Background: Phosphatidylcholine (PC) is a major phospholipid that contributes to intestinal barrier protection and is essential for hepatic secretion of lipids and bile acids. Because inflammatory bowel disease (IBD) and primary sclerosing cholangitis (PSC) are closely linked, we hypothesized that individual serum PC species would reflect disease activity. We therefore investigated whether serum PC profiling could identify clinically useful biomarkers across the gut–liver axis. Methods: Serum concentrations of 21 PC species were quantified by direct flow injection high-resolution mass spectrometry in 16 healthy controls, 57 patients with IBD, and 20 patients with PSC. Results: In IBD, multiple serum PC species were inversely associated with inflammatory activity, showing negative correlations with
serum C-reactive protein and fecal calprotectin. Patients with fecal calprotectin concentrations above the diagnostic cut-off of 120 μg/g had lower levels of PC 34:3, 36:1, 36:2, 36:3, 36:4, 36:5, 38:3, 38:4, 38:5, 38:7, 40:5, and 40:6, as well as lower total PC. In contrast, in PSC, PC 30:0, 32:0, 32:1, and 34:1 were increased compared with IBD and correlated positively with gamma-glutamyltransferase and alkaline phosphatase. Furthermore, these shorter-chain PC species as well as PC 36:1 were markedly elevated in PSC with advanced liver fibrosis compared with PSC without fibrosis. Conclusions: Serum PC species show a reciprocal disease-associated pattern in IBD and PSC. In IBD, lower concentrations of predominantly unsaturated PC species are associated with active intestinal inflammation, whereas in PSC, higher concentrations of shorter-chain PC species are associated with cholestatic injury and advanced liver fibrosis. IBD and PSC exhibit opposing serum PC signatures, suggesting that dysregulated PC metabolism is a pathophysiological feature of
intestinal inflammation and PSC-associated liver fibrosis.