REQUIREMENTS OF ANIMALS 



579 



Wliereas jS-carotene is only about one-half as effective as a source of vita- 

 min A as is the preformed vitamin A itself at a moderate intake, the relative 

 potency of the |S-carotene decreases with increased intakes, both in mam- 

 mals^'*^ and in birds. ^^^ 



(a) Rat. Since the rat has been widely used for nutrition experiments, 

 considerable data have been amassed on the vitamin A recjuirements of this 



Weeks 1 



Fig. :^. The relation of the vitamin A intake per day to the growth rate of rats.352 (From 

 the original of Fig. 1, Lewis, Bodansky, Falk, and McGuire, /. Nutrition, 23, 354 (1942).) 



species.^^^ INIost of the information is based upon the amount of vitamin A 

 required to prevent deficiencies in growth, and anomalies such as cornifica- 

 tion of the epithelial cells, xerophthalmia, reduction of the erythrocyte count 

 and of the hemoglobin content, severe anemia, infertility, and fetal resorp- 

 tion, as well as gross dental and ocular abnormalities. 



On the basis of daily requirements, the following averages (in I.U.) for 

 the vitamin A needs of rats have been recorded. Several different criteria 

 for evaluation were emploj^ed. When the requirement was calculated as 



9«« R. B. Nestler, J. V. Derbv, and J. B. DeWitt, /. Nutrition, 36, 323-329 (1948). 

 9" R. W. Little, A. W. Thomas, and H. C. Sherman, /. Biol. Chem., U8, 441-443 

 (1943). 



