VITAMINS — MUNSELL 251 



VITAMIN D 



Properties. — At least 10 different substances are known to have 

 vitamin-D activity but only two of these are of practical importance. 

 They are vitamin Dj or activated ergosterol, known also as calciferol, 

 and vitamin D3 or activated 7-dehydrocholesterol. Ergosterol which 

 is found only in plant tissue, and 7-dehydrocholesterol, which is asso- 

 ciated with cholesterol, the sterol in animal fats, are often called pro- 

 vitamins. Under the influence of ultraviolet light (irradiation) they 

 are changed into active forms of vitamin D. The commercial prep- 

 aration known as Viosterol, is a solution of activated ergosterol in 

 oil. 



The relative activity of these two forms of vitamin D is different 

 for different species of animals. A preparation of vitamin D2 or 

 calciferol, judged by tests with rats to have the same activity as a 

 given preparation of vitamin D3, will be judged to be considerably 

 less potent when examined by tests with chicks. Thus, while, for a 

 given effect, chicks may require the same amount of vitamin Ds, 

 they will require more vitamin D2. 



Vitamin D (D2 and D3) is soluble in fats and is not affected by 

 heat or oxidation. 



Food sources. — Vitamin D does not occur to any extent, if at all, 

 in foods of plant origin, but plants do contain the provitamin, ergos- 

 terol. Dried plant tissue containing ergosterol acquires properties 

 of vitamin D on exposure to ultraviolet light. Yeast contains large 

 amounts of ergosterol, and irradiated dried yeast is an important 

 source of vitamin D. 



The only significant natural sources of vitamin D are among the 

 foods of animal origin. These include milk, eggs, liver, and fish that 

 are rich in oil, like salmon and herring. The value of these foods 

 as sources of vitamin D may well be questioned, however. The 

 quantities of the vitamin that they contain are so small compared 

 to the quantities needed by children for protection against rickets as 

 to be of little practical value in this respect, and if adults require 

 vitamin D it is difficult to believe that the quantity is as small as 

 that ordinarily supplied by the use of these foods. This statement 

 does not apply to fish-liver oil, which is the richest natural source of 

 vitamin D. Since foods of animal origin are the only ones that 

 contain vitamin D naturally, and they contain only vitamin D3 this 

 form of the vitamin is sometimes referred to as natural vitamin D. 



The vitamin-D content of milk and eggs may be increased by 

 feeding the animals producing these foods some rich source of the 

 vitamin. Cows may be given irradiated yeast. "Metabolized" vita- 

 min-D milk is produced in this way. The greater proportion of 



