterol and now it has been named as cholecalciferol, of animal origin.
Vitamin D, contains four double bonds, three of which are conjugated, while vitamin D, contains three double bonds, all of which are conjugated.
Occurrence and availability of Vitamin D:
Vitamin D occurs in large quantities in fish-liver oils (e.g., cod- or halibut-liver oil) and in yolk of eggs; it is present in a small degree in animal fats (e.g.„ beef suet, milk) and is practically absent from vegetable oils (e.g., olive oil).
Milk is generally fortified with vitamin D and thus becomes an ideal food for growing children. It also contains appreciable quantities of calcium and phosphorus.
Daily requirement of Vitamin D:
It is difficult to estimate human requirements, but it is suggested that the daily requirement of vitamin D of a child is 400 International Units and that of an adult about 400 units.
The daily requirement for other animals varies to a great extent with the species, age, sex and time of year. One International Unit (IU) is equivalent to the biologic activity of 0 025 pg of pure crystalline vitamin D3.