VITAMINS — MUNSELL 241 



PROPERTIES AND FOOD SOURCES 

 GENERAL CONSIDERATIONS 



The most distinctive common characteristic of the vitamins is the 

 fact that they occur in foods in almost infinitesimal quantities and 

 are effective in the body in similarly small amounts. Beyond this 

 they have little in common since they differ markedly both in their 

 physical and chemical properties. Some are soluble in water while 

 others dissolve only in fats and fat-solvents. Some are easily de- 

 stroyed, especially at high temperatures and when oxygen is present, 

 as when foods are heated in air. Others are fairly resistant to de- 

 struction by heat even when heated for several hours at temperatures 

 well above the boiling point of water. In the case of nearly all of 

 them, however, destruction takes place more rapidly in alkaline than 

 in acid solution. 



In estimating the vitamin value of foods in the diet it is essential 

 to know and keep in mind the properties of the various vitamins in 

 order to be able to take account of possible losses. Consideration 

 of changes that occur in the vitamin content of foods during processes 

 connected with preservation and preparation, such as storage, freez- 

 ing, cooking and canning, and drying, is of as much importance as 

 consideration of the vitamin content of the fresh or untreated food. 

 A food which, in its original state, is a perfectly good source of one 

 or more of the vitamins may have its content of one or all of these 

 factors reduced to insignificance as a result of the treatments it un- 

 dergoes during preparation for consumption. Loss of vitamin value 

 may be brought about not only as a result of inactivation or destruc- 

 tion of the vitamins but also through their mechanical removal by 

 solution, the vitamin passing out of the food material into the sur- 

 rounding liquid. 



Wliile vitamins are found in foods of both plant and animal 

 origin, plants — generally speaking — should be considered the pri- 

 mary sources since animals depend upon plants for their supply of 

 most of the vitamins. This does not mean that the substance re- 

 sponsible for vitamin value in plant tissue is always the same as that 

 having a similar function in animal tissue. Vitamin A, for instance, 

 does not occur in plants, the vitamin-A value of plants being due to 

 certain orange-yellow substances called carotenoids. These are broken 

 down in the liver of the animal so that vitamin A is derived from 

 them, and for this reason the carotenoids are sometimes called the 

 "precursors" of vitamin A. 



It is now well known that foods show marked differences both in 

 the kinds and amounts of vitamins they supply. Differences in the 

 vitamin values of different foods do not constitute the only problem 



