Chapter 9 



VITAMINS 



The various vitamins differ so greatly in composition that it is impossible 

 to define them in terms of their chemical structure, as is done with other 

 classes of compounds such as carbohydrates, fats, and proteins. From 

 the physiological viewpoint, which is probably the best for an accurate 

 characterization, vitamins may be defined as organic compounds that 

 are essential constituents of the diet but are required in only minute 

 amounts for the normal functioning of the body. Thus they may be 

 seen to differ from hormones in that the body cannot synthesize them, 

 from trace elements in that they are organic compounds, and from the 

 main structural and energy-yielding materials of the diet (the carbo- 

 hydrates, fats, and proteins) in that they are required in extremely 

 minute amounts. For example, an average adult person consumes about 

 600 g. (dry basis) of the major food materials daily, whereas his total 

 vitamin requirements are only about 0.1-0.2 g. per day. The reason why 

 such small amounts are effective is that their role in the chemistry of 

 life processes is essentially catalytic. Several such as thiamine, ribo- 

 flavin, nicotinic acid, pantothenic acid, and pyridoxine have already been 

 shown to form a part of, or to act with, various enzymes, and in all 

 probability the remainder will be found to function in a similar manner. 



The development of our knowledge of the vitamins has come from 

 two main lines of investigation: the study of nutritional diseases, mainly 

 beriberi, scurvy, rickets, and pellagra, and the feeding of "simpHfied" 

 or "purified" diets to experimental animals. 



Until about 1900 it was rather generally believed that an adequate 

 diet for animals or man need contain only purified protein, carbohydrate, 

 fat, and mineral salts. However, earlier work of Lunin (1881) and, 

 particularly, the work of Hopkins (1906) demonstrated conclusively that 

 animals on such diets did very poorly unless small amounts of certain 

 additional foods, such as milk, were also given them. Thus the idea 

 arose that besides the four major constituents of the diet "accessory food 

 factors" were needed in small amounts for normal nutrition. 



What are now known to be vitamin deficiency diseases have been 

 studied for centuries, but with relatively little progress until recent times. 

 The first and one of the most difficult steps was to prove that a disease 

 was caused by faulty diet and not by some infection or other agent. 



Some of the observations which gradually led to the acceptance of this 



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