100.  Nature of the Waste Material.  An ordinarily healthy person passes daily, on an average, by the kidneys about 50 ounces of waste material, of which 96 per cent is water, and from the intestines, on an average, 5-1/2 ounces, a large proportion of which is water.  By the skin, in the shape of sweat and insensible perspiration, there is cast out about 23 ounces, of which 99 per cent is water; and by the lungs about 34 ounces, 10 of which are water and the remainder carbon dioxid.

Now if we omit an estimate of the undigestible remains of the food, we find that the main bulk of what daily leaves the body consists of water, carbon dioxid, and certain solid matters contained in solution in the renal secretion and the sweat.  The chief of these solid matters is urea, a complex product made up of four elements,—­carbon, hydrogen, oxygen, and nitrogen.  Water contains only two elements, hydrogen and oxygen; and carbon dioxid also has only two, carbon and oxygen.  Hence, what we daily cast out of our bodies consists essentially of these four elements in the form mainly of water, carbon dioxid, and urea.

These waste products represent the oxidation that has taken place in the tissues in producing the energy necessary for the bodily activities, just as the smoke, ashes, clinkers, and steam represent the consumption of fuel and water in the engine.  Plainly, therefore, if we could restore to the body a supply of these four elements equivalent to that cast out, we could make up for the waste.  The object of food, then, is to restore to the body an amount of the four elements equal to that consumed.  In other words, and briefly:  The purpose of food is to supply the waste of the tissues and to maintain the normal composition of the blood.

101.  Classification of Foods.  Foods may be conveniently divided into four great classes, to which the name food-stuffs or alimentary principles has been given.  They correspond to the chief “proximate principles” of which the body consists.  To one or the other of these classes all available foods belong[16].  The classification of food-stuffs usually given is as follows: 

    I. Proteids, or Nitrogenous Foods. 
   II.  Starches and Sugars, or Carbohydrates. 
  III.  Fats and Oils. 
   IV.  Inorganic or Mineral Foods,—­Water, Salt.

102.  Proteids; or Nitrogenous Foods.  The proteids, frequently spoken of as the nitrogenous foods, are rich in one or more of the following organic substances:  albumen, casein, fibrin, gelatine, myosin, gluten, and legumin.

The type of this class of foods is albumen, well known as the white of an egg.  The serum of the blood is very rich in albumen, as is lean meat.  The curd of milk consists mainly of casein.  Fibrin exists largely in blood and flesh foods.  Gelatine is obtained from the animal parts of bones and connective tissue by prolonged boiling.  One of the chief constituents of muscular fiber is myosin.  Gluten exists largely in the cereals wheat, barley, oats, and rye.  The proteid principle of peas and beans is legumin, a substance resembling casein.