328 THE VITAMINS 



freed from alcohol, dissolved in pure, unadulterated olive oil, and the volume 

 adjusted to produce a known concentration, in terms of ergosterol used." 



Tests of the stability of the preparation have shown that it retains 

 its activity unchanged for two years if maintained at or below 0° C, 

 but may suffer some loss in activity at higher temperatures. The recom- 

 mendations of the committee, which are intended to lead to uniform 

 procedure in Great Britain, are as follows : 



"(a) That the standard solution of irradiated ergosterol prepared and main- 

 tained by the National Institute for Medical Research be recommended for adop- 

 tion as a standard for the comparative estimation of vitamin D. 



"(b) That the unit of vitamin D be defined as the antirachitic potency of a 

 quantity of this preparation corresponding to 0.0001 mg. of the ergosterol used 

 in its production. 



"(c) That either the X-ray method, the 'Line test,' or chemical analysis of 

 the bones of the experimental animals, be recommended for use in the estimation 

 of vitamin D. 



"(d) That supplies of the standard material be made available for general 

 distribution from the National Institute for Medical Research, Hampstead. As the 

 supply is limited, it is hoped that users will themselves prepare standard solu- 

 tions of irradiated ergosterol for use in individual tests, retaining the National 

 Institute's material for reference. It is important that all such standard materials 

 be kept at temperatures not exceeding 0° C." 



Food Sources of Vitamin D 



That a dietary factor affecting calcium assimilation may appear in 

 plant tissue was shown by Hart, Steenbock and Hoppert (1922) in 

 their study of the comparative efficiency of fresh over dried leaf food 

 in relation to calcium balance of cows during lactation. But that the 

 amounts may be small was indicated by Hess and Unger (1920) who 

 reported that 30 grams spinach daily did not cure rickets in children. 



Shipley, Kinney and McCollum (1924) found that "ether, alcohol 

 and acetone extract from alfalfa leaves a substance which exerts an 

 antirachitic effect when fed in amounts corresponding to 250 grams 

 per kilogram of food" (no other levels were reported). . . . "Ether 

 extracts the antirachitic vitamin from clover blossoms." . . . "Ether 

 extracts of dry spinach, brussels sprouts, cabbage, celery, tomato and 

 sweet potato were given in amounts equivalent to feeding 250 grams 

 per kilogram of ration to animals with rickets without any effect on 

 the rachitic process in the bones. These vegetables, therefore, in the 

 dry state are free from, or contain negligible amounts of the anti- 

 rachitic vitamin." In an accompanying paper (1924a) the same authors 

 confirmed the presence of vitamin D in liver oils and butter and re- 

 ported that "alcoholic extracts of butterfat contain appreciable amounts 

 of the antirachitic substance, and that the same substance is present in 

 oil of cloves." 



