~Acid-forming and Base-forming Foods.~--Mendel[61] states: "There are foods which act as potential acids and others which function as bases in the organism. When burned up either in the laboratory or in the body cells, they have a preponderance of acid or base, as the case may be, in their ash." In this respect potatoes, apples, raisins, and cantaloupes, for example, are base-forming foods which depress the output of ammonia and increase the solubility of uric acid in the urine, whereas meal, cereals, and prunes (the latter with their content of benzoic acid) furnish acids in predominance. ~Purin Bases.~--These compounds are formed in the body as cleavage products of nucleoproteins or taken into the body in food. The chief of these products are ~adenin~, ~guanin~, ~hypoxanthin~, ~xanthin~, and ~uric acid~. The latter is the most highly oxidized of all the purin bases and is the form in which they are chiefly eliminated in the urine. ~Formation of Uric Acid.~--The formation of uric acid can in a measure be controlled by attention to the diet, eliminating those foods known to be purin-bearing. Normally from 1 to 3% of the nitrogen eliminated will be in the form of uric acid. The normal human being oxidizes about half of the purins eaten and excretes about half, mainly in the form of uric acid. According to Mendel, the formation of uric acid takes place throughout the body, and its partial destruction is accomplished by the kidneys, muscles, and liver. The formation of purins in the body and their elimination in the form of uric acid is especially significant in certain pathological conditions, gout, for example, in which the body has difficulty in eliminating these compounds. The purin bodies are both endogenous and exogenous--that is, they may be brought into the body in food as such, or they may be formed as a result of the metabolism of the body tissues. For this reason the damage wrought by these substances may to a certain extent be controlled by eliminating the purin-bearing foods from the diet. Flesh-foods are high in purins, especially the highly nucleated glandular organs, liver, thymus (sweetbreads), etc., kidney, beef, mutton, veal, pork, chicken, turkey, goose, sardines, anchovies, all kinds of fish except cod. Among the vegetable foods asparagus, beans, peas, and spinach are highest in purins. Boiling extracts much of the purins from food. Meat especially should be prepared by this method, if used in the diet of individuals suffering from gout. Eggs and milk are purin free, and may be used freely. Certain substances increase the difficulty of eliminating uric acid. Alcoholic beverages for example are especially deleterious. ~Creatinin.~--This end-product of protein metabolism is, like uric acid, endogenous and exogenous. It is one of the normal constituents of the urine. The quantity is fairly constant for the individual, averaging about 0.02 gram per kilogram of body weight per day.