266 THE VITAMINS 



possible to feed, daily, enough of the material under test to carry a unit 

 of vitamin A. By comparing the curves resulting from experiments 

 carefully conducted on the test material with any adequately estab- 

 lished set of weight curves representing different levels of feeding, the 

 quantity of the test material which would contain a unit of the vitamin 

 may be estimated approximately, if somewhat less accurately than when 

 experiments are conducted under the "standard" conditions. 



Without some such experimentally established basis of reference, 

 it is obviously misleading to try to compare quantitatively the vitamin 

 potencies of two products which, when tested, have shown different 

 rates of gain. The plan should be to compare the quantities of each 

 necessary to induce the same rate of growth, which rate should be 

 subnormal, and should be decided upon in the light of the considera- 

 tions discussed above. 



The Length of the Experimental Period. — The practice of many 

 laboratories has been to continue the test period for 8 weeks after the 

 depletion period. Laboratories devoted to control work upon cod-liver 

 oil commonly use a test period of 5 weeks and this was, therefore, 

 incorporated in the United States Pharmacopoeia method of determin- 

 ing vitamin A values. With both practices in use, and resulting data 

 recorded in terms of the same unit, it becomes important to know 

 whether this difference in length of test period will influence either the 

 uniformity of the results obtained or their average numerical values. 



Averaging the results of 104 of their experiments in which the test 

 animals showed about unit growth, Sherman and Burtis (1928a) find 

 at the end of the usual 8-weeks test period a gain of 25.89 grams (or 

 3.24 grams per week) with a coefficient of variation of 26 per cent. 

 These same experiments averaged at the end of the fifth week of the 

 test period show a gain of 18.12 grams (or 3.62 grams per week) 

 with a coefficient of variation of 45 per cent. Thus in this series of 

 determinations the shortening of the test period to 5 weeks would have 

 resulted in somewhat higher and more variable findings. The average 

 difference of 0.38 gram per week in rate of gain is about one-sixth 

 of that which results from doubling the vitamin intake under these 

 experimental conditions ; so that the shortening of the test period from 

 8 weeks to 5 weeks might be expected, as an average tendency, to in- 

 crease by about one-sixth the numerical ratings of vitamin A values 

 of materials thus tested. 



The slackening rate of growth encountered after 3 months of age 

 as noted here, and emphasized by Hume and Smith (1928) may be 

 due in part to the animal's larger need of vitamin A due to increased 



