FITNESS JFOE USE 67 



Fitness for use. — So many factors enter into this deter- 

 mination that it is difficult to generahze them. The pres- 

 ence of the necessary organs for digestion, sex, age, size, 

 occupation, hkes and dishkes, amounts necessary for 

 nourishment, and conditions of the nutrients all bear on 

 this question. If we leave out of account all personal 

 peculiarities the most important of these factors may be 

 comprised under the head of digestibility. We shall dis- 

 cuss digestion more in detail in a later chapter. For the 

 present it is sufficient to state that by digestibiUty we 

 mean the relative amount of a food that in a given time 

 our body is able to put into tissue building or energy pro- 

 ducing form. Without attempting to detail the methods 

 by means of which digestibility is determined, we may 

 state the general conclusions of such experiments as follows : 



First. Vegetable proteid is much less readily digested 

 than animal proteid. That is to say, animal foods will 

 furnish us with more energy and tissue in a given length of 

 time than wiU vegetable foods. 



Second. The amount of fat present in a given animal 

 food does not indicate exactly its energy producing power, 

 since, as ordinarily eaten, nearly five per cent of such fat 

 escapes digestion, fat requiring a long time for its digestion. 



Third. Carbohydrates are ordinarily very completely 

 digestible and hence foods rich in these nutrients become 

 important energy producers even though their caloric 

 value is less than that of fat. 



Fourth. The amount of food digested in a given time 

 (its digestibility) is not affected to the extent commonly 

 supposed, by the addition of flavors or condiments, nor 

 does the time differ much with people of different habits. 

 The statements made above are, of course, general, and 



