108 APPLIED PHYSIOLOGY 



ordinary food to supply the needs of the body. Only salt 

 needs to be added to food, but man often adds far more 

 than is necessary. Since water and mineral matters re- 

 quire no digestion, it makes little difference in what kind 

 of food they are eaten. But albumin, fat, and starch or 

 sugar require digestion, and some forms are more easily 

 digested than others, so a discussion of their forms in dif- 

 ferent foods becomes necessary. 



/* 156. Digestibility of food. In judging of the value of 

 food four things must be considered : 



First. The time and energy required. Some forms of 

 food require little or no energy in their digestion, while 

 others cannot be digested at all. Grass contains all kinds 

 of food substances, but man cannot digest it. The com- 

 bination of meat, fruit, and flour which we call mince pie 

 requires far more time and energy in its digestion than 

 the same substances in the form of roast meat, bread, 

 and fresh fruit, or in the form of a light pudding. 



Second. The amotmt of indigestible matter. All kinds 

 of food contain some matter which is wholly indigestible. 

 Only a little of fruit is digested. Careful experiments 

 show that ordinarily at least one fifth of the albumin of 

 vegetable food passes through the intestine undigested, 

 while only one thirtieth of meat is thus wasted. Animal 

 oil is easily emulsified and saponified, while vegetable oil 

 can scarcely be changed at all, but if eaten in any quantity 

 is a source of intestinal disturbance. Some wholly indi- 

 gestible matter in food is valuable, for it affords something 

 upon which the intestine can contract so as to force its 

 contents down the tube. 



Third. The amount of energy developed by the food. 

 Fat requires a large amount of oxygen in its oxidation, 

 and yields a large amount of heat and energy. Sugar 



