Thanks especially to the work of Folin,[52] these data are now apparently at hand, and the facts which he has accumulated with painstaking care seem destined to throw additional light upon our conception of proteid metabolism. It will be remembered that in the breaking down of proteid, the great bulk of its contained nitrogen is eliminated in the form of urea. In addition, a certain smaller amount of nitrogen is excreted in the forms of creatinin and uric acid. As we have seen, the total output of nitrogen, which measures the extent to which proteid is decomposed in the body, varies with the intake of proteid food; but it is found that the proportion of nitrogen excreted in the forms of urea and uric acid varies with the extent of the metabolism. In other words, quantitative changes in the daily proteid katabolism are accompanied by pronounced changes in the distribution of the excreted nitrogen. Let us take a single illustration from Folin’s results; the case of a healthy man who on one day--July 13--consumed a proteid-rich diet, and on the other day--July 20--was living on a diet containing only about 1 gram of nitrogen. The composition of the excretion through the kidneys on these two days is shown in the following table: [52] Otto Folin: Laws Governing the Chemical Composition of Urine. American Journal of Physiology, vol. 13, p. 66. A theory of Protein Metabolism. Ibid., vol. 13, p. 117. +--------------------+-----------------------+---------------------+ | | July 13. | July 20. | +--------------------+-----------------------+---------------------+ | Volume of urine | 1170 c.c. | 385 c.c. | | Total nitrogen | 16.80 grams | 3.60 grams | | Urea-nitrogen | 14.70 " = 87.5% | 2.20 " = 61.7% | | Uric acid-nitrogen | 0.18 " = 1.1% | 0.09 " = 2.5% | | Creatinin-nitrogen | 0.58 " = 3.6% | 0.60 " = 17.2% | +--------------------+-----------------------+---------------------+