De Boer et al have recently extended our observation of low systemic tryptophan (TRP) levels in inflammatory bowel disease (IBD)1 by showing increased levels of TRP and 4 other aromatic amino acids in the stools of 30 pediatric patients with IBD.2 They suggested a disturbed host–microbiota interaction as a mechanism. In their letter, they propose that colonic leakage and malabsorption rather than increased metabolism may explain the lowered systemic TRP levels in patients with IBD. In our study, we had also suggested that reduced mucosal absorption could be a contributor to reduced TRP levels in active IBD.