Researchers looked at 31 studies involving nearly 4,000 people with prediabetes to see if taking vitamin D supplements could help prevent type 2 diabetes. They found that vitamin D did help lower blood sugar levels, insulin levels, and a marker called hemoglobin A1c, which shows average blood sugar over time. It also helped reduce triglycerides, a type of fat in the blood. However, vitamin D didn’t significantly affect weight, insulin resistance, or the actual risk of developing diabetes. While these results are promising, scientists say we need more research to be completely sure about vitamin D’s benefits for people with prediabetes.

The Quick Take

  • What they studied: Whether taking vitamin D supplements could help people with prediabetes improve their blood sugar control and lower their risk of developing type 2 diabetes
  • Who participated: Nearly 4,000 people with prediabetes (blood sugar levels higher than normal but not yet diabetic) across 31 different research studies
  • Key finding: Vitamin D supplements reduced fasting blood sugar by about 0.38 mg/dL, lowered insulin levels, reduced hemoglobin A1c (a 3-month blood sugar average) by 0.48%, and decreased triglycerides (blood fats) by 0.39 mg/dL compared to people who didn’t take vitamin D
  • What it means for you: Vitamin D supplementation may help improve some blood sugar markers in people with prediabetes, but it didn’t prevent diabetes or help with weight loss in these studies. Talk to your doctor before starting supplements, as more research is still needed to confirm these benefits

The Research Details

This was an umbrella review, which means researchers looked at 14 previous meta-analyses (studies that combined results from multiple research papers). Those 14 meta-analyses had examined 31 randomized controlled trials—the gold standard type of study where some people get vitamin D and others get a placebo (fake pill) to compare results fairly. The researchers searched multiple scientific databases through July 2025 to find all relevant studies. They used special tools to check the quality of each study and to rate how confident we should be in the findings.

By combining results from many studies, researchers can see the bigger picture instead of relying on just one study, which might have been done differently or had different results by chance. This approach helps identify patterns and gives us more reliable answers about whether vitamin D really helps with blood sugar control.

The researchers used the AMSTAR2 scale to evaluate how well each study was done and the GRADE tool to assess how confident we should be in the results. The fact that they looked at randomized controlled trials (where people are randomly assigned to get vitamin D or placebo) is very strong evidence. However, the researchers noted that results varied between studies, suggesting we need more consistent research before drawing final conclusions

What the Results Show

Vitamin D supplementation showed measurable improvements in several blood sugar markers. Fasting blood sugar (the level when you haven’t eaten) dropped by about 0.38 mg/dL in people taking vitamin D compared to those taking placebo. Insulin levels decreased by about 0.17 µU/mL, which is important because high insulin can indicate your body is struggling to control blood sugar. Hemoglobin A1c, which reflects your average blood sugar over three months, decreased by 0.48%—a meaningful improvement. Triglycerides (a type of fat in your blood linked to heart disease) also decreased by about 0.39 mg/dL. All of these changes were statistically significant, meaning they’re unlikely to have happened by chance.

Vitamin D did not significantly affect insulin resistance (how well your body uses insulin), weight or body mass index (BMI), or the actual risk of developing type 2 diabetes. These negative findings are important because they suggest vitamin D’s benefits may be limited to improving certain blood sugar markers rather than preventing diabetes itself or helping with weight management.

Previous research had shown mixed results about vitamin D and diabetes risk, with some studies suggesting benefits and others finding no effect. This comprehensive review helps clarify that vitamin D does improve some specific blood sugar measurements, but the improvements are modest and don’t necessarily translate to preventing diabetes or major metabolic changes. The findings align with growing evidence that vitamin D plays a role in blood sugar control, but it’s not a standalone solution.

The researchers noted several important limitations: the improvements in blood sugar markers were small, results varied significantly between different studies, and vitamin D didn’t actually reduce diabetes risk or help with weight loss. The studies included varied in how much vitamin D they gave, for how long, and who participated, which makes it harder to draw firm conclusions. The researchers emphasized that more high-quality, consistent studies are needed before we can confidently recommend vitamin D specifically for preventing diabetes in people with prediabetes

The Bottom Line

If you have prediabetes, vitamin D supplementation may modestly help improve blood sugar control markers, but it should not replace proven strategies like exercise, healthy eating, and weight loss. Discuss with your doctor whether vitamin D supplementation is appropriate for you, especially since the benefits shown here are small and don’t prevent diabetes. Confidence level: Moderate—the evidence is promising but not definitive

People with prediabetes who are interested in additional strategies to improve blood sugar control should know about these findings. People with vitamin D deficiency may have additional reasons to supplement beyond blood sugar control. However, people without prediabetes shouldn’t expect vitamin D to prevent diabetes. Anyone considering supplements should consult their healthcare provider first

The studies reviewed typically lasted several months to a year. If vitamin D helps you, you’d likely see improvements in blood sugar markers within 3-6 months, though the changes are modest. Don’t expect dramatic results or rapid weight loss—these improvements are small and work best alongside other lifestyle changes

Want to Apply This Research?

  • Track your vitamin D supplementation (dose and frequency) alongside fasting blood sugar readings if you have a home glucose monitor. Record these weekly to monitor trends over 8-12 weeks
  • If your doctor recommends vitamin D supplementation, set a daily reminder to take it at the same time each day. Pair this habit with tracking one other prediabetes-related behavior like daily steps or servings of vegetables to maximize benefits
  • Log vitamin D intake consistency and any blood work results (fasting glucose, A1c, triglycerides) when available. Monitor for any changes in energy, mood, or blood sugar symptoms over 3-month periods to assess personal response

This research summary is for educational purposes only and should not replace professional medical advice. The findings suggest vitamin D may modestly improve certain blood sugar markers in people with prediabetes, but it is not a substitute for proven diabetes prevention strategies like exercise, healthy diet, and weight management. Before starting any vitamin D supplementation, especially if you have prediabetes or other health conditions, consult with your healthcare provider to determine if it’s appropriate for you and to discuss proper dosing. Individual results may vary, and more research is needed to confirm these findings.