IX. REQUIREMENTS 171 



B. OF 11 UMAX BEINdS 

 KARL E. xMASON 



I. The International Unit 



Dosage and requirements of vitamin A are usually expressed in terms of 

 international units (I.U.). This unit has a constant value; the standard of 

 reference for the unit, against which commercial preparations are assessed, 

 also has a constant biological value, although it has varied chemically as 

 more suitable compounds have become available. The 1931 reference stand- 

 ard of 1 7 of ciystalline carotenes was replaced in 1934 by 0.6 y of crystalline 

 /J-carotene, and in 1949 by 0.344 7 of vitamin A acetate (or 0.3 7 of vitamin 

 A alcohol) which had previously been adopted by the United States Phar- 

 macopeia to replace the older U.S.P. reference cod liver oil standard. The 

 1934 reference standard of O.G 7 of jS-carotene is retained for the measure- 

 ment of carotenes. As implied in these standards and as borne out in bio- 

 logical tests, the C40 molecule of carotene produces but one C20 molecule 

 of vitamin A. 



Vitamin A in dietary sources and in fish liver oils and concentrates exists 

 chiefly as natural esters which are hydrolyzed by pancreatic lipase and 

 absorbed as the less stable vitamin A alcohol, which is re-esterified in the 

 intestinal wall. The presence of fat and bile salts is necessary for adequate 

 absorption of both vitamin A and carotene. Concepts of the subsequent 

 fate of vitamin A ester and carotene have been considerably modified by 

 recent experiments on a variety of animals. This subject has been care- 

 fully reviewed by Kon and Thompson.^'* If these finding are applicable to 

 man, we must visualize the intestinal wall as the major site of conversion 

 of carotene to vitamin A, rather than the liver as was once believed, and 

 the lymphatic stream rather than the portal vein as the major pathway of 

 transport of these substances from the intestine to the general circulation 

 from which the liver and other tissues, under normal conditions, remove 

 the excess and maintain a reasonably constant blood level of carotene (about 

 120 I.U. per 100 ml.) and of vitamhi A (about 75 to 100 \.V. per milliliter, 

 predominantly as vitamin A alcohol). It is still undetermined whether much 

 of the carotene of the circulating blood or of that stored in various tissues 

 can be converted into \'itamin A. 



2. Requirements for Carotene 



The most valuable evidence concerning the refiuirements of young 

 adults for carotene, when this represents the sole cource of vitamin A, comes 



" S. K. Kon and S. Y. Thompson, Brit. J. Nutrition 5, 114 (1951). 



