Chemical Composition
From the standpoint of chemical composition foods are grouped into two classes, nitrogenous and non-nitrogenous. Both the animal and vegetable kingdom contribute foods to each of these classes, although the animal substances belong more to the nitrogenous, while the vegetable kingdom belongs more particularly to the non-nitrogenous foods. The nitrogenous foods consist chiefly of carbon, oxygen, hydrogen and nitrogen and are considered to be essentially tissue builders. Hygienists speak of the non-nitrogenous foods as being force producers, asserting that these foods supply energy for muscular action.