HUMAN STRUCTURE AND LIFE-ACTIVITIES 499 



More food is required in cold weather. The non-nitroge- 

 nous foods are most economically increased; but whether 

 fats or carbohydrates should be used for increased heat 

 largely depends upon availability and digestibility. The in- 

 habitants of very cold regions can easily get fat from animals, 

 while those in temperate regions can store plant food, con- 

 taining large amounts of carbohydrates, for winter use. 

 When both kinds of foods are available, there is great indi- 

 vidual variation in taste for and digestion of carbohydrates 

 and fats ; and any desired combination of the two may be 

 made provided that we remember the energy value of fats 

 as 9 calories, while sugars and starches have 4 per gram. 



422. Foods for Repair and Growth of Protoplasm. At- 

 tention has several times been directed to the fact that liv- 

 ing substance (protoplasm) is composed of protein. Also, 

 it has been stated that animal cells can make new protoplasm 

 only from protein foods, while plant cells can make protein 

 from the elements furnished by carbohydrates and the 

 nitrogen-containing substances absorbed from the soil. 

 There is abundant evidence that in the human body pro- 

 tein food is required for making new protoplasm in repair 

 and growth ; and that the foods lacking nitrogen (i.e., car- 

 bohydrates and fats) cannot answer this purpose. 



The amount of protein food required daily for repair of 

 the protoplasm in the cells of an adult man is believed to be 

 from 60 to 100 grams (28 grams to an ounce). However, 

 it should be said that some physiologists hold that 100 is 

 nearly twice the amount which is absolutely necessary; 

 but many conservative authorities regard 60 grams of 

 protein as too little for a regular daily diet intended for 

 repair and the maintenance of good health. 



If 200 grams of protein be eaten daily, the nitrogen ex- 

 cretion will be double that from the usual 100 grams. This 

 indicates (1) that protein is easily disintegrated in the living 

 cells, (2) that excess protein in the food is not stored 



