1 44 Standardisation of the Vitamins 



Ik 



response of different animals to the same dose of any vitamin. Every worker 



who has tested the same dose of a vitamin at several different times is familiar 

 with the fluctuation in the average response of the groups of animals used, 

 and hence he has realised that it is impossible to make a direct comparison be- 

 tween a result obtained with a dose of a particular preparation at one time 

 with the result obtained with a similar dose of another preparation at a 

 different time. For example, it is impossible to compare summer and winter 

 milk by determining the average responses of two groups of rats which were 

 dosed in the summer and winter months respectively. The only way of com- 

 paring summer and winter milk is by comparing each separately with a prepa- 

 ration known to be stable throughout the interval elapsing between the tests 

 and thus indirectly to make the comparison between the two samples. The 

 same method is even more necessary if results obtained in one laboratory are 

 to be compared with results obtained in another. 



A standard of reference for this purpose may be any preparation of the sub- 

 stance to be determined provided it conforms to accepted criteria of purity; but 

 if there is any doubt as to the possibility of obtaining the substance in a nearly 

 enough pure form, or, as has often happened, a standard is required before 

 even the identity of the substance has been recognised, then one preparation 

 of the substance in possibly a very impure form should be made available to 

 workers throughout the world and a particular weight of that particular prep- 

 aration should be accepted as a unit of the activity it is designed to measure. 

 When the active substance itself becomes available in acceptable form, then 

 the impure standard and its unit can be discarded. Probably by that time, the 

 chemistry and physics of the vitamin will be so far known that the potency of 

 a preparation for therapeutic use will be determinable by chemical or physical 

 methods, although the activity of a food substance may, through difficulties of 

 extracting the vitamin quantitatively, still have to be determined by a bio- 

 logical comparison with the pure substance or the original standard of refer- 

 ence. If compared with a sample of the pure substance which is not the actual 

 preparation adopted as an International Standard, then the potency of the 

 food substance should be stated as the weight of active substance per gram, not 

 as a number of International Units. 



Up to the present, four standards of reference for vitamin determinations 

 have been adopted for international use by the Health Organisation of the 

 League of Nations.^ For vitamin A, in 1931, a preparation of carotene was 

 made available. With increased knowledge of the chemistry of carotene, this 

 preparation proved to be a mixture of a- and ^-carotenes, and in 1934 it was 

 replaced by a pure sample of ^-carotene, the weight of the new preparation 

 accepted as a unit being adjusted so that the unit of activity remained the 

 same. For vitamin B,^ a preparation made by Prof. B. C. P. Jansen was adopted 

 which was an adsorption product on fuller's earth from an extract of rice 

 polishings. When the supply was nearly exhausted a similar preparation was 



