Vitamin D is obtained mainly from sun exposure (skin synthesis), food (oily fish such as trout, salmon, tuna, mackerel, and fish liver oils), and vitamin supplements. Prohormonal forms of vitamin D require hydroxylation in the liver to 25-hydroxyvitamin D (25(OH)D) and in the kidney to their active form, that is 1,25-dihydroxyvitamin D (1,25(OH)
2D
3). Vitamin D
3 exerts its biological functions by interacting with and activating the nuclear vitamin D receptor (VDR). In vitro studies point toward uterine myoma cells exhibiting lower levels of VDR expression
[15][16]. In addition, a negative correlation between decreased levels of vitamin D receptor (VDR) and increased levels of estrogen and progesterone receptors (ER-α, PR-A, PR-B) was observed in myoma tumors
[17]. It is estimated that Vitamin D deficiency affects 25–50% (possibly more) of patients
[18]. Several studies found that insubstantial levels of Vitamin D can contribute to the development of UFs in African American, Caucasian, and Asian women alike
[19][20][21][22][23][24][25][26][27]. Vitamin D
3 deficiency activates fibroid cell growth, exacerbates DNA damage, and reduces DNA repairability; it promotes uncontrolled proliferation and fibrosis, and increases chronic inflammation. In combination, these processes are highly tumorogenic
[28][29]. Othman et al. demonstrated that, in comparison to normal myometrium, myoma tissue contains significantly lower concentrations of 1,25(OH)
2D
3. Additionally, an overexpression of 24-hydroxylase was found in myoma, which may further suppress the anti-tumor effect of 1,25(OH)
2D
3 and exacerbate vitamin D deficiency in the tissue
[30]. A recent study by Ciebiera et al. revealed an inverse correlation between lower 25(OH)D serum concentrations and increased serum transforming growth factor β3 (TGF-β3) concentrations in women affected by fibroids
[31]. This growth factor can be associated with increased fibrosis and ECM accumulation in myoma
[32][33]. Findings on inverse correlation between serum levels of 25(OH)D and fibroid volume vary, ranging from significant to insignificant association
[19][34][35]. No correlation was however observed between 25(OH)D serum levels and number of fibroids
[34]. Some data suggest that Vitamin D supplementation reduces leiomyoma cell proliferation and thus prevents leiomyoma growth
[23][36][37][38]. A significant downregulation of ER-α, PR-A, PR-B, and steroid receptor coactivators in human myoma cells may be one of the mechanisms—an effect similar to that observed during the course of hormone therapy with GnRH analogues and ulipristal acetate (UPA)
[17][39][40]. Halder et al. and Li et al. point to the antifibrotic activity of Vitamin D
[41][42]. Vitamin D
3 inhibited TGF-β3-induced protein expression and all TGF-β3-mediated effects involved in the fibrotic processes in leiomyoma. Other studies indicate that increasing Vitamin D levels by one unit can reduce the risk of developing UFs by 4–8%
[26][31]. Hajhashemi et al. confirmed a significant decrease in leiomyoma size after 10 weeks of vitamin D administration
[43]. A slight, statistically insignificant reduction in fibroid volume after a short-term Vitamin D supplementation was observed by Arjeh et al.
[44], Davari Tanha et al.
[45], and Suneja et al.
[46] in patients with hypovitaminosis D (12, 16, and 8 weeks, respectively). Ciavattini et al. reported a similar effect after 12 months of Vitamin D
3 supplementation
[34]. Vitamin D
3 supplementation may inhibit the growth of UFs, reduce fibroid-related symptoms, and reduce the need for surgical or medical treatment for progression of fibroids
[34][45][46]. Especially, a long-term course of treatment can have antiproliferative, antifibrotic, and proapoptotic effects in leiomyoma, as demonstrated by Corachán et al.
[47], a finding consistent with other studies in vitro
[28][41][42]. Beside apoptosis induction, Vitamin D suppresses catechol-O-methyltransferase (COMT) expression and activity in myoma cells—an enzyme that plays a vital role in myoma formation
[41]. In fact, physiological concentrations of vitamin D can effectively inhibit the growth of myoma cells
[28]. 1,25(OH)
2D
3 can significantly reduce the expression of ECM-associated proteins and structural actin fibers in human leiomyoma cells, as observed by Halder et al.
[15]. This effect was a consequence of previous significant induction of nuclear vitamin D receptor (VDR) expression by 1,25(OH)
2D
3 in a concentration-dependent manner. In another study, Halder et al. observed a significant reduction in MMP-2 and MMP-9 mRNA levels, as well as a reduction in MMP-2 and MMP-9 protein levels in uterine fibroid cells in a concentration-dependent manner and concluded that through this mechanism, 1,25(OH)
2D
3 might limit fibroid growth and ECM deposition
[48]. Al-Hendy et al. observed that 1,25(OH)
2D
3 spontaneously induced its own VDR, while significantly downregulating the expression of sex steroid receptors (ERs and PRs) and receptor coactivators, which affected myoma formation and growth; hence, 1,25(OH)
2D
3 suppressed estrogen-induced proliferation in leiomyoma cells
[17]. Cell proliferation and extracellular matrix production in myoma tumors can be affected by Vitamin D as it can suppress tumor-promoting Wnt4/β-catenin expression and reduce activation of mTOR signaling in human UF cells
[49]. As observed by Corachán et al., Vitamin D inhibits the Wnt/β-catenin and TGFβ pathways, reducing proliferation and extracellular matrix formation, in different molecular subtypes of uterine myomas (MED12-mutated and wild-type human tumors)
[50]. Ali et al. hypothesized that myoma tumor progression might be inhibited by recovering the damaged DNA repair system
[51]. They showed in vitro that vitamin D
3 treatment significantly reduces DNA damage, restores the normal DNA damage response, and is accompanied by induction of VDR in fibroid cells
[51]. DNA repair in cells exposed to classic DNA damage inducers in UF pathogenesis (endocrine-disrupting chemicals -EDCs) was achieved by a 1,25(OH)
2D
3 treatment of myoma cells in animal models
[52]. Clinical trials show promising results in patients with UFs and hypovitaminosis D treated with Vitamin D
3 supplementation, revealing a significant decrease in tumor size and numbers
[43][53]. At the time of the review, search results pointed to a randomized trial (RCT) being conducted in women at reproductive age affected with uterine myoma, aiming to evaluate whether supplementation with Vitamin D
3 could reduce the risk and inhibit the growth of fibroids
[54]. Results of the evaluation of Vitamin D
3 effects in this particular group of women can be of value in everyday gynecological practice.