172 VITAMINS A AND CAROTENES 



from the Sheffield experiment,^* where prophylactic and curative tests on 

 human volunteers on a diet essentially devoid of vitamin A and carotene 

 indicated a minimal daily requirement of 1500 I.U. of j8-carotene, assuming 

 that all the carotene administered was absorbed. To allow for individual 

 variation and for a certain margin of safety, the original estimate was 

 doubled. The value of 3000 I.U. required further modification to allow for 

 incompleteness of absorption, which has long been recognized as varying 

 considerably with the source of the carotene. For instance, in these studies 

 it was found that fecal excretion of carotene was of such magnitude as to 

 require a daily intake of 4000 I.U. of pure j8-carotene in oil, 5000 I.U. of 

 carotene as homogenized carrots, 7500 I.U. as cabbage or spinach, 

 and 12,000 I.U. as boiled sliced carrots (as measured chemically), in order 

 to assure absorption of the estimated requirement of 3000 I.U. Conse- 

 quently, as explained in an excellent review of the standardization of vita- 

 min A and human requirements by Hume,^^ a compromise value of 7500 

 I.U., or three times the recommended vitamin A requirement, was some- 

 what reluctantly proposed as the daily requirement if provided solely as 

 carotene from average dietary sources. 



3. Requirements for Vitamins A 



Human requirements have been estimated on the general assumption 

 that only about two-thirds of the vitamin A of the average diet is provided 

 as carotenoid precursors, and the remaining as preformed vitamin A. Esti- 

 mation of these requirements has been guided by (1) experiences in deter- 

 mining daily needs for preventing impaired dark adaptation in human 

 volunteers on controlled low intakes of vitamin A, (2) measurements of 

 vitamin A storage in the liver and other tissues from cases of accidental 

 death in healthy individuals, and (3) the estimation of carotene and vitamin 

 A in the diet of well-nourished individuals. In utilizing data from the thera- 

 peutic tests, consideration has been given to the fact that an optimal intake 

 should represent two to four times the minimal intake required to prevent 

 manifestations of the deficiency state. In these computations consideration 

 has also been given to the extent to which vitamin A needs may be modi- 

 fied by age, rate of growth, caloric intake, physical expenditure and special 

 physiological needs such as arise during pregnancy and lactation. The 

 daily dietary allowances for vitamin A, as recommended by the Food and 

 Nutrition Board of the National Research Council in its 1948 report,^® are 



^^ E. M. Hume and H. A. Krebs, Med. Research Council Brit. Spec. Repi. Ser. 264 



(1949). 

 66 E. M. Hume, Brit. J. Nutrition, 5, 104 (1951). 

 66 Bull. Natl. Research Council {U. S.), Reprint and Circ. Ser. 129 (1948). 



