XXIV CONTENTS 



TX. The Metabolism and Nutritional Valiu' of the Carotenoids and 



Vitamins A 421 



1 . Introduction 421 



2. The Digestion, Absorption, and I)igestil)ility of Carotenoids and of 



the Vitamins A 424 



( 1 ) /3-Carotene and Other Carotenoids 424 



a. Factors Altering the Absorption of /3-Carotene 425 



b. The Absorption of Carotenoids Other Than /3-Carotene 428 



0. Factors Altering the Digestibility of /3-Carotene and of Other 



Carotenoids 431 



(2) Vitamins A 432 



a. Normal Absorption of Vitamins A 432 



b. Factors Altering the Absorption of Vitamins A 434 



3. Changes in the Carotenoids and Vitamin A in the Intestinal Wall .... 437 



(1) The Transformation of ;3-Carotene and of Other Provitamins A 



into Vitamin A 437 



a. /3-Carotene 437 



b. Provitamins A Other Than /3-Carotene 439 



c. Carotenoids Other Than Provitamins A 439 



d. Sites Other Than the Intestinal Wall for the Conversion of 

 Carotenoids into Vitamin A 440 



e. Factors Affecting the Conversion of Carotene into Vitamin A . . 443 



(a) The Effect of the Thyroid Gland 443 



(b) The Effect of Hypervitaminosis A 447 



(c) Miscellaneous Factors 448 



(2) The Esterification of Vitamin A 450 



(3) The Reduction of the Retinenes to Vitamins A 451 



a. The Identity of Retinene Reductase and Alcohol Dehydro- 



genase 452 



b. The Role of Alcohol Dehydrogenase in Carotene Utilization . . 453 



4. The Transport of Carotenoids and of Vitamins A from the Intestine. . 455 



(1) Transport in the Lymph 455 



5. Carotenoids and Vitamins A in the Blood 456 



(1) |3-Carotene and Other Carotenoids in the Blood 457 



a. The Nature of the Carotenoids in the Blood 459 



b. Normal Vahies for Blood Carotene 460 



c. Factors Altering the Level of Blood Carotene 461 



(a) Diet 461 



(b) Physiologic Factors 463 



(c) Pathologic Factors 464 



(2) Vitamins A in the Blood 467 



a. The Nature of Vitamins A in the Blood 467 



(a) Vitamin A-Protein Combinations 467 



(b) The Presence of Vitamin A Alcohol and of Vitamin A 



Esters 468 



b. Normal Values for Vitamin A 469 



c. Factors Altering the Level of Vitamin A 470 



