The Boston cooking-school cook book by Fannie Merritt Farmer

FOOD Food is anything which nourishes the body. From fifteen to twenty elements enter into the composition of the body, of which the following thirteen are considered: oxygen, 62½%; carbon, 21½%; hydrogen, 10%; nitrogen, 3%; calcium, phosphorus, potassium, sulphur, chlorine, sodium, magnesium, iron, and fluorine the remaining 3%. Food is necessary for growth, repair, and energy; therefore the elements composing the body must be found in the food. The thirteen elements named are formed into chemical compounds by the vegetable and animal kingdoms to support the highest order of being, man. All food must undergo chemical change after being taken into the body, before it can be utilized by the body; this is the office of the digestive system. Food is classified as follows:—