VDR is expressed in most tissues and cells of the human body, including liver, pancreas, and several immune cells including monocytes, macrophages, T lymphocytes, B lymphocytes, natural killer (NK) cells, and dendritic cells (DC), with expression most abundant on the epithelial cells of the gastrointestinal tract. As a transcription factor activated by 1α,25(OH)₂D, VDR directly or indirectly regulates the expression of more than 200 genes that influence cell proliferation, differentiation and apoptosis, as well as immunomodulation and angiogenesis 6. Studies in VDR null mice highlight the broad physiologic function of vitamin D 7.

Vitamin D deficiency

Vitamin D deficiency is broadly defined as a serum 25(OH)D level <50 nmol/L (<20 ng/ml). Levels between 75 and 125 nmol/L (30–50 ng/ml) are considered optimal as PTH levels rise when 25(OH)D is <75 nmol/L (30 ng/ml); hence, levels between 50 and 75 nmol/L (20–30 ng/ml) are increasingly recognized to represent vitamin D insufficiency 8, 9, 10 and 11. Using these definitions, it is estimated that more than 1 billion people worldwide are either vitamin D deficient or insufficient 12, with the elderly and those with chronic medical illness most at risk. However, even amongst healthy young people, vitamin D deficiency and insufficiency are still common 13, 14 and 15. A meta-analysis of 18 randomized-controlled trials involving 57,311 participants shows that subjects randomized to receive vitamin D supplementation, at a mean daily dose of 528 IU, have a statistically significant 7% reduction in all-cause mortality over a mean follow-up duration of 5.7 years 16. Vitamin D deficiency has a deleterious clinical impact on a number of important medical conditions (Table 3).

Vitamin D and chronic liver disease

The liver is a pivotal organ in the synthesis of vitamin D. It is the site where 25-hydroxylation occurs and where the vast majority of DBP is synthesized. In those with chronic liver disease (CLD) the prevalence of vitamin D insufficiency (<75 nmol/L) is almost universal, with vitamin D deficiency (<50 nmol/L) present in around two-thirds of subjects. Even in the absence of cirrhosis, vitamin D deficiency is present in the majority of subjects. In those with cirrhosis, the prevalence of severe vitamin D deficiency (<25 nmol/L) increases with increasing severity of synthetic liver dysfunction 17 and 18. Notably, in those about to undergo liver transplantation, the frequency of 25(OH)D and 1α,25(OH)₂D deficiency is 84% and 77%, respectively, with transplantation resulting in a marked increase in 25(OH)D, 1α,25(OH)₂D, and DBP levels 19.

The high prevalence of vitamin D deficiency in this population occurs regardless of the etiology of liver disease 20 and 21. Synthetic liver dysfunction is not entirely responsible, as vitamin D deficiency is still highly prevalent in those with non-cirrhotic liver disease 17. 25(OH)D levels normalize after oral or parenteral administration of vitamin D in patients with cirrhosis, indicating that 25-hydroxylation is preserved in this patient population 22 and 23. Serum DBP levels, which play a critical role in the transport and bioavailability of vitamin D, are moderately decreased in cirrhosis 24 and 25. However, as only 5% of DBP binding sites are occupied at any one time with vitamin D metabolites 2, profound liver dysfunction is required for low DBP levels to exert a significant contributing role to vitamin D deficiency in chronic liver disease.

Vitamin D deficiency in CLD is likely to result from a number of mechanisms. In addition to those described above, those patients with a chronic medical illness such as liver disease are more likely to have lower levels of sunlight exposure and/or inadequate dietary intake of vitamin D. Moreover, luminal absorption of dietary sources of vitamin D may be hindered by intestinal edema complicating portal hypertension and/or impaired bile salt dependent micellar incorporation due to cholestasis.

Vitamin D and chronic hepatitis C

Around 170 million people worldwide have chronic hepatitis C (CHC) infection 26, causing a substantial burden of chronic liver disease globally 27. Vitamin D deficiency is more prevalent in CHC subjects than healthy controls, even in those with minimal liver fibrosis. The majority of subjects with CHC are vitamin D deficient (<50 nmol/L) with 25% having severe deficiency (<25 nmol/L) 28 and 29. Current understanding of the mechanisms underlying the high prevalence of vitamin D deficiency in CHC is incomplete.

Nevertheless, recent evidence suggests that vitamin D may impact upon clinical outcomes and treatment response. Fundamental to this are several in vitro studies showing that vitamin D inhibits hepatitis C virus (HCV) replication in a dose-dependent manner 30, 31 and 32. Moreover, an association between baseline vitamin D status and treatment response to pegylated-interferon (PEG-IFN) and ribavirin (RBV) has recently been established (Fig. 2). Pre-treatment vitamin D deficiency is reportedly an independent predictor of failure to achieve a sustained virologic response (SVR) in HCV genotype 1 (HCV-1), 28 and 33, and 2/3 infection 29. However, 25(OH)D level is not associated with SVR in HCV–HIV co-infection 34. In HCV-1 infection, the rs12979860 C/T polymorphism upstream of the interleukin-28B (IL28B) gene on chromosome 19 is the strongest pre-treatment predictor of SVR 35. Baseline vitamin D status is independent of, but additive to, the IL28B genotype in predicting SVR in HCV-1. The highest SVR rate occurs in subjects who have the favorable CC genotype and 25(OH)D levels >50 nmol/L 33.

To date, there is limited data evaluating vitamin D supplementation in CHC treatment. Two small prospective randomized controlled studies from Israel showed that those subjects who received vitamin D₃ supplementation of 2000 IU/day, targeting a 25(OH)D level >80 nmol/L in addition to PEG-IFN/RBV combination therapy, had higher rates of rapid virologic response (RVR; 44% vs. 17%, p <0.001), complete early virologic response (cEVR; 94% vs. 48%, p <0.001) and SVR (86% vs. 42%; OR 2.5, 95% CI 2.0-4.9, p <0.001) in HCV-1 36 and SVR (95% vs. 77%, p <0.001) in HCV-2/3 infection 37 compared to subjects treated with standard therapy. Moreover, recipients of vitamin D₃ supplementation were less likely to be relapsers or non-responders to antiviral therapy, and had improved insulin resistance indices 36. Similarly, a small retrospective Italian study showed vitamin D₃ supplementation improved SVR rate in the treatment of recurrent hepatitis C post liver transplantation (53.3% vs. 18.5%, p = 0.02) 38. It remains unclear whether these improvements in the clearance of HCV with vitamin D supplementation are the result of an alteration in innate and/or adaptive immune function, or are mediated via improvement in insulin resistance. Large, prospective, placebo-controlled studies are thus required to assess the impact of vitamin D supplementation on viral response in CHC treatment. However, these studies now seem unlikely to occur in the new and rapidly evolving era of direct acting viral therapy.

Vitamin D status also reportedly correlates with liver histology in CHC. Patients with vitamin D deficiency have a higher grade of hepatic necroinflammation 28 and 33, more advanced fibrosis stage 28, 29 and 34 and may possibly have more rapid fibrosis progression 39. At a cellular level, vitamin D deficiency is associated with downregulation of the 25-hydroxylase enzyme CYP27A1 in liver tissue. This may have pathogenetic relevance, given the established inverse relationship between CYP27A1 expression and the severity of necroinflammatory activity 28.

The above findings highlight the potential role that proteins and enzymes involved in the synthesis and metabolism of vitamin D may have in liver inflammation and response to anti-viral therapy. Genetic variation in the rs10877012 A/C polymorphism in the promoter region of the 1α-hydroxylase enzyme CYP27B1, but not the rs10735810 FokI VDR polymorphism, is associated with SVR in HCV-1 infection. Subjects with the AA genotype have higher SVR rate and 1α,25(OH)₂D level than those with the AC or CC genotype 29, suggesting a key role of vitamin D in CHC infection. Moreover, a recently published proteomic study has shown vitamin DBP to be one of three metaproteins associated with SVR 40. DBP levels are significantly lower in subjects with significant or advanced fibrosis (METAVIR F2-4) compared with those with absent or minimal fibrosis (F0/1) and healthy controls 41 and 42.

Thus, vitamin D deficiency appears to be common in CHC and may be associated with adverse outcomes such as lower treatment response, more advanced fibrosis stage and increased severity of necroinflammation. It remains, however, uncertain as to whether vitamin D supplementation improves the SVR rate in patients receiving combination anti-viral therapy with PEG-IFN and RBV. Still, the findings of a significant association between the CYP27B1 rs10877012 A/C polymorphism, higher 1α,25(OH)₂D levels, and SVR rate, as well as the association between vitamin D-binding protein and SVR suggest that higher 25(OH)D and 1α,25(OH)₂D levels directly improve the virologic response to PEG-IFN and RBV therapy, presumably by impacting on the downstream regulation of vitamin D target gene transcription. DBP determines how much free 25(OH)D substrate is available for 1α-hydroxylase as well as the amount of free 1α,25(OH)₂D ligand available to activate the VDR and influences downstream gene transcription. Hepatic 1α-hydroxylase activity levels therefore represent a major additional factor regulating 1α,25(OH)₂D concentration in the liver.

Vitamin D and non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is the hepatic manifestation of the metabolic syndrome. It is the most common liver disease in the developed world, with a prevalence of 20–30% 43. Thirty percent of subjects with NAFLD have histologic evidence of non-alcoholic steatohepatitis (NASH) 44 and are at risk of disease progression and development of cirrhosis. The pathogenesis of NAFLD is yet to be fully elucidated, but insulin resistance (IR) is implicated as the key mechanism leading to hepatic steatosis. Apart from lifestyle modification that results in significant weight loss 45, there is currently no safe, effective therapy for NASH.

Vitamin D levels decrease by 1.3 nmol/L with each 1 kg/m² increase in body mass index (BMI) 46. Normal vitamin D status is associated with a two-thirds lower prevalence of metabolic syndrome compared to those with reduced levels 47. In non-diabetic Caucasians low vitamin D levels are independently associated with insulin resistance 48 and are a predictor of increased 10-year risk of developing hyperglycemia and insulin resistance 49. A vitamin D response element is present in the insulin gene promoter region, and 1α,25(OH)₂D activates transcription of the insulin gene 50. Both 1α-hydroxylase and the vitamin D receptor are expressed on pancreatic β cells, with an association between low vitamin D levels and impaired β cell function having been suggested 50 and 51. Two randomized placebo-controlled trials have shown that high dose vitamin D supplementation improved insulin sensitivity in non-diabetic South Asians 52 and 53. A large prospective cohort study of women demonstrated that those who received vitamin D supplementation had a significantly lower risk of developing type 2 diabetes 54.

Subjects with NAFLD have lower vitamin D levels when compared with controls. Low vitamin D levels are closely associated with histologic severity of steatosis, necroinflammation, and fibrosis in NAFLD, independent of age, gender, BMI, Homeostatic Model Assessment (HOMA)-IR score and presence of metabolic syndrome 55 and 56. These findings have been confirmed in children with NAFLD 57.

In a recent study of Lewis rats with diet-induced (choline-deficient and iron-supplemented l-amino acid or CDAA) NASH, phototherapy elevated 25(OH)D, and 1α,25(OH)₂D levels while reducing hepatocyte inflammation, fibrosis, and apoptosis when compared to controls. Phototherapy also improved insulin resistance and increased serum adiponectin in association with reduced hepatic expression of the profibrotic transforming growth factor (TGF)-β and α-smooth muscle actin (α-SMA), a marker of hepatic stellate cell activation. In addition, oral vitamin D₃ supplementation reportedly improved liver histology in a dose-dependent manner 58. Furthermore, in a rodent high fat diet model of NAFLD, vitamin D deficiency exacerbated histologic features of NAFLD, increased insulin resistance, and upregulated liver tissue expression of genes involved in hepatic inflammation and oxidative stress 59. Given the above findings, prospective studies that assess the impact of vitamin D supplementation on the histologic features of NASH are warranted as a priority, given the lack of an effective therapy for this condition.

Vitamin D and liver fibrosis

1α,25(OH)₂D has anti-fibrotic effects in lung fibroblasts and mesenchymal multipotent cells in vitro 60 and 61, as well as anti-proliferative and anti-fibrotic effects in both in vitro and in vivo rat models of liver fibrosis. VDR is expressed by hepatic stellate cells (HSC) and this expression is upregulated by 1α,25(OH)₂D. In addition, 1α,25(OH)₂D suppresses HSC proliferation, and expression of cyclin D1, tissue inhibitor of metalloproteinase 1 and collagen Iα1 in vitro. In vivo, 1α,25(OH)₂D decreases α-SMA expression and collagen levels, and prevents the development of cirrhosis by thioacetamide (TAA) 62 and 63. A vitamin D level >50 nmol/L may be associated with a decreased frequency of rapid fibrosis progression in CHC 39. However, the clinical importance of vitamin D as an anti-fibrotic agent remains to be determined.

Vitamin D receptor polymorphisms and liver disease

The vitamin D receptor (VDR) gene is located on chromosome 12. It encodes a 48 kDa soluble protein that is a member of the nuclear receptor family of ligand-activated transcription factors. Common single nucleotide polymorphisms (SNP) of the VDR gene include FokI (rs10735810), BsmI (rs1544410), ApaI (rs7975232), and TaqI (rs731236). There is a marked racial variation in the allele frequency of these VDR polymorphisms 64, but their influence on VDR function and signaling is unknown 65. The BsmI, ApaI, and TaqI SNPs are all in the 3′ region of the VDR gene and are in linkage disequilibrium with each other 66.

In CHC infection, the bAt CCA-haplotype of the BsmI, ApaI, and TaqI alleles, and the CC genotype of the ApaI allele are associated with rapid fibrosis progression, cirrhosis and increased intrahepatic expression of the fibrosis marker gene MMP-939. In chronic hepatitis B (HBV) infection, the variation in allele frequency of BsmI, ApaI, and TaqI is associated with HBeAg positivity and HBV flare 67. Variation in ApaI, and to a lesser extent TaqI, is also associated with a higher HBV viral load and more severe fibrosis and necroinflammation 68. Variation in the TaqI VDR polymorphism is also associated with both chronic HBV infection 69 and occult HBV infection 70, in which there is a low degree of HBV replication present in HBsAg negative subjects.

In hepatocellular carcinoma (HCC), complicating cirrhosis variation in the allele frequency of the BsmI, ApaI, and TaqI, but not FokI VDR polymorphisms is associated with HCC development when compared to cirrhotic patients without HCC. This association is most marked in subjects with alcohol-related cirrhosis, where carriage of the BsmI–ApaI–TaqI A–T–C and G–T–T haplotypes is independently associated with an increased risk of HCC. Furthermore, there is a significant difference in allele frequency of these VDR polymorphisms in alcohol-related cirrhosis compared to cirrhosis complicating chronic viral hepatitis 66.

Multiple studies have confirmed an association between VDR polymorphisms and autoimmune liver disease in both European and Asian populations. Variation in the allele frequency of the BsmI polymorphism is associated with primary biliary cirrhosis 71 and 72, while variation of the FokI polymorphism is associated with autoimmune hepatitis 64 and 73. Furthermore, carriage of the VDR BsmI–TaqI G–T/G–T diplotype is an independent predictor of acute cellular rejection post-liver transplantation 74. Similarly, VDR polymorphisms are associated with a variety of other autoimmune and immune-mediated diseases, including type 1 diabetes 75, leprosy 76, Crohn’s disease 77, tuberculosis 69 and 78, psoriasis 79, multiple sclerosis 80, and Graves’ disease 81 (Table 4).

Vitamin D, the immune system, and the liver

There is an increased incidence and prevalence of autoimmune diseases such as type I diabetes, multiple sclerosis (MS) and Crohn’s disease in geographic regions at higher latitude 82 and 83. This phenomenon is suggested to be related to lower 25(OH)D levels resulting from decreased ultraviolet sunlight exposure. In support of this hypothesis, the incidence of MS decreases with increasing 25(OH)D levels 84 and vitamin D supplementation decreases the risk of developing both MS in women 85 and type 1 diabetes in children by 80% 86. In this context, vitamin D has an important role in both the innate and adaptive immune system 87. Macrophages, T cells, and DCs express both 1α-hydroxylase and vitamin D receptor, and are thus direct targets of 25(OH)D and 1,25(OH)₂D 88, 89 and 90.

Innate immunity

The innate immune response is mediated by pattern-recognition receptors (PRR). Toll-like receptors (TLR) are a family of transmembrane PRRs with broad specificity, expressed on immune cells such as polymorphonuclear cells, monocytes, and macrophages. They interact with pathogen-associated molecular patterns such as viral nucleic acids, and bacterial and fungal products, to trigger an inflammatory (TNF, IL-1β, and IL-6) or antimicrobial response in the host 91. Several data from studies focusing on the immunology of mycobacterial infection suggest vitamin D and DBP play a significant part in the activation of the innate immune response. The risk of Mycobacterium tuberculosis (TB) infection is increased in subjects with vitamin D deficiency, with the greatest risk observed in subjects with the lowest 25(OH)D levels 78, 92, 93 and 94. At a cellular level, macrophages infected with M. tuberculosis initiate a TLR2/1 response that enhances 1α-hydroxylase and VDR expression and induction of the anti-microbial peptide cathelicidin. The anti-microbial activity of macrophages occurs via a vitamin D-dependent process. Addition of 1α,25(OH)₂D to M. tuberculosis-infected macrophages reduces the number of viable bacilli, while both vitamin D and TLR2/1 are required for cathelicidin production 95. DBP plays a key role in modulating monocyte responses to 25(OH)D and 1α,25(OH)₂D, that varies according to the DBP genotype. Notably, monocytes cultured in serum from DBP null mice, and in human serum with the lower affinity Gc1S and Gc2 DBP isoforms, have more potent induction of cathelicidin than monocytes cultured in serum from DBP+/− mice or human serum with the high affinity Gc1F isoform 96. The Gc2 isoform is associated with lower 25(OH)D levels 97 and 98, and carriers of this allele have an increased susceptibility to active TB in the presence of severe vitamin D deficiency (<20 nmol/L) 99. The Gc2 isoform is less able to be converted into macrophage activating factor, resulting in reduced macrophage function 4. These data indicate that vitamin D-dependent antimicrobial responses may be strongly influenced by genetic polymorphisms in DBP, especially in the presence of vitamin D deficiency. Moreover, adjunctive high-dose vitamin D significantly hastens the time to sputum culture conversion during intensive-phase antimicrobial treatment of pulmonary TB in the subset of patients with the tt genotype of the TaqI VDR polymorphism 100. In addition, variation in allele frequency of the FokI and TaqI VDR polymorphisms in the presence of vitamin D deficiency is associated with increased risk of TB 69 and 78.

The role of vitamin D in innate immunity has implications on liver disease. Chronic liver disease is characterized by ongoing increased exposure of the liver via the portal circulation to bacterial products such as lipopolysaccharide (LPS). Contributing factors include increased intestinal mucosal permeability, alcohol ingestion, and small bowel bacterial overgrowth 101, 102 and 103. Dietary factors, such as a high-fat diet that predisposes to NAFLD, may also contribute to increased intestinal permeability and result in increased hepatic exposure to LPS 104. Kupffer cells, the resident macrophages of the liver, represent 80–90% of the macrophages in the body 105, and their innate immune vitamin D-dependent antimicrobial response is also likely to be influenced by the vitamin D status and genetic polymorphisms in DBP. They also express TLR2, TLR4, and TLR9, and are responsive to LPS, the main ligand of TLR4. Hepatocytes, hepatic stellate cells, sinusoidal epithelial cells, biliary epithelial cells, and hepatic DCs also express TLR4 and are responsive to LPS. The interaction of LPS with TLR4 in the liver is crucial during hepatic fibrogenesis 101 and 106. Serum vitamin D levels are inversely proportional to TLR2 and TLR4 expression in monocytes, with administration of 1α,25(OH)₂D downregulating expression of TLR2, TLR4, and TLR9 107, 108, 109 and 110. Intestinal microbiota play an essential role in hepatic fat accumulation. TLR2, TLR4, and TLR9 are implicated in the pathogenesis of NAFLD, with TLR4 and TLR9 signaling associated with worsening steatosis, inflammation and fibrosis 111 and 112. In obese rats, vitamin D deficiency increases hepatic mRNA levels of TLR2, TLR4, and TLR9, and the endotoxin receptor CD14, which is implicated in worsening histologic features of NAFLD 59. In CHC infection, increasing hepatic necroinflammatory activity correlates with increasing hepatic mRNA expression of TLR2 and TLR4, and hepatic TNFα mRNA is also closely correlated with TLR2 and TLR4 mRNA expression 113. Furthermore, the antiviral effect of vitamin D on hepatitis C inoculated HuH7.5 hepatoma cells is mediated by innate immune system activation of the interferon-mediated signaling pathways 30.

NK cells and DCs are both important innate immune effector cells. Studies in VDR knockout mice have shown that expression of VDR is necessary for NK cell development and function 114. 1α,25(OH)₂D enhances NK cell cytotoxicity 115 and suppresses DC maturation, inducing a more tolerant DC phenotype which, at the interface of the innate and adaptive immune systems, promotes T regulatory (Treg, CD4+CD25+) cell activity 116.

Adaptive immunity

Vitamin D is an important modulator of T cell response to pathogens, which is a key component of adaptive immunity. In particular, activation of naïve T cells is a vitamin D-dependent process. In the inactivated state, naïve T cells do not express VDR and express almost no phospholipase C-γ1 (PLCγ1), which is a key molecule required for subsequent classical T cell receptor signaling and T cell activation. Following stimulus exposure, VDR is expressed on T cells through T cell receptor signaling via the alternative mitogen-activated kinase p38 pathway. The VDR complex, activated by binding of 1α,25(OH)₂D, upregulates transcription of the gene encoding PLCγ1 and results in a 75-fold increase in PLCγ1 expression, enabling activation of naïve T cells. T cells in patients with lower 25(OH)D and 1α,25(OH)₂D levels have a lower proliferation index after stimulation than T cells from patients with normal 25(OH)D and 1α,25(OH)₂D levels; this pattern is overcome by exogenous administration of 1α,25(OH)₂D 117.

1α,25(OH)₂D also has an anti-proliferative effect on adaptive immunity. It inhibits proliferation of T helper type 1 (Th1) lymphocytes, which produce interferon (IFN)-γ, interleukin (IL)-2, and activate macrophages 118, and shifts the balance to a T helper type 2 (Th2) phenotype with increased production of IL-4, IL-5, and IL-10 119. 1α,25(OH)₂D increases Treg cells 120 and 121, enhances DC secretion of IL-10, decreases DC secretion of IL-12, a critical cytokine in Th1 development 122, and inhibits Th17 development via inhibition of IL-6 and IL-23 production 121. In patients with MS, 25(OH)D, but not 1α,25(OH)₂D, levels correlate with the ability of Treg cells to suppress the proliferation of activated T responder cells and inversely correlated with Th1/Th2 ratio 123. IL-2, IL-10, and IL-12 genes in T cells have regions which bind to VDR and 1α,25(OH)₂D may directly play a role in the transcription of these cytokines in T cells 124. The ability of 1α,25(OH)₂D to modulate the adaptive immune system may explain the association of vitamin D supplementation and higher 25(OH)D levels with a lower risk of multiple autoimmune diseases.

In orthotopic liver transplant recipients, severe 25(OH)D deficiency (<12.5 nmol/L) and VDR BsmI–TaqI G–T/G–T diplotype are independent predictors of moderate-severe acute cellular rejection, whilst vitamin D₃ supplementation decreases the risk of acute rejection by 60% 74 and 125. These findings highlight the importance of optimizing the vitamin D status in liver transplant recipients, not only to prevent bone loss, but also to reduce the risk of T cell-mediated acute rejection. A lower Th1/Th2 ratio is an independent predictor of SVR in treatment of HCV-1 126, which possibly explains why vitamin D supplementation may improve therapeutic outcomes with PEG-IFN plus RBV. The immune tolerant phenotype promoted by vitamin D may also be of therapeutic benefit in NASH, where activation of both innate and adaptive immunity is implicated in its pathogenesis.

Genome wide association studies of vitamin D

Only about a quarter of vitamin D variability between individuals is explained by factors such as reported dietary intake, latitude and season of measurement 127 and 128. Twin and family studies suggest that genetic factors play a significant role in the wide variation of vitamin D levels observed within and between populations 129. Polymorphisms of the hydroxylases, DBP, and VDR may have a profound influence on serum vitamin D levels and the efficacy of vitamin D as a hormone. Two large genome wide association studies (GWAS), involving patients of European ancestry 130 and 131, of SNPs and their association with 25(OH)D levels have revealed important information about genetic variation in the enzymes and carrier proteins which are integral to the synthesis and metabolism of vitamin D.

The NADSYN1/DHCR7 locus is closely related to the de novo synthesis of vitamin D₃ in the skin from the precursor 7-dehydrocholesterol. There is an association between 25(OH)D levels and several SNPs including rs12785878, rs12800438, rs3794060, rs4945008, and rs4944957 of this locus 124. SNPs in the 25-hydroxylase CYP2R1 locus rs10741657, rs2060793, rs12794714, rs10500804, and rs7116978 are also significantly associated with 25(OH)D levels 128 and 129.