With great interest we read the study by Nikolaus et al1 concluding that increased tryptophan (TRP) metabolism is associated with disease activity of inflammatory bowel disease (IBD).1 We agree with the authors that there remains an unmet need for noninvasive, cost-effective, reproducible, and easy-to-perform tests for the diagnosis and risk assessment of IBD. Aromatic amino acids like TRP seem to be a candidates in this respect. We would, however, like to add some considerations regarding the underlying pathophysiologic mechanisms causing these differences in serum TRP levels.