IDO1 and irritable bowel syndrome: The findings in colon biopsies of IBD patients with IDO messenger RNA and KYN/TRP [78] were considerably elevated, indicating that IDO is activated and accelerates the metabolism of TRP to KYN, further reducing the serum TRP reserve [12], which may be due to the activation of IDO by increased proinflammatory cytokines in IBD, including IFN-γ, IL-1, and IL-6 [79].