Creatinine excretion
Creatinine is formed irreversibly and non-enzymically from creatine and phosphocreatinine and is usually assumed to be metabolically inert in humans (Wyss &amp; Rima Kaddurah-Daouk, 2000, Creatine and Creatinine Metabolism): no human enzyme is known to metabolize creatinine, and this compound is normally eliminated from the body via the kidneys


  In chronic liver failure, however, "a significant quantity of creatinine is excreted into the gut, as is true of urea and uric acid, and is metabolized by gut flora" (Jones &amp; Burnett, 1974, Creatinine Metabolism in Humans with Decreased Renal Function: Creatinine Deficit.


This was 'news to me', and the observation about urea is also surprising, but it looks to be a very good paper.