Folin1 as a result of his classic studies on protein metabolism in 1905 came to the conclusion that the daily creatinine output of a given person is more or less constant, is influenced by the body weight and is independent of a diet which does not contain creatine or creatinine. At this time he introduced the term "creatinine coefficient," which may be defined as the amount in milligrams of creatinine or creatinine nitrogen excreted daily per kilogram of body weight. Also, since the excretion of creatinine is independent of the diet, it is considered to represent the "endogenous metabolism" of the body, as compared to the excretion of urea, which represents the "exogenous metabolism." In Shaffer's2 view, the output of creatinine represents not necessarily the endogenous metabolism but only one special phase thereof, which takes place largely, if not wholly, in the muscles. This view was later adopted
BEARD HH, JACOB EJ. PHYSIOLOGIC SIGNIFICANCE OF THE CREATININE COEFFICIENT AND THE CREATINE TOLERANCE TEST: A CRITICAL REVIEW. Arch NeurPsych. 1939;42(1):67–82. doi:https://doi.org/10.1001/archneurpsyc.1939.02270190075004
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