254 



THE VITAMINS 



ing growth charts of rats fed by Osborne and Mendel (1921) upon 

 diets made up of foods which had been especially extracted to free them 

 from this vitamin (Fig. 9). 



When an animal placed upon a diet devoid of vitamin A ceases to 

 grow at once, as has been reported in some cases, it may be because 

 of insufficient food intake or because the previous food of the animal 

 had not been such as to induce a storage of vitamin A in the body. 

 (Storage of vitamin A in the body and in its different parts is discussed 

 more fully further on in this chapter.) As we now have much evidence 

 that such storage is a very important factor and that it depends largely 

 upon the food consumed, we must expect that the vitamin A content of 



DEPICIE5:i: 0? PaT-SOLOBLE vituiib ih liets c 



SPECIU:.^ E-TTSACTEb POOW 



Fig. 9. — Weight curves of rats on diets lacking vitamin A, from experiments 

 by Osborne and Mendel (1921). The curves show that growth continued for 

 differing lengths of time at different rates after the animals were placed upon 

 food which had been carefully freed from vitamin A. The interrupted (broken) 

 lines show periods of feeding with vitamin A after growth had stopped and 

 animals had shown pathological effects from lack of this vitamin. In most but 

 not all of these cases cures were effected and normal growth resumed. (By per- 

 mission of the Journal of Biological Chemistry.) 



the body may vary considerably with dififerent individuals of the same 

 species. This means much in the quantitative determination of vitamin A 

 inasmuch as variable stores of the vitamin in the bodies of the test 

 animals according to their age, previous feeding, and possibly other 

 conditions, will render results unreliable if they are not compensated. 

 Typical cases are shown in Fig. 10. 



The storage of vitamin A in the body is undoubtedly a very impor- 

 tant factor in the relations of this vitamin to health and will be discussed 

 further in that connection later in this chapter. 



Although much of the literature upon the occurrence of vitamin A 

 antedates the publication, or at least the general use, of quantitative 

 methods, yet it may help to clarify subsequent discussion if the con- 

 sideration of method be taken up at this point and from the quantitative 

 point of view. 



