Alloisolithocholic acid is a metabolite of the secondary bile acid metabolite 3-oxo lithocholic acid (dehydrolithocholic acid).¹ It is formed from 3-oxo lithocholic acid by gut microbiota.¹ Alloisolithocholic acid (20 µM) increases the association of the nuclear hormone receptor nerve growth factor IB (Nr4a1) to the gene encoding forkhead box protein P3 (Foxp3) in isolated mouse CD4+ T cells. It induces the differentiation of isolated mouse T cells into regulatory T cells (Tregs) and the production of mitochondrial reactive oxygen species (ROS) in isolated mouse CD4+ T cells when used at a concentration of 20 µM.² Alloisolithocholic acid (6 mg/kg) reduces hepatic bile flow and bile salt, cholesterol, and phospholipid secretion, as well as induces loss of microvilli and dilation of canaliculi in hepatocytes, markers of cholestasis, in rats.³ Fecal levels of alloisolithocholic acid are reduced in patients with Crohn’s disease or ulcerative colitis.¹