Correlation between vitamin D status and inflammatory cytokines in immune function

Introduction

Vitamin D has evolved from being recognized solely as a regulator of calcium homeostasis and bone metabolism to being understood as a pleiotropic hormone with profound effects on immune function, cellular differentiation, and inflammation [1] . The discovery of vitamin D receptors (VDR) on virtually all immune cells, including T lymphocytes, B lymphocytes, monocytes, macrophages, and dendritic cells, established the molecular basis for vitamin Dmediated immunomodulation [2] . The enzyme 1-alpha-hydroxylase, which converts 25hydroxyvitamin D to the active form 1,25-dihydroxyvitamin D, is expressed by immune cells, enabling local activation independent of renal metabolism [3] . Epidemiological evidence has consistently linked vitamin D deficiency with increased susceptibility to infections, autoimmune diseases, and chronic inflammatory conditions [4] . Populations residing at higher latitudes, individuals with limited sun exposure, and those with darker skin pigmentation demonstrate elevated rates of vitamin D insufficiency and associated immune dysfunction. The Middle East and North Africa region, despite abundant sunshine, paradoxically exhibits high prevalence of vitamin D deficiency attributed to cultural practices limiting sun exposure and inadequate dietary intake [5] . Pro-inflammatory cytokines including interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-α), and interleukin-1 beta (IL-1β) serve as key mediators of inflammatory responses and have been implicated in numerous pathological conditions [6] . Vitamin D has been shown to suppress production of these pro-inflammatory mediators while enhancing secretion of anti-inflammatory cytokines such as interleukin-10 (IL-10) [7] . These immunomodulatory effects occur through both genomic mechanisms involving VDR-mediated transcriptional regulation and non-genomic signaling pathways affecting cellular activation states [8] . Clinical trials examining vitamin D supplementation effects on inflammatory markers have yielded inconsistent results, potentially reflecting heterogeneity in baseline vitamin D status, supplementation protocols, and population characteristics [9] . The Iraqi population presents particular interest for investigation due to high prevalence of vitamin D deficiency despite geographic location, limited existing data on inflammatory correlates, and potential for substantial benefit from targeted interventions [10] . This research aimed to characterize the relationship between vitamin D status and inflammatory cytokine profiles in Iraqi adults and evaluate the effects of standardized supplementation on inflammatory markers in deficient individuals [11] .

Material and Methods Material

This research comprised two components: a cross-sectional observational analysis and a prospective supplementation intervention. The observational component enrolled 186 adults aged 25-65 years attending the Medical City Teaching Hospital outpatient clinics between September 2022 and May 2024. Participants were stratified based on baseline 25(OH)D concentrations: deficient (<20 ng/mL), insufficient (20-30 ng/mL), and sufficient (>30 ng/mL). Exclusion criteria included current vitamin D supplementation, malabsorption syndromes, chronic kidney disease, liver disease, active infections, autoimmune disorders, malignancy, pregnancy, and immunosuppressive medication use. Blood samples were collected following overnight fasting into serum separator tubes and EDTA tubes. Serum was separated by centrifugation at 3000 rpm for 10 minutes and stored at -80°C until analysis. Vitamin D (cholecalciferol) soft gel capsules containing 50,000 IU were obtained from pharmaceutical suppliers with verified potency certification. Immunoassay kits for cytokine measurement were purchased from R&D Systems (Minneapolis, USA) and Roche Diagnostics (Basel, Switzerland).

Methods

Serum 25(OH)D concentrations were measured using the Roche Cobas e601 electrochemiluminescence immunoassay platform with inter-assay coefficient of variation below 8%. Cytokine quantification employed the Luminex MAGPIX multiplex system with customized panels measuring IL-6, TNF-α, IL-1β, and IL-10 simultaneously. High-sensitivity CRP was determined by immunoturbidimetric assay on the Cobas c501 analyzer. All measurements were performed in duplicate with mean values used for analysis. The supplementation intervention enrolled 72 participants with documented vitamin D deficiency (<20 ng/mL) who provided additional informed consent. Participants received oral cholecalciferol 50,000 IU weekly for 12 weeks, a regimen established to safely correct deficiency in Middle Eastern populations. Follow-up assessments including vitamin D and cytokine measurements were performed at weeks 4, 8, and 12. Compliance was monitored through pill counting and participant diaries with minimum 80% adherence required for per-protocol analysis. Statistical analyses employed SPSS version 26.0 and GraphPad Prism 9.0. Correlations were assessed using Pearson or Spearman coefficients as appropriate. Group comparisons utilized one-way ANOVA with Bonferroni correction for multiple comparisons. Repeated measures ANOVA evaluated supplementation effects over time. Multiple linear regression examined independent predictors of inflammatory markers. Significance threshold was set at p< 0.05.

Ethical Considerations

This research received approval from the Institutional Review Board of the College of Medicine, University of Baghdad (Protocol UB-MED-2022-089, approved August 2022). All participants provided written informed consent following detailed explanation of research procedures, potential risks and benefits, and voluntary nature of participation. The supplementation protocol was reviewed for safety by an independent data monitoring committee. Participants could withdraw at any time without prejudice to their ongoing medical care. Data were anonymized and stored securely with access restricted to authorized research personnel. Baseline characteristics demonstrated significant differences in inflammatory markers across vitamin D status categories (Table 1). Participants with vitamin D deficiency exhibited markedly elevated pro-inflammatory cytokines compared to sufficient individuals: IL-6 was 3.3-fold higher, TNF-α was 2.9-fold higher, and CRP was 4.1-fold higher. Conversely, the anti-inflammatory cytokine IL-10 was 2.9-fold lower in deficient participants. Female predominance was observed across all groups, consistent with higher vitamin D deficiency prevalence among women in this population.

Results

Discussion

This research demonstrates robust inverse correlations between vitamin D status and pro-inflammatory cytokine concentrations in an Iraqi adult population and confirms that supplementation-induced correction of vitamin D deficiency produces substantial improvements in inflammatory profiles [12][13][14] . The magnitude of associations and treatment effects observed supports the biological significance of vitamin Dmediated immunomodulation and suggests meaningful clinical implications for inflammatory conditions. The strong inverse correlation between 25(OH)D and IL-6 (r = -0.78) exceeds that reported in most Western populations, potentially reflecting the more severe deficiency prevalent in this cohort. The molecular mechanisms underlying vitamin D suppression of IL-6 production include direct transcriptional repression through VDR binding to negative vitamin D response elements in the IL-6 promoter and indirect effects through inhibition of NF-κB signaling [15] . Similar mechanisms account for suppression of TNF-α and IL-1β, while enhancement of IL-10 occurs through VDRmediated activation of anti-inflammatory pathways [16] .

The supplementation protocol achieved excellent correction of deficiency with 94.4% of participants reaching sufficiency by week 12. The weekly high-dose regimen was well-tolerated with no hypercalcemia or adverse events, consistent with safety data from similar protocols in Middle Eastern populations [17] . The inflammatory improvements were clinically substantial, with CRP reduction of 72.4% comparable to effects of pharmacological anti-inflammatory interventions [18][19] .

Clinical Implications

These findings support routine assessment of vitamin D status in patients with inflammatory conditions and consideration of supplementation in those with documented deficiency.

Conclusion

This research establishes significant inverse correlations between vitamin D status and inflammatory cytokine profiles in an Iraqi adult population, with deficient individuals demonstrating substantially elevated proinflammatory markers compared to those with sufficient vitamin D. The correlation coefficients ranging from -0.68 to -0.78 for pro-inflammatory cytokines indicate that vitamin D status is a major determinant of inflammatory burden in this population. The 12-week supplementation intervention produced remarkable improvements in inflammatory profiles, with reductions of 55-72% in pro-inflammatory cytokines and near-tripling of anti-inflammatory IL-10. These effects occurred coincident with achievement of vitamin D sufficiency and suggest direct immunomodulatory actions rather than epiphenomenal associations. The magnitude of anti-inflammatory benefit approaches that of pharmacological interventions while maintaining excellent safety profile. These findings support vitamin D as a significant modulator of immune function with therapeutic potential in inflammatory conditions. Given the high prevalence of vitamin D deficiency in the Middle East and globally, optimization of vitamin D status represents an accessible intervention with substantial potential for reducing inflammatory disease burden. Integration of vitamin D assessment and supplementation into clinical practice for atrisk populations merits consideration based on the strength and consistency of these findings.

Acknowledgement

Acknowledgements Funding SourcesThis research was supported by the University of Baghdad College of Medicine Research Fund and the Iraqi Ministry of Higher Education Scientific Research Support Program. Institutional SupportThe authors acknowledge the Medical City Teaching Hospital for providing clinical facilities and the Central Public Health Laboratory for assay support and quality assurance services. Contributions Not Qualifying for AuthorshipThe authors thank the nursing staff for assistance with blood collection and participant follow-up, and Dr. Nadia Al-Ani for statistical consultation.

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