Beyond bones: Revisiting the role of vitamin D in chronic liver disease.

Beyond its traditional role in calcium and bone metabolism, vitamin D has emerged as a critical regulator of liver health. Its active form, calcitriol [1α,25(OH)2D], signals through the vitamin D receptor (VDR), which is expressed in hepatic stellate cells, Kupffer cells, and cholangiocytes. Through this pathway, vitamin D modulates fibrosis, inflammation, oxidative stress, bile acid homeostasis, and immune responses. This review explores the growing body of evidence linking vitamin D deficiency to chronic liver diseases, including autoimmune hepatitis, primary biliary cholangitis, alcoholic liver disease, viral hepatitis B and C, and metabolic-associated steatotic liver disease. Low vitamin D levels are frequently observed in these conditions and are associated with disease severity, complications (such as spontaneous bacterial peritonitis, sarcopenia, and hepatic encephalopathy), and increased mortality. Mechanistically, vitamin D-VDR signaling inhibits profibrotic TGF-β1/SMAD pathways, downregulates proinflammatory cytokines, enhances regulatory T cell differentiation, and improves insulin sensitivity. Although preclinical studies support its protective effects, clinical trials of vitamin D supplementation have produced mixed results. Overall, vitamin D appears to influence multiple pathways in liver disease pathophysiology, and correcting its deficiency may offer clinical benefits. However, its integration into clinical care will depend on identifying responsive patient subgroups and defining optimal dosing strategies to maximize therapeutic benefit.