FOODS AND THE THEORY OF DIGESTION 12 1 



between the different carbohydrates makes such a conver- 

 sion easily possible. 



Fats. The fats used as foods belong to one or the 

 other of two classes, known as solid fats and oils. The 

 solid fats are derived chiefly from animals, and the oils 

 are obtained mostly from plants. Butter, the fat of meats, 

 olive oil, and the oil of nuts are the fats of greatest im- 

 portance as foods. Fats, like the carbohydrates, are com- 

 posed of carbon, hydrogen, and oxygen. They are rather 

 complex chemical compounds, though not so complex as 

 proteids. Since neither fats nor carbohydrates contain 

 nitrogen, they are frequently classed together as non- 

 nitrogenous foods. 



Purpose Served by Carbohydrates, Fats, and Albumi- 

 noids. These classes of nutrients all serve the common 

 purpose of supplying energy. By uniting with oxygen at 

 the cells, they supply heat and the other forms of bodily 

 force. This is perhaps their only purpose. 1 Proteids 

 also serve this purpose, but they are not so well adapted 

 to supplying energy as are the carbohydrates and the 

 fats. In the first place they do not completely oxidize 

 and therefore do not supply so much energy ; and, in the 

 second place, they form waste products that are removed 

 with difficulty from the body. 



Mineral Salts and their Uses. Mineral salts are 

 found in small quantities in all of the more common food 

 materials, and, as a rule, find their way into the body 

 unnoticed. They supply the elements which are found in 

 the body in small quantities and serve a variety of pur- 



1 While awaiting oxidation at the cells, the carbohydrates and fats are stored 

 up by the body, the carbohydrates as glycogen and the fats as some form of fat. 

 In this sense they are sometimes looked upon as serving to build up certain of the 

 tissues. 



