Abstract
Calcitriol (1,25-dihydroxyvitamin D3), the active form of vitamin D, signals through the vitamin D receptor (VDR) and has been linked to colorectal cancer (CRC) risk and progression. This review summarizes current evidence on calcitriol/VDR actions relevant to CRC prevention and therapy. A structured literature search of PubMed, Scopus, and Web of Science was conducted through December 2025 for studies on calcitriol, VDR, and CRC, including mechanistic, preclinical, epidemiological, and clinical trial reports. Evidence was synthesized narratively with an emphasis on pathways connecting epithelial biology, inflammation, and the tumor microenvironment. Across model systems and patient studies, calcitriol/VDR signaling is associated with reduced proliferation, enhanced differentiation and apoptosis, and repression of oncogenic programs including Wnt/beta catenin and MYC-driven transcription. Calcitriol can also shape the tumor immune milieu by limiting pro-inflammatory signaling (for example, NF-κB, COX 2, IL-6 and IL-8), supporting epithelial tight junctions, and modulating the microbiome and bile acid metabolism, which together may enhance immune surveillance and reduce tumor-permissive inflammation. Evidence from supplementation trials is mixed, suggesting heterogeneity in baseline vitamin D status, tumor stage and VDR pathway context. Calcitriol engages convergent epithelial and immune mechanisms that plausibly limit CRC initiation and progression, but optimal patient selection and dosing strategies remain unresolved. Future trials integrating molecular biomarkers and VDR-responsive gene signatures are needed to define when vitamin D-based interventions can provide meaningful benefit in CRC.
Introduction
According to a 2023 survey (1), colorectal cancer (CRC) is the third most common cancer and the fourth leading cause of cancer-associated mortality worldwide. This increasing trend of CRC incidence is more evident in developed countries such as the United States and China (1). CRC is usually asymptomatic until it progresses to advanced stages, leading to poor prognosis; only 5% of patients with distant metastases survive for up to 5 years (2). In 1980, vitamin D was suggested to play a role in CRC prevention because the areas least exposed to sunlight had the highest CRC-associated mortality rates (3). Thereafter, several studies have described the anti-cancer effects of vitamin D, particularly against CRC (4-8). Hence, the cancer incidence and mortality rates can be effectively reduced by ensuring vitamin D levels (4-8).
In the present study, we reviewed the role of calcitriol as an antitumor agent and the underlying molecular mechanisms that promote cellular differentiation and apoptosis while reducing proliferation and angiogenesis. We focused on the progress in knowledge on the role of calcitriol as an immunomodulator of CRC and the suppression of calcitriol signaling with CRC progression towards more advanced stages. In addition, preclinical and epidemiological data have suggested the potential role of calcitriol in CRC prevention.
Methods
A structured search was performed through December 31, 2025, in PubMed, Scopus and Web of Science to identify peer-reviewed articles evaluating calcitriol (1,25-dihydroxyvitamin D3), vitamin D signaling and the vitamin D receptor in the context of colorectal carcinogenesis, tumor progression, immune regulation and the gut microbiome. Search terms included combinations of calcitriol, vitamin D, vitamin D receptor or VDR, colorectal cancer or CRC, inflammation, tumor microenvironment, immunity or immunomodulation, and microbiome or dysbiosis. Primary research studies, meta-analyses and clinical trials were considered, and relevant references were also identified by screening the bibliographies of key articles.
Vitamin D Sources and Metabolism
Vitamin D is a steroid hormone with a broken-ring structure known as a secosteroid. Vitamins D2 (ergocalciferol) and D3 (cholecalciferol) are the two primary forms present in humans. Vitamin D2 is mainly plant-derived, whereas vitamin D3 is principally produced in the skin; a small portion is obtained from vitamin D-rich foods (9). When the human skin is exposed to sunlight, provitamin D3 (7-dehydrocholesterol) is transformed into pre-vitamin D3, which is thermodynamically unstable and is further modified into vitamin D3. Vitamins D2 and D3 function as prohormones that enter the bloodstream by binding to specific proteins (vitamin D-binding proteins) and reach the liver. Hydroxylation of these vitamins by the enzyme vitamin D-25 hydroxylase (CYP2R1) in the liver results in the formation of calcidiols (25-hydroxyvitamin D or 25(OH)D3). Calcidiol is the key circulating metabolite of vitamin D, with a half-life of approximately 2 weeks, and is mostly used to predict a patient’s vitamin D status (9, 10). Calcidiol is further metabolized in the kidneys and extra-renal sites (such as the colon, brain, prostate, endothelial cells, and immune cells). The enzyme 25-hydroxyvitamin D-1α-hydroxylase (CYP27B1) metabolizes calcidiol into a hormonally active form known as calcitriol (1,25(OH)2D3) (Figure 1). Furthermore, calcidiol and calcitriol are inactivated by 25-hydroxyvitamin D-2-4-hydroxylase (CYP24A1) to 24,25(OH)2D3 and 1,24,25(OH)3D3, respectively (10).
Schematic diagram of pathways involved in vitamin D metabolism and its actions. Sun exposure and vitamin D-rich foods are the main sources of vitamin D. After synthesis, vitamin D is metabolized by the hepatic enzyme CYP27A1 into calcidiol. Calcidiol is further hydroxylated by the renal enzyme CYP27B1 into the metabolically active form, calcitriol. Calcitriol then triggers genomic and non-genomic actions. For genomic actions, calcitriol binds to the vitamin D receptor (VDR) and mediates its translocation into the nucleus, where it forms a heterodimer with the retinoid X receptor (RXR) and ultimately binds to vitamin D response elements (VDREs). This results in the transcription of various genes that reduce proliferation and angiogenesis while promoting differentiation and apoptosis. For non-genomic actions, calcitriol binds to the VDR present in the plasma membrane caveolae, which induces the opening of ion channels, activation of second messengers, and secretion of hormones. Calcitriol is deactivated by CYP24A1 to produce 1,24,25(OH)3D3.
Modes of Action of Calcitriol
Genomic actions. The vitamin D receptor (VDR), also known as the calcitriol receptor, is highly expressed in the intestine. VDR is a member of the nuclear hormone receptor family that includes estrogen, thyroid hormone, and retinoic acid receptors (11). The VDR consists of well-maintained DNA- and ligand-binding domains with elevated affinity to vitamin D (11).
Calcitriol binding to VDR induces a conformational change resulting in the formation of a complex between VDR and retinoic acid X receptor (RXR). The VDR/RXR complex is important for potentiating the binding of ligand-bound VDR specifically to vitamin D response elements (VDREs) in the promoter regions of target genes (Figure 1) (12). The VDR/RXR complex plays a vital role in initiating gene transcription and participates in chromatin remodeling, histone modification, and RNA polymerase II binding (11). Many previous studies support the anticancer activity of calcitriol in CRC through several molecular mechanisms, including inhibition of cellular proliferation and angiogenesis, and promotion of cellular differentiation and apoptosis (4, 9, 13). The genomic effects of vitamin D are described below.
Calcitriol inhibits the proliferation of CRC cells. Calcitriol inhibits cancer cell proliferation by enhancing cell cycle arrest in the G0/G1 phase. Previous data have indicated that calcitriol represses the activity of cyclins A and F and induces the activity of the cyclin-dependent kinase inhibitors p21Cip1 and p27Kip1 (14). Studies have shown that the induction of p21Cip1 in CRC cell lines depends on the calcium-sensing receptor activation by calcitriol (15). Calcitriol up-regulates the expression of the GADD45A gene, which is involved in growth arrest following DNA damage and plays a role in preserving genomic integrity (14). Calcitriol also down-regulates the expression of several genes, including c-MYC, FOS, and JUN, which are involved in cell proliferation (16). c-MYC, which is highly expressed in most cancers, is down-regulated by calcitriol (17).
Calcitriol inhibits the angiogenesis of CRC cells. Hypoxia promotes angiogenesis by up-regulating the expression of hypoxia-inducible factor-1 alpha, whereas its transcriptional activity is repressed in the presence of calcitriol (18). Interestingly, calcitriol regulates the expression of various proangiogenic factors, particularly vascular endothelial growth factor, as well as antiangiogenic factors such as thrombospondin-1, resulting in a balanced change in the angiogenic potential of colon cancer cells (19). The DICKKOPF-4 protein is essential for tumor angiogenesis and metastasis. It is a weak antagonist of Wnt signaling and its activity in cultured CRC cells is suppressed in the presence of calcitriol (19).
Calcitriol promotes the differentiation of CRC cells. Calcitriol has several pro-differentiation effects on CRC cells. Numerous studies have reported that calcitriol and its analogs stimulate the development of microvilli and enhance the expression of various brush border enzymes such as alkaline phosphatase and maltase, which are primarily used as markers of cell differentiation in CRC cells (20-22). Moreover, calcitriol plays an important role in regulating the epithelial phenotype by inducing the expression of several cell adhesion proteins, such as E-cadherin of adherens junctions, occludin, zonula occludens, claudins of tight junctions, and plectin of hemidesmosomes (23). Extensive studies have reported several mechanisms by which calcitriol induces the expression of microfilaments and intermediate filament proteins such as filamin A, keratin-13, and vinculin (24, 25). Filamin A plays an important role in cell migration and adhesion (24). Its emerging roles within the cell nucleus have been implicated in the maintenance of the nuclear shape during epithelial-mesenchymal transition and repair of DNA double-strand breaks (26).
Calcitriol potentiates CRC cell apoptosis. Calcitriol accelerates apoptosis in CRC cells by inducing and inhibiting the production of pro-apoptotic (BAK1) and anti-apoptotic (BAG1) proteins, respectively (21, 27). However, it has not been elucidated whether the expression of other pro-apoptotic proteins (BAX) or anti-apoptotic proteins (BCL-2, BCL-xL) is regulated by calcitriol in CRC (21, 28). Another study demonstrated the role of calcitriol in the induction of G0S2, a mitochondrial protein that causes apoptosis in CRC cells. G0S2 interacts with BCL-2 to prevent anti-apoptotic heterodimer formation by BAX (29). Calcitriol has also been reported to enhance the sensitivity of colon cancer cells to 5-fluorouracil, a chemotherapeutic agent, in two ways: first, by down-regulating the expression of thymidylate synthase, a molecular target of 5-fluorouracil, and second, by suppressing the activity of survivin, an anti-apoptotic protein (15). In addition, calcitriol promotes CRC sensitivity to the tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) by inhibiting macrophage-derived interleukin-1β (IL-1β) (30).
Non-genomic Actions
Classically, calcitriol mediates its genomic actions by interacting with nuclear VDR (VDRn), which requires time to elicit its effects. However, it is now recognized that calcitriol exerts certain rapid effects (mediated within seconds) at the cellular level without influencing gene expression, which are generally referred to as non-genomic actions (11). Although the non-genomic actions of calcitriol were initially unclear, it was later discovered that vitamin D exerts its non-genomic actions through the functional VDR present in the caveolae of the plasma membrane (VDRm) (31). Several hormones that function through nuclear receptors exhibit similar non-genomic effects, such as activation of certain enzymes (phosphatases and kinases) and ion channels (32). The non-genomic actions of calcitriol involve the opening of ion channels (Ca2+ and Cl−). Vitamin D also triggers the activation of several signaling molecules, such as phosphatidylinositol-3 kinase, p21ras, phospholipase A2, and phospholipase C. However, its non-genomic actions include the rapid induction of second messenger molecules (cAMP, Ca2+, and phosphatidylinositol-3,4,5-triphosphate), along with the activation of various protein kinases (protein kinase A, mitogen-activated protein kinase, protein kinase C and Ca2+-calmodulin kinase II). Calcitriol is also involved in insulin secretion via non-genomic pathways (11). These findings may explain the genomic and non-genomic functions of VDR.
Epigenetic actions. Vitamin D deficiency plays a significant role in the pathogenesis of several diseases, including diabetes, obesity, metabolic syndrome, coronary heart disease, cancer, chronic lung disorders, and inflammatory disorders, such as rheumatoid arthritis, systemic lupus erythematosus, Crohn’s disease, and multiple sclerosis, through various mechanisms, including epigenetic disruption (33). VDRs alter DNA and histone proteins in the genome promoter region (34). VDR and RXR can form heterodimers, and the VDR/RXR complex binds to specific target gene sequences in the promoter regions of transcription pathways (35, 36). Vitamin D deficiency can lead to alterations in DNA methylation at specific sites and reduce telomerase activity, both of which are associated with aging. However, vitamin D supplementation reversed these dysfunctions, aiding in the restoration of normal cellular function (Figure 2). Vitamin D is crucial for proper regulation of the invariant natural killer T (iNKT) cell number and function during prenatal development (37). Vitamin D deficiency results in irreversible epigenetic changes in the iNKT cells that cannot be reversed by subsequent vitamin D exposure (38).
DNA methylation and histone modification mediated by vitamin D receptor signaling. This figure illustrates the role of Vitamin D receptor (VDR) in modulating epigenetic mechanisms such as DNA methylation and histone modification. VDR forms heterodimers with retinoid X receptor (RXR) that regulate gene expression via binding to specific DNA sequences. The VDR/RXR complex recruits histone acetyltransferases (HATs) and histone deacetylases (HDACs) to modify chromatin structure, influencing transcriptional activity. VDR also interacts with sirtuin 1 (SIRT1), facilitating the deacetylation of nuclear factor kappa-B (NF-κB), which reduces inflammatory responses. Additionally, VDR deficiency is associated with altered DNA methylation patterns and reduced telomerase activity, contributing to aging and disease susceptibility.
Histone acetyltransferases (HATs) and other epigenetic factors are regulated by VDR/RXR dimers during transcription (39). Histone deacetylases (HDACs) and receptors that activate HATs interact directly with VDRs (39). Additionally, forkhead box proteins and their regulator sirtuin 1 (SIRT1) interact with VDRs, enhancing their function in the promoter regions of VDR target genes (40). Normally, vitamin D binds to VDRs, which then interact with SIRT1 to facilitate deacetylation of NF-κB, thus suppressing its inflammatory activity (Figure 2). This regulatory process is disrupted in vitamin D deficiency, resulting in reduced deacetylation of NF-κB. Thus, vitamin D regulates the expression of cytokines associated with NF-κB activation, thereby reducing inflammation (41).
Immunomodulatory Actions of Calcitriol
Calcitriol is known to modulate innate and adaptive immunity. The VDR is present in nearly all immune cells (42). Vitamin D deficiency is linked to autoimmune diseases (43), and the administration of vitamin D supplements has a positive impact on disease prognosis. Calcitriol acts as an immunomodulator by inducing the expression of VDR and CYP27B1.
Previous data have shown that the number of pathogenic bacteria increases in the colons of calcitriol-deficient animals compared to that in normal controls (44). Moreover, toll-like receptors (TLRs) expressed in immune cells are activated, triggering an antimicrobial response against intracellular bacteria. Such TLR activation results in the increased expression of both VDR and CYP27B1, which promotes the production of the antimicrobial peptide cathelicidin (45, 46). Cell culture studies have shown that the induction of cathelicidin requires adequate calcitriol levels (45). Furthermore, these results are supported by experimental evidence obtained from a monocyte cell culture study in African Americans, who are known to have reduced vitamin D levels. Initially, these monocytes were unable to induce cathelicidin expression; however, they later became responsive to cathelicidin induction following the addition of vitamin D (46).
Previous studies have shown that calcitriol plays preventive and therapeutic roles in inflammatory bowel disease (IBD) by decreasing the risk of colitis-associated CRC (47). Cyp27b1-knockout mice exhibit higher IL-1 and IL-17 levels in their colons than wild-type mice and are more susceptible to ulcerative colitis (47). Additionally, as calcitriol production is deficient in Il10-knockout mice, they spontaneously progress to the development of IBD symptoms (48). However, calcitriol administration to these mice prevents disease progression and ameliorates the established symptoms of IBD by inhibiting the TNF-α pathway (49). VDR is essential for the repression of gastrointestinal inflammation, particularly in Il10-deficient mice. In contrast, Il10/Vdr-double knockout mice exhibited intensified IBD symptoms and elevated expression of pro-inflammatory cytokines, such as IL-2, IL-1β, IL-12, TNF-α, and interferon (IFN)-γ (50). These findings show that calcitriol/VDR plays a significant anti-inflammatory role, both in vitro and in vivo, interfering with colonic inflammation and reducing the risk of CRC.
Vitamin D regulates the expression of Th1-promoting cytokines (IL-12 and IL-10) in antigen-presenting cells, including dendritic cells and macrophages (51). Calcitriol reportedly suppress Th1 activity by inhibiting IFN-α and IL-2 production. Th2 cell differentiation is enhanced in the presence of calcitriol; however, this was evident only at adequate calcitriol levels and Th2 cell activity was drastically reduced in the absence of calcitriol (52).
Recent data have shed light on the role of the association between gut microbiota and host immunity in colon carcinogenesis (53). Calcitriol regulates and modulates the gut microbiota. In a dextran sodium sulfate-induced colitis model, mice fed a vitamin D-sufficient diet did not exhibit symptoms of colitis and had decreased bacterial infiltration, when compared to mice fed a vitamin D-deficient diet (44). Another study using the same colitis model revealed that Helicobacter species are mainly found in Cyp27b1-knockout mice than in wild-type mice (54).
Gut Microbiome and CRC
The gut microbiome, which is a complex community of trillions of bacteria in the human digestive system, plays a crucial role in the maintenance of overall health through immunological support, digestion, and protection against pathogens. However, dysbiosis, which disrupts the balance between these microorganisms, is closely linked to the development and progression of CRC.
Composition and role of gut microbiota. The microbiome is critical for homeostasis, and several different species have been directly linked to cancer prevention (55). The best-studied probiotic defensive microbes include the Lactobacillus and Bifidobacterium species, which reportedly possess expanded anticancer strategies (56). These mechanisms influence cellular proliferation, apoptosis, host immunity, and carcinogens/xenobiotics elimination (56). However, other colonic bacteria may be pro-inflammatory or carcinogenic. A widely known group of bacteria associated with short-chain fatty acid (SCFA) production is the butyrate-producing species (e.g., Rubinococcus, Clostridium, Eubacterium, and Faecalibacterium) that ferment dietary fibers in the colon. SCFAs suppress HDAC in immune cells and colonic epithelium via G protein-coupled receptors. This accounts for histone hyperacetylation and the increase in regulatory T cell (Treg) counts and the induction of the release of two anti-inflammatory cytokines TGF-β and IL-10. At the same time, SCFAs maintain low production of pro-inflammatory cytokines, such as IL-6 and IL-12, in colonic macrophages (57). Butyrate also acts as an HDAC inhibitor and increases the expression of tumor suppressor proteins, including FAS, p21, and p27 (58).
Specific microorganisms maintain and seem to strengthen the barrier of the intestinal epithelial layer and decrease the probability and quantity of colorectal tumors including Lactobacillus acidophilus, Bifidobacteria bifidum and Bifidobacteria infantum; they increase the secretion of mucin 2 (MUC2), zonula occludens (ZO-1), and occludin (59). Specifically, the animal studies using colitis mouse models revealed that B. longum and Bacillus subtilis ameliorate the damaged intestinal barrier by decreasing the production of pro-inflammatory cytokines, such as IL-17, IL-23, and TNF-α and increasing tight junctions proteins, including claudin-1, occludin, and ZO-1 (60). One such mechanism is through the down-regulation of IL-22 by the Lactobacillus casei BL23 strain, which has been currently receiving much attention concerning the role of the tuberculosis microbiome against CRC (61).
Dysbiosis and CRC. Dysbiosis, which is an imbalance in the gut microbiota, plays a role in the development of CRC. Maintaining a balanced gut flora is essential for managing inflammation, preserving the integrity of the gut barrier, and regulating the intestinal homeostasis. However, dysbiosis disrupts this delicate balance, leading to an increased risk of CRC due to DNA damage and persistent inflammation. Dysbiosis can be triggered by factors, such as stress, antibiotic use, and dietary habits.
Table I summarizes the bacteria associated with CRC in relation to dysbiosis (62-71). Some bacteria, such as Fusobacterium nucleatum, promote tumor growth and resistance to chemotherapy, whereas Streptococcus bovis is known to trigger protective responses. Enterococcus faecalis produces oxygen species that can damage DNA, whereas the Ruminococcaceae and Anaeroplasma species are associated with varying levels of cancer risk.
Bacteria associated with colorectal cancer (CRC) in relation to dysbiosis.
Impact of Calcitriol on Gut Microbiota
Calcitriol is essential in modulating the composition and function of the gut microbiome. Calcitriol plays a critical role in gut health and cancer prevention by lowering inflammation and boosting immunological responses, which, in turn, lowers the risk of CRC.
Modulation of microbiome composition. Recent research has demonstrated that exposure to vitamin D can modify the composition of the gut microbiota (72). Research on rodents has revealed that vitamin D deficiency attributed to restricted food, CYP27B1 deficiency, or VDR deficiency stimulates the growth of Bacteroidetes and Proteobacteria phyla (73, 74). Moreover, two VDR polymorphisms were found to be important contributors to microbiota variation in a combined cohort of 2029 people from the general German population and patients with specific diseases (such as sarcoidosis, metabolic syndrome, and autoimmune disease) (75). Human VDR polymorphisms were shown to have consistent effects on the genus Parabacterioides (phylum Bacteroidetes) (76). The examination of vdr−/− mice revealed an increased abundance of Parabacteroides in comparison to wild-type mice (77).
Significant correlations between microbiota composition and vitamin D levels have been reported in human studies. Vitamin D consumption was highly positively associated with Bacteroides, a member of the phylum Bacteroidetes, and negatively associated with Prevotella abundance in a cross-sectional investigation of healthy adults (78). In contrast, Prevotella was more common in the feces of healthy individuals who reported consuming more vitamin D, whereas Veillonella (phylum Firmicutes) and Haemophilus (phylum Proteobacteria) were less common (79). Individuals with higher blood calcitriol levels in the same study showed different bacterial enrichment; they had more Megaphaera (phylum Firmicutes) but less Veillonella and Haemophilus (79). Eight weeks of vitamin D3 supplementation resulted in increased species richness in the gastric antrum; decreased Proteobacteria (specifically Gammaproteobacteria) in the upper gastrointestinal tract (gastric corpus and gastric antrum), and increased Bacteroidetes (gastric corpus and descending duodenum) (80). Notably, no difference was observed in the microbial composition of the lower gastrointestinal tract and feces before and after vitamin D3 therapy, indicating that stool sample analysis may not be the best method for examining the effects of vitamin D3 on microbial communities (80). This is corroborated by the lack of association between the relative abundance of fecal bacterial species and regular vitamin D consumption in an observational study (81). It is particularly important to determine whether vitamin D affects the microbial composition of stool along the gastrointestinal tract. This raises concerns regarding the collection of stool and fecal samples for future gut microbiome research. Furthermore, conflicting results between studies may stem from methodological discrepancies in the evaluation of vitamin D “dose” (e.g., sun exposure, reported food and nutritional supplement vitamin D consumption, serum calcitriol).
Antimicrobial peptides regulation. The in vivo mRNA and protein expression of antimicrobial peptides such as cathelicidin, defensins, lysozyme, and Ang4 are up-regulated by vitamin D (82-85). In vivo research has demonstrated the importance of antimicrobial peptides as mediators of microbiome composition. These peptides are primarily secreted by Paneth cells in the gut and have been linked to increased susceptibility to pathogen infection or colitis, as well as increased bacterial translocation following Paneth cell destruction (86). Cathelicidins have antiviral and antifungal activities and are released on surfaces that interact with the external environment. They may also generate transmembrane holes in the bacterial cell walls (87). Defensins, which play a vital role in the innate immune response of the gut, are released by immune, Paneth, and epithelial cells. The absence of VDR expression in the intestinal epithelial cells results in aberrant Paneth cells, decreased lysozyme mRNA expression, compromised autophagy, and elevated B. fragilis and E. coli levels (84). Finally, vitamin D insufficiency has been linked to a 50-fold increase in the colonic bacterial infiltration in mice and decreased colonic Ang4 expression (44).
Interplay Between Microbiome and Calcitriol in CRC Prevention
The gut microbiota plays a pivotal role in the development and progression of CRC. Invading pathogens such as Escherichia coli influence carcinogenesis by inhibiting autophagy and antimicrobial responses in colonic epithelial cells (88). Several nutrients, including the micronutrient vitamin D, influence the microbiota, which then affects the immune system. The immune system and the gut microbiome are closely related. The gut microbiota balances tolerance and immunity by promoting the growth and response of the immune system, which in turn controls intestinal homeostasis.
Role of vitamin D in shaping gut microbiota and immune health. As VDR is expressed and activated under stimulation in multiple immune cell lineages, including B cells, neutrophils, macrophages, dendritic cells, CD4 and CD8 T cells, and neutrophils, vitamin D plays a crucial role in this intricate network (83). In an 8-week human interventional study, vitamin D supplementation (a weekly dosage of 980 IU/kg body weight of vitamin D3) dramatically altered the composition of the gut microbiome, decreasing opportunistic infections, and enhancing bacterial richness. In particular, a considerable decline was noted in the class of Gammaproteobacteria, which includes Pseudomonas species and Escherichia/Shigella species. Mucosal CD8+ T cells are presumably secreting factors that affect gut microbes. These immune cells express a high level of VDR; when vitamin D is supplemented, the ratio of naive to effector T cells changes (80). CD8+ T effector cells lower the inflammatory environment by synthesizing calcitriol, which allows beneficial bacteria (such as Bacteroidetes) to surpass opportunistic pathogens (89).
Impact of vitamin D-rich diet on gut microbiome and CRC risk. Vitamin D deficiency worsens dysbiosis associated with CRC, reduces the number of bacteria that produce butyrate, and heightens chronic inflammation, which compromises immunity (90). The fecal microbiota composition of patients with CRC and control participants was investigated in a recent case-control study to assess the role of the microbiome and nutrition, including vitamin D, in the development of CRC and the regulation of inflammatory markers (91). Studies have demonstrated a negative correlation between high omega-3 polyunsaturated fatty acid and vitamin D intake, and the incidence of CRC (92). In contrast, a diet high in carbohydrates and low in fish fatty acid is strongly associated with an increased risk of CRC (93). Dietary factors have been observed to considerably moderate the influence of the gut microbiome (Bifidobacterium/Escherichia genus ratio) on CRC risk (94). Furthermore, pro-inflammatory species, such Parvimonas micra, F. nucleatum, and B. fragilis were more prevalent in the gut microbial profile of patients with CRC than in controls, which was linked to a larger abundance of Bacteroidetes and Bifidobacterium species (95).
The association between intestinal inflammation and cancer is well established (96). In individuals with IBD, the duration of colitis is correlated with the degree of inflammation and severity of the illness, increasing the risk of CRC (97). Interestingly, the onset of IBD has also been linked to vitamin D deficiency, strengthening their connection (98). In a human study, the gut microbiome composition of patients with IBD was correlated with seasonal changes in circulating calcitriol levels. One study found that during the summer/autumn period, when light exposure (and 1,25-D synthesis) is higher, the abundance of bacterial genera typical of inflammation, such as Eggerthella lenta, Fusobacterium spp., Bacteroides spp., Collinsella aerofaciens, and Helicobacter spp., is reduced (99).
Increased dietary vitamin D supplementation reduced inflammation, dysplasia, and tumor incidence in a recent mouse model of inflammation-associated colon cancer. It is also linked to lower expression of MAPK and NF-κB during the acute inflammatory stage of the disease (100). Many p53-up-regulated modulators of apoptosis (PUMA) are frequently observed in cancer cells where they interact with VDR genes (101). By suppressing PUMA and inhibiting NF-κB, VDR signaling inhibits apoptosis and prevents the proliferation of injured cells (102). SCFAs, in particular butyrate, regulate VDR activation (103). The immune system, vitamin D levels, and gut microbiota are closely associated in CRC. Further investigation into the anti-inflammatory, anti-proliferative, and chemopreventive effects of vitamin D analogs was performed in a mouse model of colitis-associated colon cancer (104). This study evaluated the down-regulation of the growth-promoting c-Myc gene, pro-inflammatory COX-2, and the inhibition of the ERK activation pathway in the premalignant phase (104). In experimental models of colitis, calcitriol has been shown to boost both antimicrobial activity and tolerogenic responses in CRC and immunological intestinal homeostasis. Calcitriol suppresses the production of IFN-γ and IL-17 from T cells both in vitro and in vivo. It also causes Foxp3+ Treg cells to produce the anti-inflammatory cytokine IL-10 and type 3 innate lymphoid (ILC3) cells to produce the antimicrobial cytokine IL-22. Although clinical studies with a prospective research design are lacking, these data suggest a preventive and anti-proliferative effect of calcitriol in CRC carcinogenesis (102, 105).
Completed Clinical Trials on Calcitriol Intervention in Patients With CRC
Studies to clarify the role of calcitriol in the prevention and management of CRC are ongoing. To date, only a small number of large-scale clinical trials have been performed to explain the efficacy of vitamin D treatment in patients with CRC. A comprehensive overview of the completed clinical trials on vitamin D intervention in patients with CRC, providing key insights into the study design, participant characteristics, intervention strategies, and notable findings, is presented in Table II (106-113). Several randomized controlled trials have explored the effects of vitamin D supplementation on the incidence, recurrence, and mortality in CRC. Some trials have demonstrated potential benefits, such as reduced CRC incidence with combined vitamin D and calcium supplementation or improved disease-free survival in specific subsets of patients with CRC. In contrast, other studies have reported no significant preventive effects. However, the optimal dose, timing, and duration of vitamin D supplementation remain unclear. A comprehensive understanding of the mode of action and potential interactions between vitamin D and other factors is essential for harnessing its full potential in the battle against CRC.
Summary of completed clinical trials on calcitriol intervention in patients with colorectal cancer (CRC).
Further large-scale, well-designed clinical trials are warranted to validate the therapeutic potential of vitamin D in CRC treatment. These trials should incorporate appropriate patient selection, standardized treatment protocols, and long-term follow-up assessments for the comprehensive evaluation of the efficacy and safety of vitamin D interventions. Rigorous clinical studies are warranted to provide robust evidence to support the integration of vitamin D as a therapeutic agent into standard CRC treatment guidelines.
Future perspectives on vitamin D treatment in patients include personalized approaches, combination therapies, nanotechnology-based delivery systems, immunomodulation, gut microbiota interactions, epigenetic approaches, and comprehensive long-term outcome assessments. Continued research and clinical trials are necessary to translate these potential avenues into clinically effective strategies, ultimately improving treatment outcomes and quality of life in patients with CRC.
Conclusion
Increasing evidence from molecular and genetic data supports the antitumor actions of vitamin D against CRC, which rely on various cellular mechanisms, including anti-proliferation, sensitization towards apoptosis, pro-differentiation, and inhibition of angiogenesis. Epidemiological studies have consistently indicated an inverse relationship between plasma calcitriol levels and the incidence of CRC. Previously available data have indicated that calcitriol is crucial in shaping the gut microbiome, affecting its composition and activity. Calcitriol reduces the risk of CRC by diminishing inflammation and boosting immune responses, establishing its importance in gut health and cancer prevention. Moreover, clinical trials revealing the antitumor effects of vitamin D on the survival of patients with CRC are scarce; therefore, further research is warranted from this perspective.
Footnotes
Authors’ Contributions
Asma Rafique: Data curation, Formal analysis, Investigation, Methodology, Visualization, Writing – original draft, Writing – review and editing. Young-Sang Koh: Conceptualization, Formal analysis, Funding acquisition, Methodology, Project administration, Resources, Supervision, Validation, Visualization, Writing – original draft, Writing – review and editing. All Authors have critically read and approved the final manuscript.
Conflicts of Interest
No potential conflicts of interest were reported by the Authors.
Funding
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (RS-2023-00270936).
Artificial Intelligence (AI) Disclosure
No artificial intelligence (AI) tools, including large language models or machine learning software, were used in the preparation, analysis, or presentation of this manuscript.
- Received February 20, 2026.
- Revision received April 13, 2026.
- Accepted April 14, 2026.
- Copyright © 2026 The Author(s). Published by the International Institute of Anticancer Research.
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
References
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- Article
- Abstract
- Introduction
- Methods
- Vitamin D Sources and Metabolism
- Modes of Action of Calcitriol
- Non-genomic Actions
- Immunomodulatory Actions of Calcitriol
- Gut Microbiome and CRC
- Impact of Calcitriol on Gut Microbiota
- Interplay Between Microbiome and Calcitriol in CRC Prevention
- Completed Clinical Trials on Calcitriol Intervention in Patients With CRC
- Conclusion
- Footnotes
- References
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