In spite of these facts, it is safe to assert that the welfare of the human organism is best promoted by a mixed diet, including all three classes of food. The larger part of man’s food is used for the production of energy, and it is physiologically economic that this energy be supplied by the non-nitrogenous nutrients, particularly the carbohydrates, and to allocate to protein, so far as practicable, its special rôle of building material. _Nitrogenous Waste Products._—The end-product of proteins is urea, which is formed from protein decomposition products in relatively large amounts in the liver cells, and, being readily soluble and diffusible, is easily eliminated by the kidneys. Besides urea there are other smaller quantities of nitrogen compounds, the one most deserving of notice being uric acid. When the proteins are broken down to supply energy, there is always a definite proportion of urea and uric acid residue that must be eliminated through the kidneys. An excessive protein diet would burden these organs beyond their accustomed habit, and flooding the system with these nitrogenous wastes increases the tendency to rheumatism and gout. Uric acid is of general interest, and when present in the system in abnormally large amounts, as in gouty persons, has a pathologic significance. It is more complex in its composition, and, what is of particular importance, is that it does not readily dissolve. It forms very insoluble salts which have unpleasant tendencies to settle in the joints, causing great pain. Did its metabolism proceed properly, it should be resolved into urea and carbon dioxid. The quantity of uric acid that appears in the urine is the residue that has escaped this oxidation. Uric acid is built up from purin bodies in the food, so that it would be well for those with a gouty diathesis to abstain, at least when an attack of gout threatens, from all aliment in which purin is at all abundant. The most valuable of the purin-free foods are eggs and milk, and to these the uric-acid sufferer has to turn in times of trouble. _Carbohydrates Physiologically Economic._—This group of compounds comprises starches, sugars, and gums; the elements of which they are composed—namely, carbon, hydrogen, and oxygen—are so balanced that if all of the carbon were removed the residual hydrogen and oxygen would be in the proportions to form water. Carbohydrates are usually characterized as the fuel portions of the food, or, in other words, that part which is burned to produce the various forms of energy. They are also essential for the well-being of the organism; reduction of the intake below the essential point frequently leads to acetonuria.