600 IX. CAROTENOIDS AND VITAMINS A 



which decrease the abihty of the body to transform carotene to vitamin A 

 (diabetes melhtus aad hypothyroidism), and (S) factors which cause an 

 unusually rapid utihzation of the vitamin or an increased rate of loss of 

 vitamin A (acute or chronic infections which are associated with a high and 

 sustained fever). These conditions exert their effect by reducing the vita- 

 min A content of the plasma and of several tissues, rather than by depleting 

 the liver stores of vitamin A.^°^^ 



The vitamin A stores ordinarily present are so extensive that two or 

 three years on a vitamin A-deficient diet would be required to deplete the 

 reserve supplies of vitamin A in the case of the adult. The deficiency can 

 be brought about more rapidly in infants and young children, in whose case 

 the prior storage of vitamin A is less extensive, and a rapid rate of utiliza- 

 tion due to growth normally takes place. Although outright vitamin A defi- 

 ciency is more common in the young, it does not often occur in these cases 

 as a simple deficiency state. The most recent authoritative discussion of 

 avitaminosis A in man is that of Mason. ^"^^ 



b. Experimental Avitaminosis A in Man. Mason ^"-^ points out that 

 eight series of experiments were performed between 1937 and 1943 in 

 which an attempt was made to demonstrate the symptoms which supervene 

 in man following a vitamin A-free regimen, and the effects of vitamin A 

 on this condition. Most of the assessment of vitamin A deficiency has 

 been made by the use of dark adaptation tests. As an example of this 

 type of study, Hume and Krebs^^* studied the dark adaptation time in the 

 case of twenty-three subjects (twenty men and three women) who Hved on 

 a diet essentially devoid of vitamin A and of carotene. One subject main- 

 tained this dietary regimen for as long as two years, and several followed it 

 for one and one-half years "wdthout any appreciable signs of vitamin A de- 

 ficiency. There was shght e\idence of deficiency symptoms in three of the 

 twenty-three subjects, as indicated by reduced plasma vitamin A levels and 

 a shghtly increased dark adaptation time. It is thus evident that there is 

 a marked variation between different subjects with regard to their ability 

 to resist vitamin A deficiency when they are continued on a vitamin A-free 

 diet over a prolonged period. For example, Blanchard and Harper'"^^ 

 reported that, although only moderate changes in dark adaptation time 

 were observed in the majority of their ten young male subjects, as a result 

 of a stringent vitamin A-low diet over a period of forty-two to forty-five 

 days, in at least one case the dark adaptation time was increased from a nor- 

 mal value of two or three minutes to a period of over nineteen minutes on 

 the twenty-ninth day of deprivation of vitamin A. Hume and Krebs^«* 

 reported that eight months of depletion were required before most of the 



