Vitamins, Blood Sugar and Diabetes: Evidence for D, B12, C, E and Niacin
What the American Diabetes Association and trials say about vitamins D, B12, C, E and niacin for blood sugar and diabetes, including metformin and B12 monitoring.
Content last updated . This page has not been medically reviewed.
Vitamins do not replace diabetes medicine, blood sugar checks or regular visits with your care team. Never stop or change a prescribed diabetes medicine on your own. If you have symptoms of very high or very low blood sugar, such as extreme thirst, confusion or fainting, get urgent medical help (911 in the US). This page is information only. A clinician or pharmacist can say whether a supplement suits you, especially if you take several medicines.
Metformin and vitamin B12
The ADA states that metformin is associated with vitamin B12 deficiency, based on findings from the Diabetes Prevention Program Outcomes Study. [1] It advises that periodic testing of B12 levels should be considered in people taking metformin, particularly those with anemia or peripheral neuropathy. [1] Its medication table lists the potential for B12 deficiency and says to monitor and replete as appropriate. [2] This is a monitoring point to raise with your clinician. It is not a reason to start B12 on your own. See vitamin B12 and medicines.What research found, by vitamin
| Vitamin and question | What the source found | Label |
|---|---|---|
| Vitamin D, preventing type 2 diabetes in people with prediabetes | In the D2d trial (2,423 adults, 4000 IU of vitamin D3 a day or placebo, median 2.5 years), the hazard ratio for diabetes was 0.88 (95% CI 0.75 to 1.04), not statistically significant. [3] A later pooled analysis of three trials reported a 15% lower risk in adjusted analyses (hazard ratio 0.85). [4] | Mixed |
| Vitamin D, ADA view | The ADA says universal vitamin D supplementation for people with diabetes without deficiency is not recommended at this time. [1] | No good evidence of benefit (for treating diabetes) |
| Vitamin C and blood sugar in type 2 diabetes | A meta-analysis of 22 trials (1,447 people) found lower HbA1c, fasting insulin and fasting glucose, with long-term (12 weeks or more) and high-dose (1,000 mg a day or more) use; these are study amounts, not advice, and the authors said more high-quality trials are needed. [5] | Promising but limited |
| Vitamins C and E together | A meta-analysis found they had comparable effects on glycemic measures. [6] The ADA says routine antioxidant supplementation, such as vitamins E and C, is not recommended because of lack of evidence of efficacy and long-term safety concerns. [1] | Insufficient evidence |
| B vitamins for diabetic nerve pain (neuropathy) | A meta-analysis of 13 trials (834 people) found some improvement on neuropathy scores, but one measure did not reach the minimum clinically important difference and pain scales showed no consistent effect. [7] | Mixed |
| Niacin (vitamin B3) | In 25,673 adults with vascular disease, niacin-laropiprant was linked with more serious disturbances in diabetes control (absolute excess 3.7 percentage points) and more diabetes diagnoses. [8] | Evidence of harm (that product, that group) |
An umbrella review of 14 meta-analyses suggested vitamin C may improve glycemic control and folic acid may improve insulin resistance in type 2 diabetes, and called for better trials. [9] See vitamin C, vitamin D and niacin flush, high-dose niacin and safety.
Who may need a multivitamin
The ADA says that for special populations, including pregnant or lactating individuals, older adults, vegetarians, vegans and people on very-low-calorie or low-carbohydrate eating patterns, a multivitamin may be necessary. [1] Ask your care team. See multivitamins and vegetarian and vegan supplements.Frequently asked questions
Do vitamins lower blood sugar?
Not reliably. The ADA says that without underlying deficiency there is no benefit from vitamin or mineral supplementation for people with diabetes. [1] Some small studies of vitamin C found lower HbA1c. [5]
Should people on metformin take vitamin B12?
The ADA advises considering periodic B12 testing in people taking metformin, especially with anemia or neuropathy. [1] Whether to treat is a clinician's decision.
Can vitamin D prevent type 2 diabetes?
The D2d trial did not show a statistically significant reduction. [3] A pooled analysis of three trials reported a 15% lower risk in people with prediabetes. [4] The ADA does not recommend universal vitamin D supplementation. [1]
Does niacin affect blood sugar?
In a large trial of niacin-laropiprant, there were more serious disturbances in diabetes control and more diabetes diagnoses. [8]
Can B vitamins treat diabetic neuropathy?
A meta-analysis found limited, inconsistent benefit. [7] Persistent numbness or pain needs medical assessment.
More details: evidence, limits and cautions
The details behind this page: cautions, limits of the evidence and where sources disagree. Open any note to read it.
How to read the evidence labels
Labels used on this page: established nutritional role / deficiency treatment, promising but limited, mixed, insufficient evidence, no good evidence of benefit, evidence of harm. Blood sugar markers such as HbA1c are biomarkers; complications and diabetes onset are clinical outcomes.
Limits and uncertainty
- Many vitamin and blood sugar studies are small, short, and measure markers rather than complications.
- The pooled vitamin D analysis noted that studies of people with prediabetes do not apply to the general population. [4]
- The ADA notes that post hoc analyses of D2d and some meta-analyses suggest potential benefit in specific populations, while other studies found no benefit or mixed results. [1]
- Studies in people without diabetes, or with type 1 diabetes, children or pregnancy: Proper evidence not found in the sources read for this page.
Safety and who should ask a clinician
- Medicines: supplements can interact with diabetes and other medicines. See medication interactions.
- Kidney disease: ask before taking high doses. See vitamins and kidney or liver disease and high-dose vitamin C safety.
- Beta-carotene: the ADA counsels against it because of evidence of harm for certain individuals and no benefit. [1]
- Product quality: see quality testing and certifications.
References
- American Diabetes Association Professional Practice Committee. 5. Facilitating Positive Health Behaviors and Well-being to Improve Health Outcomes: Standards of Care in Diabetes-2026. Diabetes Care 2026 (full text on PubMed Central). Read 2026-10-23.
- American Diabetes Association Professional Practice Committee. 9. Pharmacologic Approaches to Glycemic Treatment: Standards of Care in Diabetes-2026. Diabetes Care 2026 (full text on PubMed Central). Read 2026-10-23.
- Vitamin D Supplementation and Prevention of Type 2 Diabetes. The New England journal of medicine 2019. PubMed abstract. Read 2026-10-23.
- Vitamin D and Risk for Type 2 Diabetes in People With Prediabetes : A Systematic Review and Meta-analysis of Individual Participant Data From 3 Randomized Clinical Trials. Annals of internal medicine 2023. PubMed abstract. Read 2026-10-23.
- The effects of vitamin C supplementation on glycemic control in patients with type 2 diabetes: A systematic review and meta-analysis. Diabetes & metabolic syndrome 2023. PubMed abstract. Read 2026-10-23.
- Effects of Vitamin C and/or E Supplementation on Glycemic Control and Cardiovascular Risk Factors in Type 2 Diabetes: A Systematic Review and Subgroup Meta-analysis. Nutrition reviews 2026. PubMed abstract. Read 2026-10-23.
- Vitamin B Supplementation for Diabetic Peripheral Neuropathy: A Review and Meta-Analysis. Journal of clinical medicine 2026. PubMed abstract. Read 2026-10-23.
- Effects of extended-release niacin with laropiprant in high-risk patients. The New England journal of medicine 2014. PubMed abstract. Read 2026-10-23.
- Effects of water-soluble vitamins on glycemic control and insulin resistance in adult type 2 diabetes: an umbrella review of meta-analyses. Asia Pacific journal of clinical nutrition 2025. PubMed abstract. Read 2026-10-23.