264 THE VITAMINS 



son for several reasons. The term means various rates in different 

 laboratories, and it does not permit the results to be verified by means 

 of larger as well as smaller gains in weight resulting from feeding 

 correspondingly larger or smaller amounts of vitamin-A-containing 

 food. Moreover, with foods poor in the vitamin it is frequently difficult 

 and sometimes impossible to feed enough to induce fully average nor- 

 mal growth. 



In general, the less one need feed of the food under test, the less 

 is the danger that the rate of growth may be influenced by changes in 

 other factors through partial displacement of the basal diet by the test 

 food, and hence the better is the prospect that the resulting weight 

 curves will furnish an accurate quantitative measure of the vitamin 

 value of the material under investigation. 



While the procedure here recommended aims to secure a gain of 

 weight of 3 grams per week during the test period, it is recognized 

 that, in the judgment of some investigators, it may be possible to 

 secure more uniform results at a somewhat higher level of feeding and 

 rate of growth because the animals will then be more vigorous. 

 Dutcher et al. (1927) while recognizing that quantitative differences 

 may be more discernible at a rate of growth of about 3 grams per 

 week are inclined to favor the Javillier (1925) standard of half-normal 

 growth (about 6 grams per week) since under this condition they 

 encountered less infection among their test animals. Sherman and 

 Burtis (1928a) studied variability with the following results: In 104 

 cases, when the level of vitamin intake during the experimental period 

 was twice that needed for a gain of 3 grams per week, the data showed 

 coefficients of variability of 32 and 30 per cent at the ends of the fifth 

 and eighth weeks, respectively, when the influence of differing response 

 of the sexes was ruled out ; while at a level of vitamin intake inducing 

 a gain of 3 grams per week, the coefficients of variability in the same 

 number of cases were 45 and 26 per cent respectively. The variability 

 is slightly (perhaps not significantly) greater at the higher level when 

 the tests are continued for 8 weeks, but less when the test period is 

 shortened to 5 weeks. 



While undoubtedly the higher level of vitamin A intake does result 

 in more vigorous animals, the rates of gain are not increased in full 

 arithmetical proportion to the vitamin intake as was plainly shown by 

 data of Sherman and Munsell (1925), so that quantitative comparisons 

 at higher rates of gain necessarily make the method less delicate. Also, 

 the higher rates of growth are attended with much less uniform results 

 between male and female test animals. 



