746 



PHYSIOLOGY OF DIGESTION AND SECRETION. 



very much among themselves. Some idea of the limits of variation 

 may be obtained from the following table, taken chiefly from 

 Konig's analyses: 



The vegetable foods are distinguished, as a rule, by their large 

 percentage in carbohydrates and the relatively small amounts of 

 proteins and fats, as seen, for example, in the composition of rice, 

 corn, wheat, and potatoes. Nevertheless, it will be noticed that the 

 proportion of protein in some of the vegetables is not at all insignifi- 

 cant. They are characterized by their excess in carbohydrates 

 rather than by a deficiency in proteins. The composition of peas 

 and other leguminous foods is remarkable for the large percentage 

 of protein, which exceeds that found in meats. Analyses such as 

 are given here are indispensable in determining the true nutritive 

 value of foods. Nevertheless, it must be borne in mind that the 

 chemical composition of a food is not alone sufficient to determine 

 its precise value in nutrition. It is obviously true that it is not what 

 we eat, but what we digest and absorb, that is nutritious to the 

 body; so that, in addition to determining the proportion of food- 

 stuffs in any given food, it is necessary to determine to what extent 

 the several constituents are digested. This factor can be obtained 

 only by actual experiments. It may be said here, however, that 

 in general the proteins of animal foods are more completely digested 

 than are those of vegetables, owing chiefly to the fact that the 

 latter may contain a considerable amount of indigestible cellulose, 

 which tends to protect the protein from the action of the diges- 

 tive secretions. In the animal foods, therefore, chemical analysis 

 comes nearer to expressing directly the nutritive value. 



It must be borne in mind also, as will be explained more in 

 detail in the chapters on nutrition, that our various foods, animal 

 and vegetable, may contain substances other than the proteins, 

 fats, and carbohydrates, which play an accessory or even essential 

 role in the complex chemical processes of nutrition. This idea is 

 illustrated by the existence of what are known as food accessories or 

 viiamines (p. 912), and their mysteriously important relation to 

 normal nutrition. The proteins also, it will be found, are not all of 

 equal value in metabolism, and the same fact is true probably to 

 * Atwater, "The Chemistry of Foods and Nutrition," 1887. 



