VITAMIN A 285 



of the constituents of the diet in the exact proportions stated. The food 

 mixture and distilled water were furnished ad libitum. Rats which had 

 been separated from their mothers at the age of 28 days were con- 

 tinued either upon the above-mentioned diet alone, or upon this diet 

 plus 1, 2, or 4 per cent of cod-liver oil and then at the age of 3, 4, or 6 

 months transferred to the vitamin- A- free diet above described and kept 

 under identical conditions for the determination of their survival periods. 

 Taking the survival periods as measures of the relative amounts of 

 vitamin A which had been stored under these different conditions of 

 age and feeding, it is clear that at each age studied the animals which 

 had received the richest intake of vitamin A had acquired the largest 

 bodily store, and that at any given level of feeding, the bodily store 

 became larger at 4 months of age than at 3, and larger at 6 months 

 than at 4. It is not to be inferred, however, that the body stores quanti- 

 tatively the entire surplus received in the intake, for the increases in 

 survival period found are, while very striking and doubtless extremely 

 important, not arithmetically proportional to the concentration of vita- 

 min A in the food. 



Evidently the limit of the body's capacity for storage of vitamin 

 A is not quickly reached, whether regarded from the standpoint of 

 the level of intake or the length of time during which a liberal intake 

 is continued; and it is possible, by building up a bodily store through 

 previous feeding, to prolong very greatly the length of time that an 

 animal can survive upon a diet devoid of vitamin A. 



Experiments of Sherman and Burtis (1928) have shown that the 

 level of intake of vitamin A (and perhaps certain other of the chemical 

 factors in nutrition) during early life, may markedly influence subse- 

 quent susceptibility to infection. They found that this influence of 

 the level of intake upon the incidence of infection may be pronounced 

 and long continued even when the differences of diet are no greater 

 than may readily occur within the range of ordinary normal or adequate 

 nutrition. 



In young rats taken when 4 weeks of age, from families belonging 

 to the same strain, and continued under conditions identical in all re- 

 spects except as to diet, 38 animals coming from a diet consisting of 

 a mixture of one-sixth dried whole milk and five-sixths ground whole 

 wheat and 37 animals from a diet composed of the same food mate- 

 rials but with a larger proportion of milk (one-third of the dry weight 

 instead of one-sixth), were alike placed upon vitamin-A-free food for 

 periods of about 1 month, i.e., until in each case the surplus store of 

 this vitamin in the body was depleted, and then received during a 



