Effects of vitamin D and exercise on glycemic control and circulating 25(OH)D in adults with metabolic disorders: A systematic review and meta-analysis with GRADE assessment

Background However, vitamin D and exercise are both key factors in regulating glycemic control; no previous study has examined their combined effects in adults with metabolic disorders. Hence, the current systematic review and meta-analysis aimed to evaluate the impact of vitamin D supplementation and exercise, both individually and in combination, on glycemic control and circulating 25(OH)D levels in adults with metabolic disorders. Methods This systematic review and meta-analysis followed PRISMA guidelines. SCOPUS, PubMed, Web of Science, and Google Scholar were searched until August 2025 for randomized controlled trials examining combined vitamin D supplementation and exercise in adults with metabolic disorders. Data extraction and risk of bias assessment were performed independently using the Cochrane RoB 2.0 tool, and evidence certainty was graded via GRADE. Random-effects models were applied using Comprehensive Meta-Analysis (version 3). Results Fifteen randomized controlled trials were analyzed. Vitamin D supplementation plus exercise significantly improved fasting blood glucose (FBG) (WMD: −16.59 mg/dL, 95% CI: −20.69 to − 12.48, P < 0.001), fasting insulin (WMD: −1.54 µU/mL, 95% CI: −2.34 to − 0.73, P < 0.001), HbA1c (WMD: −0.97%, 95% CI: −1.46 to − 0.48, P < 0.001), HOMA-IR (WMD: −0.98, 95% CI: −1.54 to − 0.42, P = 0.001), vitamin D (WMD: 14.39 ng/mL, 95% CI: 10.39 to 18.38, P < 0.001) compared to control. Vitamin D increased significantly in the aerobic group, while FBG, HOMA-IR, and fasting insulin declined in both aerobic and anaerobic subgroups. FBG, HOMA-IR, and vitamin D improved after 8- and 12-week interventions, whereas fasting insulin decreased only after 8 weeks. Conclusion The results of this study indicated that vitamin D supplementation alongside exercise can reduce FBS, fasting insulin, HbA1c, and HOMA-IR more effectively than either intervention alone or the control group. Furthermore, vitamin D supplementation combined with exercise, aerobic exercise can increase circulating 25(OH)D levels more than the other comparison groups. Nevertheless, additional well-designed studies are needed to strengthen and confirm these findings. Trial registration: PROSPERO, registration number CRD420251131187, retrospectively registered on 24 August 2025.