VITAMINS — ^MUNSELL 257 



required to produce a given response in the animal used and under 

 the conditions specified for the test. Such a quantity was known as 

 a "unit." Several of these biological units have been defined and 

 used but the best known are probably the Sherman units for vita- 

 mins A, Bi, and C, and vitamin G or Bg (riboflavin). 



As interest in the importance of the vitamins increased, attempts 

 were made to devise more satisfactory methods of evaluating them. 

 A committee appointed by the Health Organization of the League 

 of Nations has established standards of reference called Interna- 

 tional Standards of Keference for vitamins A, Bi, C, D, and E to be 

 used in determining the content of these vitamins in foods and other 

 materials. A definite quantity of each standard was specified as the 

 International unit in terms of which the content of the respective 

 vitamin was to be expressed. 



Definitions of the International Units for Vitamins A, Bi, C, and D 



Vitamin A. — The International unit of vitamin A is the vitamin-A activity 

 of 0.6 microgram (0.0006 milligram) of the International Standard beta-carotene. 

 One U. S. P. (United States Pharmacopoeia) unit of vitamin A presumably 

 has the same value as 1 International unit (I. U.) of vitamin A. 



Vitamin Bi. — The International unit of vitamin Bi is the vitamin-Bi activity 

 of 3.0 micrograms (0.003 milligram) of the International Standard crystalline 

 thiamin chloride (vitamin Bi). One U. S. P. (United States Pharmacopoeia) 

 unit of vitamin Bi has the same value as 1 International unit (I. U.) of 

 vitamin Bi. 



Vitamin C. — The International unit of vitamin C is the vitamin-C activity of 

 0.05 milligram of the International Standard crystalline ascorbic acid (vitamin 

 C). One U. S. P. (United States Pharmacopoeia) unit of vitamin C has the 

 same value at 1 International unit (I. U.) of vitamin C. 



Vitamin D. — The International unit of vitamin D is the vitamin-D activity of 

 1 milligram of the International Standard solution of irradiated ergosterol in oil. 

 One U. S. P, (United States Pharmacopoeia) unit of vitamin D presumably has 

 the same value as 1 International unit (I. U.) of vitamin D. 



Enumeration of vitamin potency in terms of International units 

 is now the accepted mode of expression. As more satisfactory chemi- 

 cal and physical methods of measuring vitamin content are developed, 

 this somewhat cumbersome device will doubtless be abandoned for 

 the more usual procedure of giving composition on the basis of 

 weight of chemical substance. This is already the case with vitamin 

 C where values are given more often in terms of milligrams of as- 

 corbic acid per gram or per 100 grams of material than in terms of 

 International units. 



No International Standard for riboflavin has been established. 

 The Sherman or Sherman-Bourquin unit is frequently used for de- 

 noting vitamin-G potency, otherwise riboflavin is given directly as 

 milligrams or micrograms of riboflavin. 



