VITAMINS 209 



Occurrence 



The carotenoid pigments possessing vitamin A activity are synthesized 

 in the plant, where they are found associated with such pigments as 

 xanthophyll and chlorophyll, which have no vitamin A potency. From 

 available information it appears that all animals are ultimately depend- 

 ent upon the carotenoid pigments for their supply of vitamin A, for in 

 no instance has the real vitamin been reported to occur in plant prod- 

 ucts. Neither has any animal been observed to possess the ability to 

 synthesize the vitamin from other compounds. The potency of animal 

 products may be due either to stored carotenoids or to the vitamin that 

 the body has formed from these ingested pigments. 



The most potent sources of vitamin A are the fish-liver oils. Of these 

 oils, that of the cod has been most widely used. The oil from the liver 

 of the halibut has been shown to possess 75 to 125 times the potency 

 of cod-liver oil. Foods containing relatively high concentrations of 

 vitamin A include: butter, cream, cheese, milk, egg yolk, liver, the green 

 vegetables such as spinach, escarole, string beans, and leaf lettuce, apri- 

 cots, bananas, oranges, peaches, prunes, pumpkins, sweet potatoes, and 

 tomatoes. The green outer leaves of lettuce are reported to contain 30 

 times as much vitamin A as the crisp inner portion. The potency of 

 cow's milk has been found to vary with the breed and ration. ]\Iilk 

 remains a good source of the vitamin, even in winter, since the cow is 

 able to draw upon her reserves when confined to feeds deficient in vitamin 

 A. In some milk {e.g., Guernsey) the larger part of the potency is due 

 to carotene, while in others {e.g., Holstein) there is more vitamin A than 

 carotene. Since the vitamin A activity of colostrum (the first milk after 

 parturition) is 10-100 times that of ordinary milk, it is apparent that 

 nature has provided for immediate supplementation of the meager supply 

 with which the young are provided at birth. Human milk is 3-5 times 

 richer in this factor than cow's milk. Glandular organs, like liver, con- 

 tain much more vitamin A than muscles. Cereals and cereal grains (with 

 the exception of yellow corn) , roots, and tubers are generally conspicuously 

 deficient in the vitamin. 



Requirements 



According to the Food and Nutrition Board of the National Research 

 Council, the vitamin A requirement of the average adult or adolescent 

 is satisfied by the consumption of approximately 5000 international units 

 daily. The needs of smaller children are somewhat less. Pregnancy 

 may increase the demand for vitamin A to 8000 international units per 

 day. Quantities of representative foods that would supply approximately 



