VITAMINS 203 



An interesting sidelight on this question is furnished by the estimate 

 that during 1948-1950 the people of the United States spent over $500,- 

 000,000 annually for vitamin preparations. This figure does not include 

 the value of vitamins used in enriched foods and food supplements, which 

 is also estimated to approximate $500,000,000 a year. 



VITAMIN A 

 Physiological function 



Vitamin A was first recognized and studied as a growth-promoting 

 substance, but it was soon discovered that it plays other equally im- 

 portant roles in the body. In the absence of adequate amounts of vitamin 

 A the epithelial cells of the mucous membranes lose their ability to secrete 

 normally and, as a consequence, become dry and hardened. This harden- 

 ing process is known technically as cornification or keratinization (pro- 

 duction of a horny layer). In many species the eye seems to be par- 

 ticularly susceptible to this change, and a disease known as xerophthalmia 

 is produced. When the tear glands cease to secrete, the eyes are not 

 cleansed properly, they become noticeably irritated, and infection ensues. 

 As the exuding pus dries, movement of the eyelids becomes increasingly 

 difficult until eventually they become closed tightly. In the later stages 

 ulcers form on the cornea. When these ulcers rupture, the lens pops 

 out, thus permanently destroying sight. Keratinization likewise occurs 

 extensively in the respiratory tract (including the lungs), the alimentary 

 tract, the urino-genital tract, and the salivary and ductless glands. In- 

 fants suffering from xerophthalmia also exhibit a dry, scaly skin. 



Because keratinization of the mucous membranes opens those parts of 

 the body to bacterial invasion, some have ascribed to vitamin A an anti- 

 infective role, particularly valuable in warding off colds and other respira- 

 tory diseases. However, vitamin A does not appear to be toxic to 

 organisms that cause these diseases. Only to the extent that it helps main- 

 tain a healthy condition of the mucous surfaces and increases the general 

 resistance of the body is it "anti-infective." 



In addition to keratinization of epithelial tissue there are a number 

 of other symptoms of vitamin A deficiency. Urinary calculi and deposits 

 of calcium phosphate have been observed in rats suffering from vitamin 

 A deficiency. Although xerophthalmia seldom develops in the chick, 

 immense deposits of urates accumulate in the kidney and renal tubules 

 as a result of vitamin A deficiency. The vitamin is also essential to 

 reproduction, for on diets deficient in this vitamin, female rats cease 

 to ovulate normally, and males show testicular degeneration, even though 

 the vitamin E intake may be adequate. Sherman has shown that fer- 

 tility and longevity of rats is greater on a ration rich in vitamin A than 

 on one that is low in this factor. 



