VIII. VITAMIN A DEFICIENCY IN' HUMAN BEINGS 139 



lived oil a diet eisseiitially devoid ol' vituiniii A and providing little more 

 than traces of /3-carotene. Although several subjects were depleted for more 

 than II2 years, and 1 subject for more than 2 years, there was no more 

 than slight evidence of deliciency manifestations in 3 subjects, based upon 

 diminished plasma vitamin A levels and slight impairment of dark adapta- 

 tion. This herculean effort emphasizes the capacity of the healthy adult to 

 retain and utilize stores of vitamin A pre\dously acquired by the body. It 

 also confirms previous observations that the first effect of vitamin A de- 

 jiletion is a prompt and pronounced decrease in carotenoid content of the 

 blood plasma. On the other hand, it required about 8 months of depletion 

 before an appreciable diminution in plasma vitamin A levels occurred, and 

 14 months or more before levels of less than 40 I.U. per 100 ml., generally 

 regarded as a critical level, were observed in 3 subjects. 



3. Nutrition Surveys 



Representative individuals, or representative families, of a community 

 may be assessed on the basis of: (1) the daily intake of vitamin A and of 

 carotene provided by their average diet; (2) the incidence of recognized 

 symptoms of vitamin A deficiency (e.g., night blindness, xerophthalmia, 

 keratomalacia, skin lesions); and (3) plasma levels of vitamin A. Unfor- 

 t unately, avitaminosis A in man is often difficult to diagnose and to evaluate 

 ([ualitatively. Much uncertainty still exists as to the reliability of methods 

 for measuring night V)lindness, which are difficult to apply in field surveys; 

 and there is lack of accord among investigators concerning the specificity 

 of conjunctival and cutaneous lesions and their differentiation from similar 

 lesions due to other causes. These criteria have increased significance if 

 there is opportunity to test their response to vitamin A therapy. Serum 

 vitamin A levels have definite meaning when below certain critical levels, 

 but they seem not to afford a reliable criterion of subclinical deficiency. 

 Data obtained by survey studies provide useful information regarding 

 incidence of deficiency and possible merits of diagnostic criteria used but 

 usually add little to our knowledge of the pathologic features of the defi- 

 ciency manifestations. 



4. Clinicopatiiologic Studies 



Symptoms or manifestations suggestive of inadequacy of vitamin A occa- 

 sionally are noted in routine examination of patients attending large out- 

 patient clinics or in nutritional surveys. Where the clinician supplements 

 this evidence with other data designed either to eliminate or to confirm 

 the suspected implication of vitamin A (e.g., dietary history, evaluation of 

 dark adaptation, plasma levels of vitamin A, biopsy and microscopic study 

 of tissues, conditioning factors, response to vitamin A therapy), important 

 contributions can be made to an understanding of pathologic manifesta- 



