The uric acid present in the urine is, as mentioned above, of twofold origin, partly derived from the metabolism of the tissues endogenous and part derived from the decomposition of the foods exogenous. The former quantity would seem to vary in amount in different individuals, but to be constant for the same individual and to be uninfluenced by diet. The exogenous moiety, on the other hand, can be profoundly influenced by diet. Formerly it was thought that the quantity of uric acid was directly dependent on the amount of the nitrogenous ingesta, in the same manner that urea is, and that a relationship existed between the quantity of urea and the quantity of uric acid excreted. It is now recognized that the uric acid of exogenous origin is derived from certain protein constituents of the food, but not from all, in other words, that it is a disintegration product of the purin bases such as xanthin, hypoxanthin, etc. These purin bases are not present in equal amount in all varieties of protein food. They are, however, especially abundant in certain glands, as, for instance, the thymus and the pancreas, and to a considerable but less degree in ordinary muscle or flesh. Thus a meat diet may greatly increase the excretion of uric acid, not simply because it is rich in protein, but because it contains considerable quantities of purin bases. On the other hand, a diet of milk or eggs, although it may contain an equal amount of protein matter will not yield a corresponding excretion of uric acid, as such a diet is poor in purin bases. Vegetable diet may also, although containing an equivalent quantity of protein matter, fail to yield any notable uric acid excretion because it is poor in purin bases. The uric acid excretion may thus be considerably cut down, by giving a diet in which purin bases are scanty, and yet such a diet may contain considerable quantities of protein.
In the treatment of the uric acid diathesis, whether giving rise to gout or to gravel and stone, it is a point of great practical importance that the kind of protein matter in the food should be regulated and that attention should not be concentrated merely on the amount. Milk, eggs, cheese, bread, rice and vegetables generally, and fruit, form the main ingredients of such diet, and the quantity of meat should be reduced to a minimum. In this way the uric acid excretion may be cut down to a very considerable extent. It must, however, be remembered that the deposition of uric acid in the urine does not depend solely on the percentage amount present, but also very largely on the chemical relationships determining the formation in which the uric acid is excreted, and that in order for it to remain in the urine in a state of solution the reaction of the urine must not be unduly acid, and further, salts must be present in abundance to provide the necessary bases to combine and form biurates. A diet rich in vegetables is peculiarly useful in the treatment of uric acid gravel, inasmuch as vegetables are rich in alkaline salts, especially potash salts. With a liberal diet of vegetables, the reaction of the urine can be rendered far less acid or even neutral, and the deposition of uric acid greatly hindered. In cases of gravel it is advisable for the patient to drink large quantities of water in order to flush out the kidneys as much as possible. At the same time this water should be taken at other times than with the meals, inasmuch as it is not advisable for a dilute urine to be passed at a time when the products of metabolism have to be excreted. The free ingestion of water is also desirable in the treatment of pyelitis, and pyelitis, especially in its slighter forms, may play a great part in determining the production of a stone.