The nutritive part of foods or "food principles" are the proteins, fats, carbohydrates, water and salt.

Proteins contain nitrogen, carbon, hydrogen and oxygen, and sometimes phosphorus and sulphur. They make up the greater part of the muscles and organs of the body and are needed in food for the growth and repair of body tissue. Proteins are also used by the body as "fuel" to keep the body warm and give muscle and other kinds of energy or power.

The proteins make up a considerable proportion of meat, eggs, ■cheese, skim milk, beans, peas and nuts, and are present in lesser proportion in all food products except the pure fats, sugars and starches.

Fats contain carbon, hydrogen and oxygen only. The fats and oils are concentrated fuel foods and yield to the body 2 1/4 times the heat and energy given by like weights of dry protein or starch or sugar.

Carbohydrates or starch, sugar and a few gums and substances like them contain carbon, hydrogen and oxygen only. The hydrogen and oxygen are in the same proportion as is found in water, hence the name carbo-hydrates. Cellulose or woody fibre is also a carbohydrate, but as it is not digestible it is not a food for man.

The carbohydrates, like the fats, are fuel foods and yield warmth and the power to do the work to the body when oxidized by the air brought in by the lungs.

Excess of food over the requirements of the body, if digested, is stored as fatty tissue for use, if need be, as fuel.

Water is needed for the fluids of the body, to carry away wastes and to regulate the temperature through perspiration.

Salts are needed in the formation of bones, digestive fluids and as a small but necessary part of every living cell in the body. they are present in nearly all foods, being most abundant in vegetables, fruits, milk and eggs.