276 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



Water ; 



[norganic salts; 



Proteids (or proteid-containing bodies); 

 Food-stuffs. { Albuminoids (a group of bodies resembling proteids, but 

 having in some respects a different nutritive value) ; 

 Carbohydrates ; 

 Fats. 



The main facts with regard to the specific nutritive value of each of these 

 substances will be given later on, after the processes of digestion have been 

 described. A few general remarks, however, at this place will serve to give 

 the proper standpoint from which to begin the study of the chemistry of 

 digestion and nutrition. 



Wafer and Salts. — Water and salts we do not commonly consider as foods, 

 but the results of scientific investigation, as well as the experience of life, 

 show that these substances are absolutely necessary to the body. The tissues 

 must maintain a certain composition in water and salts in order to function 

 normally, and, since there is a continual loss of these substances in the various 

 excreta, they must continually be replaced in some way in the food. It is to 

 be borne in mind in this connection that water and salts constitute a part of 

 all our solid foods, so that the body gets a partial supply at least of these 

 substances in everything we eat. 



Proteids. — The composition and different classes of proteids are described 

 from a chemical standpoint in the section ou The Chemistry of the Body. 

 Different varieties of proteids are found in animal as well as in vegetable 

 foods. The chemical composition in all cases, however, is approximately the 

 same. Physiologically, they are supposed to have equal nutritive values out- 

 side of differences in digestibility, a detail that will be given later. The 

 essential use of the proteids to the body is that they supply the material from 

 which the new proteid tissue is made or the old protcid tissue is repaired, 

 although, as we shall find when we come to discuss the subject more thor- 

 oughly (p. 345), proteids are also extremely valuable as sources of energy to 

 the body. Inasmuch as the most important constituent of living matter is the 

 proteid part of its molecule, it will be seen at once that proteid food is an 

 absolute necessity. Proteids contain nitrogen, and they arc frequently spoken 

 of as the nitrogenous foods; carbohydrates and fats, on the contrary, do not 

 contain nitrogen. It follows immediately from this fact that fats and carbo- 

 hydrates alone could not suffice to make new protoplasm. If our diet con- 

 tained no proteids, the tissue s of the body would gradually waste away 

 and death from starvation would result. All the food-stuffs are necessary 

 in one way or another to the preservation of perfect health, but proteids, 

 together with a certain proportion of water and inorganic salts, are absolutely 

 necessary for the bare maintenance of animal life — that is, for the formation 

 and preservation of living protoplasm. Whatever else is contained in our 

 food, proteid of some kind must form a part of our diet. The use of 



