Fecal Zonulin as a Non-Invasive Marker of Intestinal Permeability: Findings from a Prospective Cohort Study

Background and Objectives: Metabolic dysfunction-associated steatotic liver disease (MASLD) is now widely acknowledged as belonging to the broader category of metabolic disorders, being closely associated with obesity, insulin resistance, and chronic systemic inflammation. Recent evidence indicates that in MASLD, alterations in the gut–liver axis—particularly increased intestinal permeability may serve as a crucial mechanistic link between metabolic dysfunction and hepatic steatosis. Zonulin, a physiological modulator of intestinal tight junctions, has been suggested as an indicator of compromised barrier integrity; however, its specific role in MASLD remains to be fully elucidated. Materials and Methods: We conducted a prospective observational study including 52 adult patients diagnosed with MASLD. Hepatic steatosis was evaluated using the SteatoTest (FibroMax panel), while fecal zonulin levels were measured by ELISA at baseline. Clinical, anthropometric, and metabolic parameters were assessed. We used ROC curve analysis to explore zonulin’s predictive value for moderate-to-severe steatosis (≥S2). Results: Elevated fecal zonulin (>107 ng/mL) occurred in 26.9% of participants. In a binary logistic model with SteatoTest ≥ S2 as outcome, zonulin was independently associated with clinically significant steatosis (OR per 1 ng/mL = 1.017; 95% CI 1.002–1.032; p = 0.029). Discrimination for ≥S2 was AUC = 0.680 (95% CI 0.535–0.825; p = 0.015). The Youden-optimal cut-off was 57.0 ng/mL (sensitivity 68.2%, specificity 63.3%) versus 40.9%/83.3% at the manufacturer’s 107 ng/mL threshold. Conclusions: Fecal zonulin shows modest discriminatory ability for steatosis and is best used as an adjunct to non-invasive assessment; cohort-specific calibration (57.0 ng/mL) outperformed the generic 107 ng/mL threshold.