284 IV. DIGESTION OF LIPIDS OTHER THAN FATS 



tenoids as a whole, more information is available concerning its absorption 

 and metabolism than is the case with any other carotenoid. Since carotenes 

 are unsaturated hydrocarbons with aliphatic chains somewhat resembling 

 those of squalene, one might expect them to be absorbed and deposited in 

 the liver in a somewhat similar manner. 



A number of observations indicate that carotene can be absorbed from 

 the gastrointestinal tract. Thus, the color of the milk and of the butter- 

 fat made from it is to a large extent directly dependent upon the carotene 

 present in the feed. The same is to some degree also true for egg yolk, 

 although in this case carotenoids other than /3-carotene may contribute 

 a large proportion of the chromogenic material. The same relationship 

 exists between dietary carotenoids and plasma carotenoids, particularly 

 in the case of man and cattle. The abnormal condition in man referred 

 to as carotenemia, which resembles jaundice in appearance, is due to the 

 unusual deposition of carotene under the skin. This excess carotene 

 content of the blood and tissues is directly related to an excessive ab- 

 sorption of the pigment due to the continued high consumption of such 

 carotene-high vegetables as Hubbard squash and carrots. 



There is likewise a considerable amount of indirect evidence of the high 

 absorbability of /3-carotene, at least when it is fed in small amounts to the 

 rat. The fact that exceedingly small amounts of /3-carotene can furnish 

 the necessary vitamin A for the animal organism demonstrates its ex- 

 ceedingly efficient absorption and transformation into vitamin A. It is 

 generally agreed that, on the weight basis, the requirement for /3-carotene 

 as compared with that for vitamin A does not exceed a factor of 2:1. 

 Since vitamin A is known to be completely absorbed, a weight factor of 

 2:1 Avould denote absorption of a minimum of 50% of the /3-carotene, 

 provided that /3-carotene were transformed quantitatively to vitamin A. 

 However, a number of workers have suggested that a higher percentage 

 of carotene is converted to vitamin A than would be indicated by a weight 

 relationship of 2:1. Moreover, if one assumes that, within the organism, 

 the maximum possible yield of vitamin A does not result from carotene, 

 then the minimum absorption value of 50% must be appreciably increased 

 and, in all probability, may approximate 100%. 



Other indirect methods have likewise been used to study the absorption 

 of /3-carotene. These include quantitative bioassay based upon the 

 amount of vitamin A laid down in the liver. Another procedure involves 

 the determination of the effect of ingested carotene on the level of blood 

 carotene. However, the latter test can be used only in the case of ani- 



