FOOD 343 



where their composition and properties are tabulated, and grouped 

 as inorganic and organic matter. Here we shall take up their 

 functions in relation to the life and growth of animals, especially 

 as food for man. 



Functions of Inorganic Foods. Water constitutes about sixty 

 per cent of all animal tissue, usually more than that in plants. It 

 is a necessity to plants in starch making and in both plants and 

 animals as a transporter and solvent for other foods. Though not 

 oxidized in the body it is a very essential part of all foods. 



Mineral salts compose about five per cent of all animal tissue. 

 They are essential in formation of bone, teeth, blood, digestive 

 fluids, and are used to supply nitrogen, sulphur, phosphorus, 

 and iron for making protoplasm. Table salt, sulphate and phos- 

 phate of lime, and various nitrates are important examples. 



Functions of Organic Foods. Proteids are the only food stuffs 

 containing nitrogen, and are therefore absolutely essential in pro- 

 duction of living tissue. They include some of man's most valuable 

 foods, such as lean meat, white of eggs, cheese, gluten in wheat, 

 legumin in peas and beans, etc. Proteid matter constitutes about 

 eighteen per cent of the weight of man's body. The chief function 

 of proteid foods is to build tissue. They build anew and repair 

 muscle and tendon, bone, cartilage, and skin and also compose 

 the corpuscles of the blood. Proteids may also be oxidized directly 

 and thus may be used to furnish energy. While this actually 

 takes place to some extent, it would be an expensive source of fuel 

 and it would also put too great a strain upon the digestive and 

 excretory organs if all energy were sought from this class of foods. 



The fats and carbohydrates are the chief energy producers. The 

 former occur in fat meats, butter, fish, and eggs among animal 

 foods, and in olive and cotton seed oils, nuts, corn, and cocoa from 

 the vegetable world. The amount of fat needed varies with age, 

 occupation, and other conditions but if more is taken than is re- 

 quired, it may be stored, almost unchanged, to be drawn upon if 

 the energy supply becomes short. About fifteen per cent of the 

 human body is fat tissue and much of our energy is derived from 

 other amounts that are oxidized directly. 



