High-dose vitamin D3 supplementation (4,000 IU/day for 12 weeks) significantly improved vitamin D status and was associated with modest improvements in glycemic control in obese patients with T2DM, without affecting blood pressure, calcium, or body weight.
Key Findings
Results
Serum 25(OH)D concentration increased significantly in the supplementation group compared to controls after 12 weeks.
The supplementation group received 4,000 IU/day of vitamin D3 for 12 weeks; the control group received no supplementation.
Mean change in serum 25(OH)D was +23.7 ng/mL in the supplementation group versus +1.3 ng/mL in the control group.
The difference was statistically significant (p < 0.001).
Study included 200 patients with T2DM allocated to supplementation (n = 100) or control (n = 100) groups.
Results
Fasting serum glucose (FSG) decreased significantly in the vitamin D supplementation group after 12 weeks.
Mean change in FSG in the intervention group was Δ −0.4 mmol/L (p = 0.02).
No significant changes in FSG were observed in the control group.
All patients were treated with metformin monotherapy.
The study design was a 12-week prospective cohort study.
Results
HbA1c decreased significantly in the vitamin D supplementation group after 12 weeks.
Mean change in HbA1c in the intervention group was Δ −0.6% (p = 0.01).
No significant changes in HbA1c were observed in the control group.
The target HbA1c threshold used in the analysis was ≤ 6.5%.
Results
Vitamin D3 supplementation did not significantly affect systolic or diastolic blood pressure, serum calcium, or BMI.
No significant changes were observed in systolic BP, diastolic BP, serum calcium, or BMI in either group.
These parameters were prespecified as secondary outcomes.
The lack of effect on BMI indicates the glycemic improvements were not mediated by weight loss.
Results
Higher baseline FSG, longer diabetes duration, and higher BMI were identified as independent predictors of failure to achieve target HbA1c ≤ 6.5%.
Higher baseline FSG: OR 1.34 (95% CI 1.12–1.61).
Longer diabetes duration: OR 1.28 (95% CI 1.07–1.54).
Higher BMI: OR 1.21 (95% CI 1.01–1.47).
Predictors were identified using logistic regression analysis.
Hoffmann K, Bryl W, Bhongade B, Avagimyan A, El-Tanani M, Rabbani S, et al.. (2026). Vitamin D supplementation and selected metabolic parameters in patients with type 2 diabetes and obesity: a prospective observational study.. Frontiers in endocrinology. https://doi.org/10.3389/fendo.2025.1750040