However, “these effects are often modest and may diminish over time,” wrote researchers at Universitas Diponegoro in Indonesia in Nutrients.
The review assessed the impact of vitamin D supplementation on glycemic control and inflammation in patients with T2DM, focusing on key biomarkers such as fasting plasma glucose, insulin levels and HOMA-IR.
“The variability in outcomes across the studies highlights the importance of considering factors such as baseline vitamin D levels, dosage, duration of treatment and individual patient characteristics,” the scientists wrote while calling for further research to fully understand the implications of the supplement on the management of T2DM.
Vitamin D and diabetes
Vitamin D plays a role in regulating insulin secretion and sensitivity, and deficiency has been observed in metabolic syndrome, obesity and T2DM.
Hypovitaminosis D—a lack of vitamin D—has been linked to decreased insulin secretion in diabetic populations.
“In older males with hypovitaminosis D, insulin secretion is hyperresponsive to a glucose challenge,” the current study noted.
The researchers explained that there are several mechanisms by which vitamin D may support insulin production and secretion. These include inducing cellular growth in β cells and affecting calcium concentrations, triggering insulin release.
Although evidence suggests the positive outcomes of vitamin D in T2DM, the researchers noted that a few studies have assessed both “glycemic and inflammatory markers within a single comprehensive evaluation,” in order to “provide the most current and detailed qualitative summary of the evidence.”
Study details
The systematic review sourced clinical trials involving T2DM patients undergoing treatment with vitamin D, published between 2014 and 2024. The meta-analysis included nine studies with these criteria, with findings based on biomarkers for diabetes and inflammation.
The researchers noted that the quality of the included studies was generally moderate, with three showing a potential risk of bias.
The results indicated significant differences between the experimental and control groups for insulin levels, HOMA-B, hs-CRP and HbA1c levels at the 12-week follow-up. At the 24-week follow-up, only HOMA-IR was observed to be statistically significant. No significant effects were found for fasting plasma glucose (FPG) at either time point.
The researchers pointed out that previous research has found that vitamin D supplementation can significantly reduce fasting plasma glucose, and the heterogeneity observed in the included studies may account for the insignificant result.
They also commented that while the reduction in insulin levels was significant at 12 weeks, the effects did not persist over time, “suggesting a potential short-term benefit of vitamin D.”
“While vitamin D supplementation shows potential benefits in managing T2DM, these effects are complex and may require individualized approaches,” the study concluded.
“High-dose vitamin D regimens might be necessary for rapid correction in certain populations, while maintenance doses could sustain benefits in the longer term.”
The researchers called for future stratified studies to understand the mechanism of action.
Source: Nutrients 2025, 17(15), 2489. doi: 10.3390/nu17152489. “The Impact of Vitamin D Supplementation on Fasting Plasma Glucose, Insulin Sensitivity, and Inflammation in Type 2 Diabetes Mellitus: A Systematic Review and Meta-Analysis”. Authors E. Probosari et al.