VITAMINS 205 



through which dim light is transmitted. Pronounced inabiUty to see in 

 dim light is termed "night-blindness." 



Prevalence of vitamin A deficiency 



Because the young of mammals, including man, are born with limited 

 stores of vitamin A, we find them more susceptible to a deficiency than 

 adults. Consequently, xerophthalmia is most often observed in young 

 children of the extremely poor classes, or in children at times of great 

 food shortage. Although considered to be a rare disease in the United 

 States, 13 cases were reported in 1931. Of every five children of the 

 poor classes in Yucatan one is said to be suffering from xerophthalmia. 

 Reports from Ceylon state that the disease is very prevalent there, espe- 

 cially in asylums and charity boarding schools. If, as is reported, the 

 diets in the latter institutions furnish only 30 per cent of the estimated 

 minimum requirement of vitamin A, one can readily understand why 

 two-thirds of the children are suffering to some extent. In China, India, 

 Java, and Sumatra the disease is said to be very common. Biophotometer 

 tests reveal that diets of the poorer classes in our country frequently 

 are inadequate with respect to vitamin A, and some indicate that diets 

 of those in the higher economic brackets are likely to show suboptimal 

 amounts of vitamin A. 



Chemical nature 



The vitamins A are closely related to a group of yellow, orange, and 

 red pigments called carotenoids, present in many plants and animals. The 

 group name is derived from one of the best known and most important 

 members, carotene, the orange coloring matter of carrots. Lycopene of 

 the red tomato, zeaxanthine of yellow corn, and xanthophyll of egg yolk 

 are other common carotenoids. There are several isomers of carotene, but 

 the one most closely related to vitamin Ai is beta-carotene, of which the 

 formula is given below: 



HaC .CH3 CHa CH, CH3 CHa Haa .CH, 



/C:; II II ^cC 



U^C^ ^C-CH:CH-C:CH-CH:CH-C:CH-CH:CH-CH:CCH:CH-CH:CCH:CHC^ ^CH, 



I II II I 



H.C^^X:-CH, CHa~G,^^CH» 



Hi Hj 



Beta-carotene, C^oHi* 



A high degree of unsaturation, in which the double bonds occur alternately 

 with single bonds along the carbon chain, is characteristic of carotene, as 

 well as of vitamin A and the whole carotenoid group. Pure beta-carotene 

 is a dark red, crystalline solid (Fig. 9-1). 



The relationship of vitamin A to carotene was first established by 



