The influence of the ingestion of methylated xanthines on the excretion of uric acid.

In 1916 Benedict 1 reported a single experiment in which the ingestion of caffeine lead to an increased output of uric acid as determined by the then new Benedict-Hitchcock colorimetric method. A year later Mendel and Wardell 2 studied this question with the same method, employing not only caffeine, but also coffee and tea. They observed an unmistakable increase in the uric acid output in all cases. The statements in the older literature are quite contradictory. Many texts still state that the ingestion of methylated xanthines is without effect on the excretion of uric acid. For this reason it seemed desirable to again study this problem.

In the present studies metabolism experiments have been carried out with the three common methylated xanthines, viz., caffeine, theobromine and theophylline, employing several methods for the estimation of uric acid. In all experiments the subjects were placed on purine-free constant diets until the uric acid excretion reached its endogenous level. The methylated purine to be studied was then added to the diet for a period of three or four days and finally the purine-free diet was continued for three days longer. Estimation of uric acid excretion were always made by the Benedict-Franke colorimetric method; in certain cases they were made also by the Benedict-Hitchcock colorimetric method and bly the Krüger-Schmidt precipitation method.

With two subjects the excretion of uric acid was markedly increased throughout the period of caffeine ingestion, but when the caffeine was discontinued, the uric acid fell back to its endogenous level within 48 hours. With two other subjects the excretion of uric acid was markedly increased on the first day of caffeine ingestion, then gradually decreased until on the last day of caffeine ingestion the uric acid was back to its endogenous level, where it remained practically constant throughout the final period.