Vitamin D is a fat-soluble vitamin that plays an essential role in maintaining the calcium and phosphorus balance, promoting bone health. In recent years, it has been recognized for its essential biological functions in immune regulation, inflammation inhibition, cell proliferation, and differentiation [1, 2, 3]. Recurrent pregnancy loss (RPL) is a common pregnancy complication that has generated significant concern [4]. Although the exact etiology remains unclear, more and more evidence shows that vitamin D deficiency (VDD) may be closely related to the occurrence and development of RPL [5, 6, 7]. Previous studies have found that with the increase of vitamin D level, the clinical pregnancy rate (CPR) also increases, while the miscarriage rate (MR) decreases [6, 8]. These findings support the crucial role of vitamin D in pregnancy, especially in avoiding RPL.

Glucose metabolism plays a vital role during pregnancy, providing necessary energy and nutrition for the normal development of embryos [9, 10]. Some studies have found a complex interaction between vitamin D and glucose metabolism [11, 12, 13, 14]. Vitamin D receptor (VDR) is widely expressed in islet cells, suggesting that vitamin D may affect glucose metabolism through insulin secretion and resistance [15, 16]. Therefore, exploring the relationship between VDD and RPL, as well as its potential mechanistic involvement in glucose metabolism, is essential for understanding the etiology of RPL and finding preventative and therapeutic strategies.

This study explores the potential role of vitamin D in the occurrence and development of RPL and whether there is a mechanism related to glucose metabolism disorder. The result of the study will help provide a theoretical basis for clinical intervention research and individualized treatment in the future.

RPL is a common adverse outcome in pregnancy with a multifactorial etiology. Although the exact mechanisms remain unclear, increasing evidence suggests that VDD may play a crucial role in the onset and progression of RPL [22]. In this study, we observed lower serum 25(OH)D levels and a higher prevalence of VDD among women with RPL compared to control subjects. Concurrently, FINS levels and HOMA-IR were elevated in the RPL group and negatively correlated with serum 25(OH)D concentrations, suggesting a potential link between vitamin D status and glucose metabolism in the pathogenesis of RPL.

Previous studies have reported a high prevalence of VDD among women with RPL [23]. A meta-analysis demonstrated that VDD is associated with an increased risk of miscarriage, with women receiving vitamin D supplementation having significantly lower odds of miscarriage (OR: 1.94, 95% CI: 1.25–3.02) [8]. In our study, patients in the RPL group exhibited lower serum 25(OH)D levels compared to controls (31.16 nmol/L vs. 35.05 nmol/L), along with a higher prevalence of VDD (46.10% vs. 34.90%). These findings align with previous research and highlight the potential role of vitamin D in maintaining pregnancy health. Furthermore, multivariate logistic regression analysis revealed that increasing serum 25(OH)D levels were significantly associated with a decreased risk of RPL (aOR: 0.98, 95% CI: 0.97–0.99), suggesting that VDD may act as a risk factor for RPL.

However, the underlying mechanisms linking low vitamin D levels to increased miscarriage risk remain unclear. One widely accepted hypothesis is that vitamin D exerts immunomodulatory effects that are critical for successful pregnancy. Ota et al. [24] reported that 1,25(OH)₂D₃ significantly inhibits the cytotoxicity of natural killer (NK) cells, downregulates pro-inflammatory cytokines such as interferon-$\gamma$ and tumour necrosis factor-$\alpha$, and upregulates anti-inflammatory and implantation-supportive cytokines, including interleukin (IL)-10, IL-1$\beta$, vascular endothelial growth factor, and granulocyte colony-stimulating factor. This promotes a shift from a type I to a type II immune response conducive to maintaining pregnancy [24]. Consistently, previous research has demonstrated a heightened type I immune response in women with RPL [22], implying that VDD may exacerbate NK cell activity and contribute to pregnancy failure. Although the immunomodulatory role of vitamin D is widely recognized and likely relevant to RPL, our study did not measure immune-related indicators, which represents a limitation. Future research, including immune biomarkers, is necessary to clarify the potential influence of vitamin D on immune regulation in RPL.

In addition to peripheral immune modulation, vitamin D is also thought to influence the maternal-fetal interface. A study has found significantly reduced levels of 25(OH)D, transforming growth factor-beta, and VDR in the decidual tissue of RPL patients, alongside elevated levels of IL-17 and IL-23 [25]. Given that 25(OH)D negatively regulates the IL-23/IL-17 axis, these findings suggest that insufficient vitamin D may impair local immune tolerance at the maternal-fetal interface, further increasing the risk of pregnancy loss [26].

Furthermore, glucose metabolism disturbances may represent another pathway linking VDD and RPL. Previous research has identified elevated insulin levels and HOMA-IR in women with RPL, both in natural conception and in those who used assisted reproduction [27, 28, 29]. In our cohort, RPL patients had higher FINS and HOMA-IR, and their increase was associated with greater RPL risk. Previous literature suggests that insulin resistance may impair endometrial vascularization, embryo development, and implantation, or lead to hypercoagulability and androgen excess, all of which may contribute to adverse pregnancy outcomes [30, 31, 32].

Importantly, our mediation analysis revealed that FINS and HOMA-IR partially mediated the association between 25(OH)D levels and RPL risk, accounting for 10.9% and 10.7% of the total effect, respectively. These findings suggest that glucose metabolism may serve as a mechanistic bridge between VDD and RPL. One potential mechanism is that VDD leads to reduced expression of insulin receptors on insulin-responsive cells, thereby decreasing insulin sensitivity and resulting in elevated circulating insulin levels [33]. Elevated insulin, in turn, may reduce the expression of glycoproteins at the implantation site and increase homocysteine levels, both of which could impair endometrial receptivity [34]. These alterations may disrupt endometrial blood flow and vascular integrity, ultimately compromising successful embryo implantation. Although subgroup analysis indicated that patients with VDD had higher FINS levels and HOMA-IR, and slightly worse reproductive outcomes, these trends did not reach statistical significance, likely due to limited sample size. Larger studies are needed to validate these associations.

Generally speaking, the results of this study emphasize the relationship between low levels of vitamin D and RPL and its potential regulation mechanism of glucose metabolism. However, it should be pointed out that this study has some limitations, including the relatively small sample size and the cross-sectional characteristics of the research design. Although this study is cross-sectional in design, all serum 25(OH)D measurements were conducted before pregnancy, and pregnancy outcomes were collected retrospectively or through follow-up. This temporal sequence helps mitigate, although not eliminate, the concern of reverse causality. Nevertheless, prospective studies are still warranted to clarify the causal direction of the association between vitamin D status and RPL. Therefore, further large-scale research and clinical trials are necessary to deeply understand the role of vitamin D in RPL and provide more powerful theoretical support for future intervention and treatment strategies. To sum up, vitamin D may be a potential therapeutic target, which is expected to play an essential role in preventing and managing RPL.

In summary, our study supports the hypothesis that VDD is associated with an increased risk of RPL and may act, at least in part, through dysregulation of glucose metabolism. FINS and HOMA-IR partially mediated the association between 25(OH)D and RPL, suggesting a novel mechanistic link. While these findings are promising, they are observational and cross-sectional. Future large-scale, prospective, and mechanistic studies are needed to confirm causality and elucidate the precise role of vitamin D and insulin pathways in the pathogenesis of RPL. Vitamin D may serve as a potential target for preventive or therapeutic interventions aimed at improving reproductive outcomes in women at risk of RPL.

aOR, adjusted odds ratio; CPR, clinical pregnancy rate; CI, confidence interval; E2, estradiol; FBG, fasting blood glucose; FINS, fasting insulin; FSH, follicle-stimulating hormone; HOMA-IR, homeostasis model assessment of insulin resistance; HDL-C, high-density lipoprotein cholesterol; IR, insulin resistance; LBR, clinical live birth rate; LDL-C, low-density lipoprotein cholesterol; LH, luteinizing hormone; MR, miscarriage rate; NK, natural killer; OR, odds ratio; RCS, restricted cubic spline; RPL, recurrent pregnancy loss; TC, total cholesterol; TG, triglycerides; VDD, vitamin D deficiency; VDR, vitamin D receptor.

The corresponding author can provide data with a reasonable request.

KW and FW jointly designed the study. RW and CW jointly collected the data; RW performed data analysis and wrote the statistical results section; CW verified the data, generated the figures, and contributed to writing the results description. All authors contributed to editorial changes in the manuscript. All authors read and approved the final manuscript. All authors have participated sufficiently in the work and agreed to be accountable for all aspects of the work.

The study was conducted in accordance with the Declaration of Helsinki, and the protocol was approved by the Ethics Committee of the Second Hospital of Lanzhou University (No: 2023 A-532). The need for written informed consent was waived by the Ethics Committee of the Second Hospital of Lanzhou University due to the retrospective nature of the study.

The research was supported by the Medical Innovation and Development Project of Lanzhou University (Grant No. lzuyxcx-2022-137), The Science Foundation of Lanzhou University (Grant No. 054000229) and A Real-World Study on RPL in China (Grant No. YJS-BD-19).

The authors declare no conflict of interest.