FOODS AND THEIR USES 51 



either directly or indirectly dependent on plants for their 

 food supply. They use these food materials, however, for 

 the same purposes as do plants. The use of the individual 

 food substances will now be considered. 



54. Uses of proteins. — Protein is an essential constit- 

 uent of plant protoplasm (P. B., 43). This class of nutrients 

 is also essential for the growth and repair of the living sub- 

 stance in muscle, nerve, and all other tissues of the human 

 body. Proteins may also be oxidized in the body and give 

 heat and muscular energy. 



55. Uses of fats and carbohydrates. — The chief fuel in- 

 gredients of food, however, are the fats and carbohydrates. 

 Most of the fat in our foods is probably oxidized soon after 

 it reaches the cells to furnish heat and power, and this class 

 of nutrients possesses two and a half times the fuel value of 

 any other kind of food substance. This is the reason why 

 the inhabitants of arctic countries eat such large quantities 

 of fatty foods. 



The starches and sugars of bread, potato, fruits, and milk 

 are also used as fuel. The fat which we stated (44) is 

 stored in various parts of the body, is derived partly from the 

 carbohydrates and partly from the fats in our food, and this 

 acts as a reserve fuel. That portion of the fat which is stored 

 in the deeper layers of the skin helps to keep our bodies warm 

 by preventing the escape of heat. 



56. Comparison of uses of the nutrients. — It is evident 

 that the three nutrients thus far studied may be used to supply 

 the body with energy. If our diet is deficient in any one of 

 the three, the others supply the need, and are burned instead. 

 For growth and repair, however, proteins are absolutely 

 essential ; neither carbohydrates nor fat can be transformed 



