Effect of vitamin D supplementation on metabolic syndrome parameters in adults: a systematic review of randomized controlled trials exploring the role of baseline vitamin D status and supplementation dose.

Introduction: Metabolic syndrome (MetS) is a cluster of cardiometabolic abnormalities including central obesity, dysglycemia, hypertension, and dyslipidemia. Vitamin D deficiency is highly prevalent among adults with MetS and may contribute to insulin resistance, inflammation, and adverse lipid profiles. Although vitamin D supplementation has been reported to improve MetS parameters, limited information is available on whether these effects differ by baseline vitamin D status or supplementation dose, information essential for clinical decision-making. Methods: This systematic review evaluated the effect of oral vitamin D supplementation on MetS components in adults and examined whether treatment effects appeared differ according to baseline vitamin D status and supplementation dose. A comprehensive search of PubMed/MEDLINE, Embase, Web of Science, and CENTRAL was conducted following PRISMA guidelines. Given the anticipated clinical and methodological heterogeneity across eligible trials, a narrative synthesis rather than meta-analysis was planned a priori. Risk of bias was assessed using the Cochrane Collaboration Risk of Bias Tool. Eligible RCTs included those evaluating oral vitamin D supplementation for at least 8 weeks in adults diagnosed with metabolic syndrome or presenting at least 3 of 5 MetS components. In addition, comparator groups included placebo or dietary-instruction-only controls. Outcomes of interest included waist circumference, triglycerides, HDL-cholesterol, blood pressure, fasting glucose, homeostatic model assessment of insulin resistance (HOMA-IR), glycated hemoglobin (HbA1c), and body mass index (BMI). Results: Eight randomized controlled trials (634 participants) met the full eligibility criteria and were included in the narrative synthesis. Vitamin D supplementation consistently increased serum 25(OH)D concentrations across trials. Significant improvements were reported in triglycerides and insulin resistance indices in a subset of trials, and in selected inflammatory markers, waist circumference, and BMI in individual studies, while blood pressure, HDL-cholesterol, and fasting glucose showed no consistent change across the majority of trials. Findings were most favorable among participants with baseline vitamin D deficiency and in trials using higher supplementation doses, although these patterns are exploratory, as no trial was powered for formal subgroup comparison. Risk-of-bias assessment indicated that seven of the eight evaluated trials had some concerns or high risk of bias in at least one domain, limiting the overall certainty of the evidence. Conclusions: Vitamin D supplementation shows slight benefit for specific MetS components, particularly among vitamin D-deficient adults, but effects were inconsistent across trials and metabolic parameters. Substantial variability in study design, dosing strategies, and population characteristics limits definitive conclusions. Adequately powered, standardized randomized trials are needed to clarify vitamin D's role in metabolic syndrome management. Systematic review registration: CRD420261421121.