By food elements the reader is not to understand, chemical elements. The chemical elements--nitrogen, carbon, iron, calcium, etc.--in foods were named in the preceding chapter. By food elements is meant the various distinct compounds that exist in foods and that are useful, after digestion, as nourishment for the body. The chemical elements of the body do not exist in the body in their "free" (uncombined) or pure state. They are always present in various complex combinations, both in the human body and in the simplest forms of food-stuffs. The animal body does not make use of the "free" elements, with the exception of oxygen, but employs only certain acceptable compounds prepared by the synthetic processes of the plant. The plant is the ultimate source of all animal food.

Foods are materials which supply the "elements" necessary for promoting growth of the body and repairing its waste, yield energy for muscular work, yield heat, regulate the body processes, and make reproduction possible. According to their chemical composition, foods are classified as:

Proteins.
Carbohydrates (starches and sugars).
Hydrocarbons (fats and oils).
Salts (organic).
Vitamins.

According to their functions, foods are divided into:

1. Fuel and energy foods--carbohydrates, fats, proteins.
2. Building foods--proteins, salts, water.
3. Regulators of body processes--minerals, vitamins, water.

According to their sources, foods are classified as:

1. Vegetable foods.
2. Animal foods.

Food substances as they come from the plant and animal contain: (1) Nutritive matter; (2) Water; (3) Refuse--or waste. The different articles of food are placed into different classes according to the food elements that predominate. Thus, there are protein foods, carbohydrate foods, hydrocarbon foods and foods rich in organic salts, vitamins and water. Let us briefly discuss each of these classes of food substances and notice the chief sources of each.