All the varieties of food with which we are supplied will be found to contain some of these substances: protein, fat, carbohydrate, mineral matter, water; and to these we give the name foodstuffs. Some food materials (like the milk and beans just studied) contain all the foodstuffs, some only one, as in the case of sugar. We can now define food as something that contains one or more of the substances known as foodstuffs. But what are the foodstuffs themselves ?
Although we are not chemists, and may not even have taken a course in chemistry, yet through our nature study or physiology lessons, we are familiar with the fact that all the materials about us, including our own bodies and our food, are made up of simple substances that we call "elements." We know, for instance, that coal is chiefly carbon, and we are familiar with such sub-stances as sulphur, calcium, phosphorus, and iron. We -know that the air contains oxygen, which we inhale, and that we breathe out a combination of carbon and oxygen called "carbon dioxide." Since our bodies are composed of these and other elements, these elements must be supplied by our foods, and therefore, the foodstuffs in turn are composed of these same elements.
The term "foodstuff" is used in place of "food principle," as being the later and better term.
Proteins, fats, and carbohydrates all contain large amounts of carbon, and on this account are called fuel foods. But proteins are distinguished because they contain nitrogen in addition, which is found in no other foodstuff. Sulphur, too, we get only from protein, but we need less of it than of nitrogen, so we think about the nitrogen and let the sulphur take care of itself. The nitrogen that we draw in from the air with every breath, we breathe out again without being able to use it. This element is necessary to every living cell, but we can make it ours only through our protein food. Nitrogen is cheapest when obtained from the grains, from dried beans and peas. We pay a higher price for it in milk, eggs, fish, meat, and nuts. Carbon, which is found in all foodstuffs except water and some kinds of mineral matter, costs much less, especially when we take it in the form of carbohydrates such as starches and sugars. Oxygen is also abundant in our foods, but we get it even more cheaply in water and by breathing it in from the air. Phosphorus, iron, and calcium are very important elements, but we do not need them in very large quantities. We can get them cheaply in whole grains, peas and beans, some fruits and green vegetables, but they are worth paying for in milk and eggs. The elements last mentioned are present in the food partly as constituents of certain proteins and fats, partly as mineral salts. Other elements found as mineral matter are sodium and chlorine (which we take as common salt), potassium, magnesium, and traces of iodine and fluorine. These are all necessary to keep our bodies in good working order. We shall see later how to select our food materials so as to have all the different elements in the foodstuffs present in sufficient amounts.