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Vitamin D for your patients with chronic hepatitis C?

Open AccessPublished:August 06, 2012DOI:https://doi.org/10.1016/j.jhep.2012.07.026

      Summary

      Vitamin D is increasingly becoming recognized as an important physiological regulator with pleiotropic functions outside of its classical role in skeletal homeostasis. A growing body of clinical evidence highlights the prevalence and risks of vitamin D deficiency in patients suffering from chronic hepatitis C infection, and vitamin D supplementation has been proposed as an adjunct to current standards of care. This review considers the experimental evidence for the anti-inflammatory, antifibrotic and antiviral effects of vitamin D, and discusses the therapeutic potential of vitamin D supplementation to protect against liver disease progression and improve responses to treatment.

      Abbreviations:

      HCV (hepatitis C virus), SVR (sustained virological response), 25(OH)D (25-hydroxyvitamin D), 1,25(OH)2D (1,25-dihydroxyvitamin D), VDR (vitamin D receptor), RXR (retinoid X receptor), VDREs (vitamin D response elements), PTH (parathyroid hormone), TLR (toll-like receptor), TNFR (tumor necrosis factor receptor)

      Keywords

      Introduction

      Advances in hepatitis C virus (HCV) pharmaceutical development are being made at a blistering pace; however, highly effective, non-toxic therapies remain a hope for the future. This leaves an immediate need for interventions that can minimize disease progression and/or improve sustained virological response (SVR) rates in the short term. The aging of the HCV-positive population is creating an epidemic of end stage liver disease. Many patients cannot wait for second and third generation direct acting antiviral drugs to reach the clinic. As an interim measure, vitamin D supplements have been proposed as an adjunct to pegylated-interferon and ribavirin. This review integrates the known biological effects of the vitamin D system with recent clinical findings and discusses the therapeutic potential of vitamin D supplementation in HCV-positive patients.

      Vitamin D metabolism

      Unlike most vitamins, vitamin D is neither an enzyme co-factor nor an essential nutrient that must be obtained from food. Rather, it is a precursor of a seco-steroid hormone. Vitamin D can be manufactured endogenously from 7-dehydrocholesterol when skin is exposed to ultraviolet B radiation (Fig. 1). Historically, sun exposure was the main source of vitamin D, but food and supplements are now important sources, especially among urban populations and people who work indoors. During its conversion from a precursor to an active hormone, vitamin D is first modified in the liver by microsomal vitamin D 25-hydroxylases, which form 25-hydroxyvitamin D [25(OH)D], a stable metabolite that is the best single indicator of vitamin D status [
      • Holick M.F.
      Vitamin D: physiology, molecular biology, and clinical applications.
      ]. A second hydroxylation step, mediated by the mitochondrial cytochrome P450 oxidase, CYP27B1, produces the most biologically active metabolite, 1,25-dihydroxyvitamin D [1,25(OH)2D]. In individuals with adequate renal function, most of the circulating 1,25(OH)2D is produced by the kidney; however, CYP27B1 activity occurs in many extra-renal tissues, including innate immune cells, such as macrophages and dendritic cells. The local metabolism of 25(OH)D by these cells is likely to be an important factor in generating the high local concentrations of 1,25(OH)2D needed for its paracrine and autocrine activities.
      Figure thumbnail gr1
      Fig. 1Vitamin D is obtained from dietary sources and through the photochemical conversion of 7-dehydrocholesterol in the skin. It binds to vitamin D-binding protein and is transported to the liver, where it is hydroxylated by 25-hydroxylases to form 25(OH)D, a stable metabolite that is the best single indicator of vitamin D status. A second hydroxylation step, mediated by the 1a-hydroxylase CYP27B1 in the kidneys and other extrarenal tissues, produces the most active metabolite 1,25(OH)2D, which signals primarily through the VDR, resulting in pleiotropic physiological effects, as highlighted in the ,25(OH)2D is catabolized by CPY24A1 to its inactive metabolite, calcitroic acid.
      1,25(OH)2D mediates most of its biological effects by binding to the vitamin D receptor (VDR), which is expressed at some level in almost all human tissues. In the absence of its ligand, the VDR largely exists as an inactive homodimer. Upon binding 1,25(OH)2D, the VDR is phosphorylated and forms a heterodimer with its preferred binding partner, the retinoid X receptor (RXR), forming a nuclear transcription factor. This VDR/RXR heterodimer binds vitamin D response elements (VDREs) in DNA and recruits co-regulatory protein complexes to modulate the expression of hundreds of genes. In addition to acting as a ligand-activated transcription factor, the VDR is also thought to activate cell signaling pathways independent of its genomic effects [
      • Messa P.
      • Alfieri C.
      • Rastaldi M.P.
      Recent insights into vitamin D and its receptor.
      ].
      The multiple steps in vitamin D bioactivation are controlled by intricate regulatory pathways [
      • Holick M.F.
      Vitamin D: physiology, molecular biology, and clinical applications.
      ]. CYP27B1 expression in the renal proximal tubule is stimulated by the parathyroid hormone (PTH), which is regulated by free serum calcium levels. 1,25(OH)2D itself can directly and indirectly inhibit CYP27B1 expression, thereby providing a tight negative feedback loop. CYP27B1 expression in keratinocytes is stimulated by both PTH and inflammatory cytokines such as TNFα and IFNγ. 1,25(OH)2D negatively regulates its own activity in these cells by inducing the expression of the 1,25(OH)2D catabolic enzyme, CYP24A1. The functional expression of CYP27B1 and intracellular synthesis of 1,25(OH)2D in macrophages are induced by both inflammatory cytokines, such as IFNγ, and toll-like receptor (TLR) ligands, such as lipopolysaccharide. Because vitamin D metabolism is controlled by multiple factors, the amount of vitamin D consumed in the diet is only one of many variables that determine the local activity of the vitamin D system. The levels of vitamin D binding protein and VDR are additional variables that strongly influence the magnitude of the biological effects of vitamin D.
      VDR activation by 1,25(OH)2D has long been known to increase intestinal calcium and phosphate absorption, fostering healthy bones. A growing number of recent studies reveal pleiotropic roles of 1,25(OH)2D beyond bone and calcium metabolism, including the induction of antimicrobial genes and the reduction of inflammation and fibrogenesis [
      • Holick M.F.
      Vitamin D: physiology, molecular biology, and clinical applications.
      ]. Given the prevalence of bone disease, inflammation, and fibrosis in HCV-positive patients, both classical and newly-discovered effects of vitamin D may be relevant to disease management.

      Vitamin D deficiency in patients with liver disease

      Knowledge of the widespread physiological importance of vitamin D raises concern about the risks of vitamin D deficiency. Focusing on bone, several groups have attempted to identify markers that can be used to determine whether a patient’s vitamin D status is optimal. A panel assembled by the Institute of Medicine (IOM) determined that a serum 25(OH)D concentration of 20 ng/ml is sufficient for the majority of healthy adults, but also determined that a concentration of 16 ng/ml is insufficient for about 50% of healthy adults, suggesting that there is very little margin of error at a 25(OH)D level of 20 ng/ml [
      ]. A panel from the Endocrine Society concluded that 32 ng/ml should be used as the threshold of 25(OH)D sufficiency in patients with various disease conditions [
      • Holick M.F.
      • Binkley N.C.
      • Bischoff-Ferrari H.A.
      • Gordon C.M.
      • Hanley D.A.
      • Heaney R.P.
      • et al.
      Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline.
      ]. It is not clear whether the optimal level of 25(OH)D is the same for adults of all racial groups, which may be an important issue when considering these guidelines.
      While serum levels of 25(OH)D are the best single surrogate marker of vitamin D status, PTH levels provide a second useful marker, and skeletal homeostasis is influenced by the combined effect of these two hormones. There is high inter-individual variability in the relationship between 25(OH)D and PTH levels. With serial testing, a patient’s vitamin D status can be optimized by increasing vitamin D supplements until a 25(OH)D level is reached that maximally suppresses PTH. This approach is time consuming and costly, and the financial burden of widespread vitamin D testing is becoming recognized as an increasingly significant issue [
      • Sattar N.
      • Welsh P.
      • Panarelli M.
      • Forouhi N.G.
      Increasing requests for vitamin D measurement: costly, confusing, and without credibility.
      ]. Some have advocated that screening efforts should be focused on specific high risk individuals; however, a recent study has shown that 25% of a group of young physicians had 25(OH) D levels below 20 ng/ml, despite their lack of established risk factors [
      • Growdon A.S.
      • Camargo Jr, C.A.
      • Clark S.
      • Hannon M.
      • Mansbach J.M.
      Serum 25-hydroxyvitamin D levels among Boston trainee doctors in winter.
      ]. Whatever the cost-effectiveness of vitamin D testing in the general public, the benefits to HCV-positive patients are likely to be greater because of their susceptibility to bone disease and increased risk of bone fracture [
      • Gutierrez J.A.
      • Parikh N.
      • Branch A.D.
      Classical and emerging roles of vitamin D in hepatitis C virus infection.
      ,
      • Carey E.J.
      • Balan V.
      • Kremers W.K.
      • Hay J.E.
      Osteopenia and osteoporosis in patients with end-stage liver disease caused by hepatitis C and alcoholic liver disease: not just a cholestatic problem.
      ,
      • Collin F.
      • Duval X.
      • Le Moing V.
      • Piroth L.
      • Al Kaied F.
      • Massip P.
      • et al.
      Ten-year incidence and risk factors of bone fractures in a cohort of treated HIV1-infected adults.
      ,
      • Arase Y.
      • Suzuki F.
      • Suzuki Y.
      • Akuta N.
      • Kobayashi M.
      • Sezaki H.
      • et al.
      Virus clearance reduces bone fracture in postmenopausal women with osteoporosis and chronic liver disease caused by hepatitis C virus.
      ,
      • Nanda K.S.
      • Ryan E.J.
      • Murray B.F.
      • Brady J.J.
      • McKenna M.J.
      • Nolan N.
      • et al.
      Effect of chronic hepatitis C virus infection on bone disease in postmenopausal women.
      ].
      It is important to keep in mind that the estimates of the 25(OH)D levels needed for optimal health are based exclusively on skeletal outcomes. The levels required for the non-classical functions of vitamin D have yet to be established and are under dispute [
      ,
      • Holick M.F.
      • Binkley N.C.
      • Bischoff-Ferrari H.A.
      • Gordon C.M.
      • Hanley D.A.
      • Heaney R.P.
      • et al.
      Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline.
      ,
      • Rosen C.J.
      • Abrams S.A.
      • Aloia J.F.
      • Brannon P.M.
      • Clinton S.K.
      • Durazo-Arvizu R.A.
      • et al.
      IOM committee members respond to Endocrine Society vitamin D guideline.
      ]. The first solid information about the level for a non-skeletal outcome is likely to come from the infectious disease field where intervention trials can be completed in a relatively short period of time. Intriguingly, active tuberculosis increases seasonally as 25(OH)D levels fall [
      • Martineau A.R.
      • Nhamoyebonde S.
      • Oni T.
      • Rangaka M.X.
      • Marais S.
      • Bangani N.
      • et al.
      Reciprocal seasonal variation in vitamin D status and tuberculosis notifications in Cape Town.
      ], suggesting that optimal 25(OH)D levels could be determined for this disease by conducting randomized trials of vitamin D supplements. Meanwhile, a growing body of evidence indicates that higher levels of vitamin D and calcium may protect against colon cancer, and possibly other cancers, including hepatocellular carcinoma; however, definitive trials have not been carried out [
      ,
      • Gutierrez J.A.
      • Parikh N.
      • Branch A.D.
      Classical and emerging roles of vitamin D in hepatitis C virus infection.
      ].
      A large number of studies have examined the relationship between the vitamin D status of patients with chronic hepatitis C and disease outcome [
      • Gutierrez J.A.
      • Parikh N.
      • Branch A.D.
      Classical and emerging roles of vitamin D in hepatitis C virus infection.
      ,
      • Cholongitas E.
      • Theocharidou E.
      • Goulis J.
      • Tsochatzis E.
      • Akriviadis E.
      • Burroughs A.K.
      Review article: the extra-skeletal effects of vitamin D in chronic hepatitis C infection.
      ,
      • Kitson M.T.
      • Roberts S.K.
      D-livering the message: the importance of vitamin D status in chronic liver disease.
      ]. The majority found that HCV-positive patients have depressed 25(OH)D levels. A large study conducted in Sicily found that mean serum 25(OH)D levels were significantly lower than in healthy, age, and sex-matched controls (25.1 ± 9.9 ng/ml vs. 43.1 ± 10.2 ng/ml; p<0.0001), and that 73% of the HCV-positive patients had 25(OH)D levels below 30 ng/ml, in contrast to only 6% of the control subjects [
      • Petta S.
      • Cammà C.
      • Scazzone C.
      • Tripodo C.
      • Di Marco V.
      • Bono A.
      • et al.
      Low vitamin D serum level is related to severe fibrosis and low responsiveness to interferon-based therapy in genotype 1 chronic hepatitis C.
      ]. It is possible that HCV depresses 25(OH)D levels by altering lipid metabolism; a recent study has shown that HCV reduces production of 7-dehydrocholesterol, the precursor of endogenously-produced vitamin D [
      • Clark P.J.
      • Thompson A.J.
      • Vock D.M.
      • Kratz L.E.
      • Tolun A.A.
      • Muir A.J.
      • et al.
      Hepatitis C virus selectively perturbs the distal cholesterol synthesis pathway in a genotype-specific manner.
      ]. Most studies indicate that the severity of vitamin D deficiency correlates with the severity of liver disease. In addition to low 25(OH)D levels, a recent study of VDR genetic polymorphisms has found that the bAt (CCA) haplotype is significantly associated with fibrosis progression rate as compared to the bat (CAA) and BaT (TAG) haplotypes [
      • Baur K.
      • Mertens J.C.
      • Schmitt J.
      • Iwata R.
      • Stieger B.
      • Eloranta J.J.
      • et al.
      Combined effect of 25-OH vitamin D plasma levels and genetic vitamin D receptor (NR 1I1) variants on fibrosis progression rate in HCV patients.
      ]. While observational studies cannot establish a causal link between low 25(OH)D levels and liver disease progression, there are many ways in which vitamin D deficiency might plausibly play a role in pathogenesis.

      Vitamin D as a down-modulator of inflammation

      Chronic HCV infection is associated with intrahepatic inflammation and enhanced circulating levels of multiple inflammatory cytokines including members of the TNF superfamily, a complex group of ligands and receptors that are involved in cell survival, death and differentiation. Circulating TNFα and soluble TNF receptor (TNFR) levels are elevated in patients with chronic HCV relative to controls, and serum TNFR levels correlate with the grade of liver inflammation [
      • Zylberberg H.
      • Rimaniol A.C.
      • Pol S.
      • Masson A.
      • De Groote D.
      • Berthelot P.
      • et al.
      Soluble tumor necrosis factor receptors in chronic hepatitis C: a correlation with histological fibrosis and activity.
      ,
      • Itoh Y.
      • Okanoue T.
      • Ohnishi N.
      • Sakamoto M.
      • Nishioji K.
      • Nakagawa Y.
      • et al.
      Serum levels of soluble tumor necrosis factor receptors and effects of interferon therapy in patients with chronic hepatitis C virus infection.
      ]. TNFα is strongly linked to insulin resistance, and the elevated levels of this cytokine in HCV-positive patients are thought to promote the development of type 2 diabetes, which is associated with adverse outcomes [
      • Knobler H.
      • Schattner A.
      TNF-{alpha}, chronic hepatitis C and diabetes: a novel triad.
      ].
      Hepatic TNFα expression is highly correlated with hepatic toll-like receptor (TLR)2 and TLR4 expression in HCV-infected patients [
      • Berzsenyi M.D.
      • Roberts S.K.
      • Preiss S.
      • Woollard D.J.
      • Beard M.R.
      • Skinner N.A.
      • et al.
      Hepatic TLR2 & TLR4 expression correlates with hepatic inflammation and TNF-α in HCV & HCV/HIV infection.
      ], and a similar correlation exists between serum levels of TNFα and TLR2 expression on circulating monocytes [
      • Riordan S.
      • Skinner N.
      • Kurtovic J.
      • Locarnini S.
      • McIver C.
      • Williams R.
      • et al.
      Toll-like receptor expression in chronic hepatitis C: correlation with pro-inflammatory cytokine levels and liver injury.
      ]. Some data suggest that a proinflammatory cytokine milieu promotes HCV replication; siRNA-based silencing of multiple members of the proinflammatory NFκB signaling pathway inhibits HCV RNA replication [
      • Ng T.I.
      • Mo H.
      • Pilot-Matias T.
      • He Y.
      • Koev G.
      • Krishnan P.
      • et al.
      Identification of host genes involved in hepatitis C virus replication by small interfering RNA technology.
      ]. Further evidence of the importance of TNFα in HCV infection is provided by a double-blinded randomized, placebo controlled trial of 50 patients with chronic HCV, in which treatment with etanercept, a TNFα antagonist, improved the virologic response to IFN and ribavirin therapy [
      • Zein N.N.
      Etanercept as an adjuvant to interferon and ribavirin in treatment-naive patients with chronic hepatitis C virus infection: a phase 2 randomized, double-blind, placebo-controlled study.
      ].
      Another proinflammatory factor that is strongly associated with HCV infection is the chemokine CXCL10 (a.k.a., interferon gamma-inducible protein 10), which exhibits strong chemotactic effects on effector T cells. CXCL10 levels are elevated in HCV-positive patients compared with healthy controls and correlate with viral load, ALT elevations, and the extent of hepatic inflammation [
      • Larrubia J.R.
      • Benito-Martínez S.
      • Calvino M.
      • Sanz-de-Villalobos E.
      • Parra-Cid T.
      Role of chemokines and their receptors in viral persistence and liver damage during chronic hepatitis C virus infection.
      ]. There is a highly statistically significant association between serum CXCL10 levels and the likelihood of treatment failure; however, it is not known if the relationship is causal. A truncated antagonistic form of CXCL10 was recently discovered [
      • Casrouge A.
      • Decalf J.
      • Ahloulay M.
      • Lababidi C.
      • Mansour H.
      • Vallet-Pichard A.
      • et al.
      Evidence for an antagonist form of the chemokine CXCL10 in patients chronically infected with HCV.
      ], and more information about this new form is needed to understand the relationship between CXCL10 and treatment outcome.
      If proinflammatory cytokines and chemokines promote HCV persistence and disease progression, vitamin D’s anti-inflammatory effects may be beneficial. These effects are illustrated by studies showing that treatment of human monocytes, macrophages and myeloid dendritic cells with 1,25(OH)2D downregulates TLR expression, reduces the production of multiple inflammatory factors, including TNFα and CXCL10, and promotes a more tolerogenic phenotype [
      • Almerighi C.
      • Sinistro A.
      • Cavazza A.
      • Ciaprini C.
      • Rocchi G.
      • Bergamini A.
      1Alpha, 25-dihydroxyvitamin D3 inhibits CD40L-induced pro-inflammatory and immunomodulatory activity in human monocytes.
      ,
      • Kuo Y.
      • Kuo C.
      • Lam K.
      • Chu Y.
      • Wang W.
      • Huang C.
      • et al.
      Effects of vitamin D3 on expression of tumor necrosis factor-α and chemokines by monocytes.
      ,
      • Sadeghi K.
      • Wessner B.
      • Laggner U.
      • Ploder M.
      • Tamandl D.
      • Friedl J.
      • et al.
      Vitamin D3 down-regulates monocyte TLR expression and triggers hyporesponsiveness to pathogen-associated molecular patterns.
      ,
      • Penna G.
      • Amuchastegui S.
      • Giarratana N.
      • Daniel K.C.
      • Vulcano M.
      • Sozzani S.
      • et al.
      1,25-Dihydroxyvitamin D3 selectively modulates tolerogenic properties in myeloid but not plasmacytoid dendritic Cells.
      ]. Consistent with these results, circulating 25(OH)D levels inversely correlate with monocyte TLR expression and with seasonal differences in TLR-induced TNFα production by peripheral blood mononuclear cells [
      • Do J.E.
      • Kwon S.Y.
      • Park S.
      • Lee E.-S.
      Effects of vitamin D on expression of Toll-like receptors of monocytes from patients with Behçet’s disease.
      ,
      • Khoo A.
      • Chai L.Y.A.
      • Koenen H.J.P.M.
      • Sweep F.C.G.J.
      • Joosten I.
      • Netea M.G.
      • et al.
      Regulation of cytokine responses by seasonality of vitamin D status in healthy individuals.
      ]. Due in part to these tolerogenic effects in antigen presenting cells, vitamin D also inhibits inflammatory T cell responses, reducing the production of IFNγ and IL-17 in favor of IL-4 and IL-10 [
      • Jeffery L.
      • Raza K.
      • Filer A.
      • Sansom D.
      25-hydroxyvitamin D3 conversion by dendritic cells and T cells drives 1,25-dihydroxyvitamin D3 mediated anti-inflammatory CD4+ T cell responses.
      ]. These findings suggest that vitamin D deficiency is likely to exacerbate chronic inflammation in HCV-positive patients.

      Vitamin D as a downregulator of fibrogenesis

      Progressive liver fibrosis is a dreaded consequence of HCV infection. Advanced fibrosis/cirrhosis is associated with reduced virologic response rates [
      • Strader D.B.
      • Wright T.
      • Thomas D.L.
      • Seeff L.B.
      Diagnosis, management, and treatment of hepatitis C.
      ]. Several association studies demonstrate that vitamin D levels inversely correlate with the liver fibrosis stage [
      • Gutierrez J.A.
      • Parikh N.
      • Branch A.D.
      Classical and emerging roles of vitamin D in hepatitis C virus infection.
      ,
      • Cholongitas E.
      • Theocharidou E.
      • Goulis J.
      • Tsochatzis E.
      • Akriviadis E.
      • Burroughs A.K.
      Review article: the extra-skeletal effects of vitamin D in chronic hepatitis C infection.
      ]. This could conceivably be related to the anti-inflammatory effects of vitamin D, given the contribution of inflammatory cell recruitment to hepatic fibrosis progression. In vitro studies establish that 1,25(OH)2D treatment also directly inhibits the proliferation and pro-fibrotic phenotype of hepatic stellate cells and reduces thioacetamide-induced liver fibrosis in rats [
      • Abramovitch S.
      • Dahan-Bachar L.
      • Sharvit E.
      • Weisman Y.
      • Ben Tov A.
      • Brazowski E.
      • et al.
      Vitamin D inhibits proliferation and profibrotic marker expression in hepatic stellate cells and decreases thioacetamide-induced liver fibrosis in rats.
      ]. Vitamin D analogues have been found to similarly reduce renal fibrosis through multiple mechanisms in animal models of obstructive nephropathy [
      • Tan X.
      • Li Y.
      • Liu Y.
      Paricalcitol attenuates renal interstitial fibrosis in obstructive nephropathy.
      ]. These findings suggest that vitamin D deficiency may contribute to liver fibrosis and that vitamin D supplementation may have anti-fibrotic effects in HCV-positive patients.

      Vitamin D and the response to antiviral treatment

      Figure thumbnail fx1

      Conclusions

      Vitamin D is increasingly becoming recognized as an important physiological regulator with pleiotropic effects. A growing body of experimental and clinical evidence suggests that vitamin D deficiency is a risk factor in HCV-infected patients and that vitamin D supplementation might protect against liver disease progression and improve responses to treatment. There is still a lack of consensus on optimal 25(OH)D target levels and dosing strategies. The existing evidence highlights the need for additional well-designed clinical trials to evaluate the effects of vitamin D supplementation. The outcomes should include effects on the fibrosis progression rate (in patients with ongoing HCV replication and in patients who achieve a SVR), the incidence of hepatocellular carcinoma, and the incidence of bone fractures. In light of data showing protective effects of vitamin D supplementation in preventing influenza virus infection [
      • Urashima M.
      • Segawa T.
      • Okazaki M.
      • Kurihara M.
      • Wada Y.
      • Ida H.
      Randomized trial of vitamin D supplementation to prevent seasonal influenza A in schoolchildren.
      ], studies of vitamin D supplementation in HCV patients should also examine vaccine responses and susceptibility to infectious diseases. Given that the relationships between vitamin D and chronic inflammation and progressive hepatic fibrosis are not unique to HCV infection, and that vitamin D deficiency may also be a factor in other liver diseases [
      • Gutierrez J.A.
      • Parikh N.
      • Branch A.D.
      Classical and emerging roles of vitamin D in hepatitis C virus infection.
      ], clinical trials to study the effects of vitamin D supplementation in HCV patients are likely to be broadly relevant to the field of hepatology. Although dose-response data are limited, many liver disease patients will likely require relatively high doses of nutritional vitamin D to achieve 25(OH)D levels above 20 ng/ml. Until clinical data are available, 4000 IU/day is a reasonable daily dose for patients with baseline 25(OH)D levels below 10 ng/ml and 2000 IU/day is an appropriate starting dose for patients with levels between 10 and 20 ng/ml.

      Conflict of interest

      The authors do not have anything to disclose regarding funding or conflict of interest with respect to this manuscript.

      References

        • Holick M.F.
        Vitamin D: physiology, molecular biology, and clinical applications.
        Springer, 2010
        • Messa P.
        • Alfieri C.
        • Rastaldi M.P.
        Recent insights into vitamin D and its receptor.
        J Nephrol. 2011;
      1. Ross A.C.A.C. Taylor C.L.C.L. Yaktine A.L.A.L. Del Valle H.B.H.B. Dietary reference intakes for calcium and vitamin D [Internet]. National Academies Press (US), Washington (DC)2011
        • Holick M.F.
        • Binkley N.C.
        • Bischoff-Ferrari H.A.
        • Gordon C.M.
        • Hanley D.A.
        • Heaney R.P.
        • et al.
        Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline.
        J Clin Endocrinol Metab. 2011; 96: 1911-1930
        • Sattar N.
        • Welsh P.
        • Panarelli M.
        • Forouhi N.G.
        Increasing requests for vitamin D measurement: costly, confusing, and without credibility.
        Lancet. 2012; 379: 95-96
        • Growdon A.S.
        • Camargo Jr, C.A.
        • Clark S.
        • Hannon M.
        • Mansbach J.M.
        Serum 25-hydroxyvitamin D levels among Boston trainee doctors in winter.
        Nutrients. 2012; 4: 197-207
        • Gutierrez J.A.
        • Parikh N.
        • Branch A.D.
        Classical and emerging roles of vitamin D in hepatitis C virus infection.
        Semin Liver Dis. 2011; 31: 387-398
        • Carey E.J.
        • Balan V.
        • Kremers W.K.
        • Hay J.E.
        Osteopenia and osteoporosis in patients with end-stage liver disease caused by hepatitis C and alcoholic liver disease: not just a cholestatic problem.
        Liver Transpl. 2003; 9: 1166-1173
        • Collin F.
        • Duval X.
        • Le Moing V.
        • Piroth L.
        • Al Kaied F.
        • Massip P.
        • et al.
        Ten-year incidence and risk factors of bone fractures in a cohort of treated HIV1-infected adults.
        AIDS. 2009; 23: 1021-1024
        • Arase Y.
        • Suzuki F.
        • Suzuki Y.
        • Akuta N.
        • Kobayashi M.
        • Sezaki H.
        • et al.
        Virus clearance reduces bone fracture in postmenopausal women with osteoporosis and chronic liver disease caused by hepatitis C virus.
        J Med Virol. 2010; 82: 390-395
        • Nanda K.S.
        • Ryan E.J.
        • Murray B.F.
        • Brady J.J.
        • McKenna M.J.
        • Nolan N.
        • et al.
        Effect of chronic hepatitis C virus infection on bone disease in postmenopausal women.
        Clin Gastroenterol Hepatol. 2009; 7: 894-899
        • Rosen C.J.
        • Abrams S.A.
        • Aloia J.F.
        • Brannon P.M.
        • Clinton S.K.
        • Durazo-Arvizu R.A.
        • et al.
        IOM committee members respond to Endocrine Society vitamin D guideline.
        J Clin Endocrinol Metab. 2012; 97: 1146-1152
        • Martineau A.R.
        • Nhamoyebonde S.
        • Oni T.
        • Rangaka M.X.
        • Marais S.
        • Bangani N.
        • et al.
        Reciprocal seasonal variation in vitamin D status and tuberculosis notifications in Cape Town.
        S Afr Proc Natl Acad Sci USA. 2011; 108: 19013-19017
        • Cholongitas E.
        • Theocharidou E.
        • Goulis J.
        • Tsochatzis E.
        • Akriviadis E.
        • Burroughs A.K.
        Review article: the extra-skeletal effects of vitamin D in chronic hepatitis C infection.
        Aliment Pharmacol Ther. 2012; 35: 634-646
        • Kitson M.T.
        • Roberts S.K.
        D-livering the message: the importance of vitamin D status in chronic liver disease.
        J Hepatol. 2012; 57: 897-909
        • Petta S.
        • Cammà C.
        • Scazzone C.
        • Tripodo C.
        • Di Marco V.
        • Bono A.
        • et al.
        Low vitamin D serum level is related to severe fibrosis and low responsiveness to interferon-based therapy in genotype 1 chronic hepatitis C.
        Hepatology. 2010; 51: 1158-1167
        • Clark P.J.
        • Thompson A.J.
        • Vock D.M.
        • Kratz L.E.
        • Tolun A.A.
        • Muir A.J.
        • et al.
        Hepatitis C virus selectively perturbs the distal cholesterol synthesis pathway in a genotype-specific manner.
        Hepatology. 2012; 56: 49-56
        • Baur K.
        • Mertens J.C.
        • Schmitt J.
        • Iwata R.
        • Stieger B.
        • Eloranta J.J.
        • et al.
        Combined effect of 25-OH vitamin D plasma levels and genetic vitamin D receptor (NR 1I1) variants on fibrosis progression rate in HCV patients.
        Liver Int. 2012; 32: 635-643
        • Zylberberg H.
        • Rimaniol A.C.
        • Pol S.
        • Masson A.
        • De Groote D.
        • Berthelot P.
        • et al.
        Soluble tumor necrosis factor receptors in chronic hepatitis C: a correlation with histological fibrosis and activity.
        J Hepatol. 1999; 30: 185-191
        • Itoh Y.
        • Okanoue T.
        • Ohnishi N.
        • Sakamoto M.
        • Nishioji K.
        • Nakagawa Y.
        • et al.
        Serum levels of soluble tumor necrosis factor receptors and effects of interferon therapy in patients with chronic hepatitis C virus infection.
        Am J Gastroenterol. 1999; 94: 1332-1340
        • Knobler H.
        • Schattner A.
        TNF-{alpha}, chronic hepatitis C and diabetes: a novel triad.
        QJM. 2005; 98: 1-6
        • Berzsenyi M.D.
        • Roberts S.K.
        • Preiss S.
        • Woollard D.J.
        • Beard M.R.
        • Skinner N.A.
        • et al.
        Hepatic TLR2 & TLR4 expression correlates with hepatic inflammation and TNF-α in HCV & HCV/HIV infection.
        J Viral Hepat. 2011; 18: 852-860
        • Riordan S.
        • Skinner N.
        • Kurtovic J.
        • Locarnini S.
        • McIver C.
        • Williams R.
        • et al.
        Toll-like receptor expression in chronic hepatitis C: correlation with pro-inflammatory cytokine levels and liver injury.
        Inflamm Res. 2006; 55: 279-285
        • Ng T.I.
        • Mo H.
        • Pilot-Matias T.
        • He Y.
        • Koev G.
        • Krishnan P.
        • et al.
        Identification of host genes involved in hepatitis C virus replication by small interfering RNA technology.
        Hepatology. 2007; 45: 1413-1421
        • Zein N.N.
        Etanercept as an adjuvant to interferon and ribavirin in treatment-naive patients with chronic hepatitis C virus infection: a phase 2 randomized, double-blind, placebo-controlled study.
        J Hepatol. 2005; 42: 315-322
        • Larrubia J.R.
        • Benito-Martínez S.
        • Calvino M.
        • Sanz-de-Villalobos E.
        • Parra-Cid T.
        Role of chemokines and their receptors in viral persistence and liver damage during chronic hepatitis C virus infection.
        World J Gastroenterol. 2008; 14: 7149-7159
        • Casrouge A.
        • Decalf J.
        • Ahloulay M.
        • Lababidi C.
        • Mansour H.
        • Vallet-Pichard A.
        • et al.
        Evidence for an antagonist form of the chemokine CXCL10 in patients chronically infected with HCV.
        J Clin Invest. 2011; 121: 308-317
        • Almerighi C.
        • Sinistro A.
        • Cavazza A.
        • Ciaprini C.
        • Rocchi G.
        • Bergamini A.
        1Alpha, 25-dihydroxyvitamin D3 inhibits CD40L-induced pro-inflammatory and immunomodulatory activity in human monocytes.
        Cytokine. 2009; 45: 190-197
        • Kuo Y.
        • Kuo C.
        • Lam K.
        • Chu Y.
        • Wang W.
        • Huang C.
        • et al.
        Effects of vitamin D3 on expression of tumor necrosis factor-α and chemokines by monocytes.
        J Food Sci. 2010; 75: H200-H204
        • Sadeghi K.
        • Wessner B.
        • Laggner U.
        • Ploder M.
        • Tamandl D.
        • Friedl J.
        • et al.
        Vitamin D3 down-regulates monocyte TLR expression and triggers hyporesponsiveness to pathogen-associated molecular patterns.
        Eur J Immunol. 2006; 36: 361-370
        • Penna G.
        • Amuchastegui S.
        • Giarratana N.
        • Daniel K.C.
        • Vulcano M.
        • Sozzani S.
        • et al.
        1,25-Dihydroxyvitamin D3 selectively modulates tolerogenic properties in myeloid but not plasmacytoid dendritic Cells.
        J Immunol. 2007; 178: 145-153
        • Do J.E.
        • Kwon S.Y.
        • Park S.
        • Lee E.-S.
        Effects of vitamin D on expression of Toll-like receptors of monocytes from patients with Behçet’s disease.
        Rheumatology. 2008; 47: 840-848
        • Khoo A.
        • Chai L.Y.A.
        • Koenen H.J.P.M.
        • Sweep F.C.G.J.
        • Joosten I.
        • Netea M.G.
        • et al.
        Regulation of cytokine responses by seasonality of vitamin D status in healthy individuals.
        Clin Exp Immunol. 2011; 164: 72-79
        • Jeffery L.
        • Raza K.
        • Filer A.
        • Sansom D.
        25-hydroxyvitamin D3 conversion by dendritic cells and T cells drives 1,25-dihydroxyvitamin D3 mediated anti-inflammatory CD4+ T cell responses.
        Ann Rheum Dis. 2011; 70: A45
        • Strader D.B.
        • Wright T.
        • Thomas D.L.
        • Seeff L.B.
        Diagnosis, management, and treatment of hepatitis C.
        Hepatology. 2004; 39: 1147-1171
        • Abramovitch S.
        • Dahan-Bachar L.
        • Sharvit E.
        • Weisman Y.
        • Ben Tov A.
        • Brazowski E.
        • et al.
        Vitamin D inhibits proliferation and profibrotic marker expression in hepatic stellate cells and decreases thioacetamide-induced liver fibrosis in rats.
        Gut. 2011; 60: 1728-1737
        • Tan X.
        • Li Y.
        • Liu Y.
        Paricalcitol attenuates renal interstitial fibrosis in obstructive nephropathy.
        J Am Soc Nephrol. 2006; 17: 3382-3393
        • Liu P.T.
        • Stenger S.
        • Li H.
        • Wenzel L.
        • Tan B.H.
        • Krutzik S.R.
        • et al.
        Toll-like receptor triggering of a vitamin D-mediated human antimicrobial response.
        Science. 2006; 311: 1770-1773
        • Gal-Tanamy M.
        • Bachmetov L.
        • Ravid A.
        • Koren R.
        • Erman A.
        • Tur-Kaspa R.
        • et al.
        Vitamin D: an innate antiviral agent suppressing hepatitis C virus in human hepatocytes.
        Hepatology. 2011; 54: 1570-1579
        • Matsumura T.
        • Kato T.
        • Sugiyama N.
        • Tasaka-Fujita M.
        • Murayama A.
        • Masaki T.
        • et al.
        25-hydroxyvitamin D3 suppresses hepatitis C virus production.
        Hepatology. 2012; ([Epub ahead of print])
        • Baur K.
        • Mertens J.C.
        • Schmitt J.
        • Iwata R.
        • Stieger B.
        • Frei P.
        • et al.
        The vitamin D receptor gene bAt (CCA) haplotype impairs the response to pegylated-interferon/ribavirin-based therapy in chronic hepatitis C patients.
        Antivir Ther. 2012; 17: 541-547
        • Falleti E.
        • Bitetto D.
        • Fabris C.
        • Fattovich G.
        • Cussigh A.
        • Cmet S.
        • et al.
        Vitamin D binding protein gene polymorphisms and baseline vitamin D levels as predictors of antiviral response in chronic hepatitis C.
        Hepatology. 2012; ([Epub ahead of print])
        • Bitetto D.
        • Fabris C.
        • Fornasiere E.
        • Pipan C.
        • Fumolo E.
        • Cussigh A.
        • et al.
        Vitamin D supplementation improves response to antiviral treatment for recurrent hepatitis C.
        Transpl Int. 2011; 24: 43-50
        • Abu-Mouch S.
        • Fireman Z.
        • Jarchovsky J.
        • Zeina A.-R.
        • Assy N.
        Vitamin D supplementation improves sustained virologic response in chronic hepatitis C (genotype 1)-naïve patients.
        World J Gastroenterol. 2011; 17: 5184-5190
        • Nimer A.
        • Mouch A.
        Vitamin D improves viral response in hepatitis C genotype 2–3 naïve patients.
        World J Gastroenterol. 2012; 18: 800-805
        • Urashima M.
        • Segawa T.
        • Okazaki M.
        • Kurihara M.
        • Wada Y.
        • Ida H.
        Randomized trial of vitamin D supplementation to prevent seasonal influenza A in schoolchildren.
        Am J Clin Nutr. 2010; 91: 1255-1260