142 BIOLOGY AND HUMAN LIFE 



out for yourself with respect to every kind of food, and then 

 use your knowledge. You will have to find out how much you 

 can best eat at one time, or how often you have to eat. 



Digestibility. Aside from individual peculiarities of the di- 

 gestive system, some foods are more easily digested than others. 

 For example, milk contains the proteins, fats, carbohydrates, 

 and salts in a very easily digested form. Meat proteins and fats 

 are in general more easily digested than vegetable proteins and 

 fats ; but the meat proteins are inclosed in materials that may 

 not be so easily digested. 



Nutritive value. We usually know immediately whether our 

 food pleases us, or whether we have stopped our hunger ; and if 

 something goes wrong with the digestion, we soon discover it. 

 But we may continue a very long time on a diet that is seriously 

 lacking in essentials without realizing it. For this reason we must 

 see that everybody acquires tastes and habits guided by reliable 

 knowledge of daily needs. Such knowledge rests upon studies 

 of what people do actually eat, and upon experiments with the 

 diet and its effects on college students, soldiers and other people, 

 and on various animals. Some of these experiments are made 

 with a very elaborate and very accurate apparatus called the 

 respiration calorimeter (see Fig. 79). 



120. Daily needs. From these studies and experiments it has 

 been possible to make out tables of daily needs for men and 

 women, boys and girls, at different ages, in different occupa- 

 tions, at different seasons of the year. The age is important 

 because ( i ) the digestive system of a young person may not be 

 able to tolerate what an older person can stand ; ( 2 ) a young 

 person is usually smaller and so uses up less proteins in the 

 basic metabolism each day; and (3) the young person is still 

 growing, and so uses up more proteins for building new tissues 

 than does the older person. The occupation is important be- 

 cause the amount of energy used up in the day's work must be 

 supplied by the fats and carbohydrates, in addition to the nu- 

 trients required for the basic metabolism. So in the winter we 

 need more fuel to keep up the body temperature. 



