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Shaffer (1), Myers and Fine (2), and Hahn and Meyer (3) adduced evidence that muscle creatine is the precursor of urinary creatinine. This was questioned by Chanutin and Kinard (4) but was conclusively proved by Bloch, Schoenheimer, and Rittenberg (5, 6) by isotope tracer evidence, which also showed that creatinine was the only normal urinary constituent containing any significant amount of body creatine nitrogen (1).
Borsook et al. (Thu,) studied this question.