The chemical constitution of the human body is made up of a large variety of elements and compounds. From fifteen to twenty elements are found in it, chief among which are oxygen, hydrogen, carbon, nitrogen, calcium, phosphorus, sodium, and sulphur. The most important compounds are protein, hydrocarbons, carbohydrates, organic mineral matter, and water. The food which nourishes the body is composed of the same elements and compounds. Food serves two purposes,--it builds and repairs the body tissues, and it generates vital heat and energy, burning food as fuel. Protein and mineral matter serve the first purpose, and hydrocarbons (fats) and carbohydrates (sugars and starches) the second, although, if too much protein be assimilated it will be burnt as fuel, (but it is bad fuel as will be mentioned later), and if too much fat is consumed it will be stored away in the body as reserve supply. Most food contains some protein, fat, carbohydrates, mineral matter, and water, but the proportion varies very considerably in different foods. Water is the most abundant compound in the body, forming on an average, over sixty per cent. of the body by weight. It cannot be burnt, but is a component part of all the tissues and is therefore an exceedingly, important food. Mineral matter forms approximately five or six per cent. of the body by weight. Phosphate of lime (calcium phosphate), builds bone; and many compounds of potassium, sodium, magnesium and iron are present in the body and are necessary nutrients. Under the term protein are included the principal nitrogenous compounds which make bone, muscle and other material. It forms about 15 per cent. of the body by weight, and, as mentioned above, is burnt as fuel for generating heat and energy. Carbohydrates form but a small proportion of the body-tissue, less than one per cent. Starches, sugars, and the fibre of plants, or cellulose, are included under this term. They serve the same purpose as fat. All dietitians are agreed that protein is the essential combined in food. Deprivation of it quickly produces a starved physical condition. The actual quantity required cannot be determined with perfect accuracy, although estimates can be made approximately correct. The importance of the other nutrient compounds is but secondary. But the system must have all the nutrient compounds in correct proportions if it is to be maintained in perfect health. These proportions differ slightly according to the individual's physical constitution, temperament and occupation.