Foods are classified according to source, chemical composition, physical properties and function.
In classifying food as to its source we have three classifications: animal, plant and mineral. Meat, fowl, fish, shell-fish, eggs, milk and its products, animal fat and gelatin help to constitute the animal foods. Seeds, roots, leaves, cereals, vegetables, fruits, sugar and vegetable oils are plant foods. From the mineral there are: iron, potassium, phosphorus, iodin, sulphur. These are obtained from the animal and plant foods. Water is not classed as a food, but it enters into the diet as a very important constituent.
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.
Physical Properties
Under this heading foods are classified as: (a) solids, semi-solids and liquid foods; (b) fibrous, gelatinous, starchy, oleaginous, crystalline and albuminous foods. The indigestible residue is called roughage.
Function
Foods are classified as: Proteins, carbohydrates, fats, condiments, inorganic salts and vitamins.
Proteins are the tissue builders of the body. They are derived from meat, milk, eggs, peas and beans for example. Some foods are much richer in protein than others, and there is also a difference in the quality of the protein. Some protein is more valuable from the standpoint of food than others. Casein is very rich in protein which is of great food value. The protein found in meat is also of good quality, but it is not as valuable as that found in milk. Such cellular organs as the liver and pancreas furnish a good source of protein. There is a very small amount of protein of excellent quality found in leafy plants. There is a difference of opinion among authors as to the amount of protein needed per day by healthy individuals. Some advise a very small amount of protein while others recommend a diet containing as much as 125 grams, approximately four and one-half ounces, per day.
The carbohydrates furnish the body with the necessary material for forming adipose tissue and heat through oxidation. It is very essential that the body be provided with sufficient starches. This is evidenced by the fact that starch constituents are so universally found in food from whatever source taken. For example, carbohydrates or starchy foods are found in cereals, tubers, such as potatoes, and sugars of cane, beets and fruits, and glycogen in flesh.