242 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1941 



of variation that must be considered, however. There is the equally 

 important matter of variation from sample to sample of a single 

 food item. While it may generally be taken for granted that samples 

 of a given food, selected at different times, will contain the same 

 kind or kinds of vitamins, it does not necessarily follow that they 

 will contain equal quantities of any kind. The idea must not be 

 held with respect to any natural food, that it has a definite and fixed 

 content of any vitamin — unless, perchance, it is zero. 



The problem of sample variation in vitamin content of foods is 

 responsible for some of the newer phases of vitamin research, espe- 

 cially in connection with studies related to food production. Some 

 of the factors associated with this variation have been identified but 

 there is still much to be learned. In foods of plant origin, variety in 

 a given kind is very often an important factor in relation to vitamin 

 content. Age and maturity of the product, its size, the amount and 

 kind of fertilizer used in cultivation, the amount of moisture present 

 in the soil, and the degree of exposure to sunlight may also have 

 considerable influence. In foods of animal origin the breed of the 

 animal from which the food comes, as well as its age and physical 

 condition, is sometimes of significance, but the most important fac- 

 tors are the vitamin content of the animal's food and, in the case of 

 vitamin-D value, the length of time the animal was exposed to sun- 

 shine. This sums up to the conclusion that values for vitamin con- 

 tent can in no sense be considered exact unless correlated with an 

 adequate knowledge of the conditions that might have had an 

 influence on them. 



A point of considerable practical importance in dealing with vita- 

 min values for foods is the fact that relative vitamin potency may 

 easily be discussed by reference to food groups or food types. A diet 

 can be planned on the basis of food groups rather than individual 

 foods, thus lessening the tendency to place undue emphasis on one 

 food that may have been shown to be very rich in a particular 

 vitamin. 



VITAMIN A 



Properties. — Vitamin A belongs to the group of fat-soluble vita- 

 mins and is practically insoluble in water. The pure vitamin, pre- 

 pared by freezing it out of solution, is a pale yellow, viscous, oily 

 substance. It is not readily broken down by heat but is inactivated 

 by oxidation, especially when heated in a medium where there is 

 free access of oxygen. 



As already explained, the vitamin-A value of foods of plant origin 

 is due not to vitamin A, since this substance does not occur in plants, 

 but to the presence of orange-yellow pigments called carotenoids — 

 "precursors" of vitamin A. There are four of these substances: 



