CHAPTER V 



VITAMIN A 



While the conception of the presence in certain food materials of 

 the water-soluble vitamins B and C developed primarily from obser- 

 vations upon disease, the conception of the fat-soluble vitamin A arose, 

 as has been noted in Chapter I, from the failure to secure normal 

 growth in experimental animals for a long period of time on purified 

 food materials furnishing adequate proteins, fats, carbohydrates, and 

 salts. The plan of studying the nutritional needs of animals through 

 the feeding of mixtures of purified foodstuffs instead of natural foods 

 had early impressed itself as a logical method of procedure but one 

 beset with difficulties. Voit (1881) in his treatise on nutrition in Her- 

 mann's Handbuch der Physiologic wrote : "Unquestionably it would 

 be best for the purpose if one could feed only pure chemical com- 

 pounds (the pure foodstuffs), for example, pure protein, fat, sugar, 

 starch, ash constituents, or mixtures of the same. However, inasmuch 

 as men and animals only rarely tolerate continuously such tasteless 

 mixtures, it is necessary in most cases to choose foods as they are 

 provided by nature." Several workers contributed links to the chain 

 of evidence of the existence in natural food materials of previously 

 unknown growth-promoting substances as noted in the introductory 

 chapter. Lunin's experiments conducted primarily to determine the 

 nutritional significance of certain inorganic substances led to the sug- 

 gestion that milk must contain unknown substances indispensable in 

 nutrition. In 1909, Stepp published the first of a series of experiments 

 concerned with the indispensability of lipoids for normal nutrition. His 

 procedure was to feed mice with milk-bread subjected to prolonged 

 extraction with alcohol and ether for the purpose of removing the 

 lipoids. In commenting on the use of this food instead of an artificial, 

 "synthetic," lipoid-free mixture he stated that, as there was no exact 

 knowledge of the materials necessary for life, nutritive failure on a 

 "synthetic" ration lacking in lipoids might be attributed not to lack 

 of lipoids but of other (unknown) substances. Stepp found that mice 

 could not live on the extracted bread but that, if the total extract were 

 fed with the bread, normal growth ensued. In the light of our present 



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