Vitamin D and its active forms can help fight hepatitis C by blocking virus production in different ways, and they also boost a natural antimicrobial peptide called LL‑37 that can directly break apart the virus. While this suggests vitamin D might improve standard interferon treatments, the findings are mostly from lab studies, so the real‑world impact is still uncertain.

Until the development of direct-acting antivirals (DAAs), interferon (IFN)-based therapy had been the primary treatment strategy for patients with chronic hepatitis C, even though this therapy has a therapeutic limitations and considerable side effects. Therefore, many efforts have been made to improve the efficacy of treatment. Several clinical studies have clearly shown that supplementation with vitamin D of IFN-based therapy improves treatment efficacy. To clarify the molecular mechanisms of the effect of vitamin D on IFN-based therapy, several researchers have performed basic research with cell culture models of hepatitis C virus (HCV). Consequently, two vitamin D₃ metabolites, 25-hydroxyvitamin D₃ (25-(OH)D₃) and 1α,25-dihydroxyvitamin D₃ (1α,25-(OH)₂D₃), have been suggested to have anti-HCV effects. 25-(OH)D₃ inhibits HCV production by suppressing infectious virus assembly through reducing apolipoprotein expression, while 1α,25-(OH)₂D₃ inhibits HCV production by modulating IFN signaling and/or inducing various host factors associated with the inhibition of viral genome replication. In addition, an antimicrobial peptide, LL-37, which is known to be partly regulated by vitamin D, was also reported to exhibit an anti-HCV effect by disrupting infectious viral particles directly. In conclusion, vitamin D₃ supplementation improves the response rate of IFN-based therapy via the direct and/or indirect anti-HCV effects of vitamin D₃ metabolites.