CHAPTER IX 



THE METABOLISM AND NUTRITIONAL 



VALUE OF THE CAROTENOIDS 



AND VITAMINS A 



1. Introduction 



The metabolism of fats and oils is intimately linked with that of the fat- 

 soluble vitamins. This is because the fats usually serve as the solvents for 

 these vitamins, and the two foodstuffs frequently appear in the gastro- 

 intestinal tract simultaneously for digestion and absorption. The physical 

 nature of the fat-soluble vitamins is so similar to that of the fats that they 

 follow the same pathway of absorption from the gastrointestinal tract, and 

 of transport to the liver and tissues, namely by way of the lacteals and the 

 thoracic duct, and then via the blood stream. In some cases fats also aid 

 the absorption of the fat-soluble vitamins. They may likewise protect 

 the vitamins from oxidation and destruction. On the other hand, the to- 

 copherols (vitamins E) act to protect the fats from oxidation by virtue of 

 their antioxidant action. 



The chemical nature of the fat-soluble vitamins was described in The 

 Lipids, Vol. I, while their absorption, transport, and blood levels, as well 

 as the transformation of carotene to \'itamin A, were discussed in The 

 Lipids, Vol. II. The present chapter will outline the metabolism of the 

 carotenoids and vitamins A, and will hst the functions and the nutritional 

 importance of each. Chapters X to XII will deal with the other fat-soluble 

 vitamins in a similar mamier. For a more complete discussion of the fat- 

 soluble vitamins, the reader is referred to the monographs of Rosenberg' 

 and to that of Sebrell and Harris.- The textbooks on biochemistry by West 

 and Todd* and of White, Handler, Smith, and Stetten^ likewise present 



1 H. R. Rosenberg, Chemistry and Physiology of the Vitamins, Interscience, New 

 York, 1945. 



* W. H. Sebrell, Jr., and R. S. Harris, The Vitamins, Chemistry, Physiology, Path- 

 ology, Vols. I, II, III, Academic Press, New York, 1954. 



' E. S. West and W. R. Todd, Textbook of Biochemistry, 2d. ed., ^Macmillan, New 

 York, 1955. 



* A. White, P. Handler, E. L. Smith, and DeW. Stetten, Jr., Principles of Bio- 

 chemistry, McGraw-Hill, New York, 1954. 



421 



