FOODS AND DIETARIES 273 



the growth or repair of the body of a plant or animal. The mil 

 lions of cells of which the body is composed must be given material 

 which will form more living matter or material which can be oxi 

 dized to release energy when muscle cells move, or gland cells 

 secrete, or brain cells think. 



Nutrients. Certain nutrient materials form the basis of food 

 of both plants and animals. These have been stated to be proteins 

 (such as lean meat, eggs, the gluten of bread), 

 carbohydrates (starches, sugars, gums, etc.), fats 

 and oils (both animal and vegetable), mineral 

 matter and water. 



Proteins. Protein substances contain the 

 element nitrogen. Hence such foods are called 

 nitrogenous foods. Man must form the proto 

 plasm of his body (that is, the muscles, tendons, 

 nervous system, blood corpuscles, the living parts 

 of the bone and the skin, etc.) in part at least 

 from nitrogenous food. Some of this he ob- The""composition of 

 tains by eating the flesh of animals, and some milk- Why is it 



he obtains directly from plants (for example, 



peas and beans). Proteins are the only foods 



available for tissue building. They may be oxidized to release 



energy if occasion requires it. 



Fats and Oils. Fats and oils, both animal and vegetable, 

 are the materials from which the body derives part of its energy. 

 The chemical formula of a fat shows that, compared with other 

 food substances, there is very little oxygen present; hence the 

 greater capacity of this substance for uniting with oxygen. The 

 rapid burning of fat compared with the slower combustion of a 

 piece of meat or a piece of bread illustrates this. A pound of butter 

 releases over twice as much energy to the body as does a pound of 

 sugar or a pound of steak. Human fatty tissue is formed in part 

 from fat eaten, but carbohydrate or even protein food may be 

 changed and stored in the body as fat. 



Carbohydrates. We see that the carbohydrates, like the fats, 

 contain carbon, hydrogen, and oxygen. Carbohydrates are essen 

 tially energy-producing foods. They are, however, of use in build- 



HUNTER, CIV. BI. - 18 



