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Evidence-Based Supplement Research
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Vitamin D and Reduced Hemoglobin A1c

Research synthesisLow evidenceMixed effect size7 studies · 2 beneficial · 5 neutral · 0 harmful

Across 7 studies, 2 reported beneficial moderate-sized reductions in hemoglobin A1c, while 5 found neutral effects. The predominant effect size across studies is small to moderate, with the majority showing no significant impact. Most studies focused on clinical populations, including type 2 diabetes patients, but results were inconsistent and the median study duration was approximately 98 days (based on 2 studies).

  • Studied populations: Adults with type 2 diabetes mellitus

Caveats: Many of the included studies did not reach statistical significance — effect may be smaller than the predominant direction suggests. The beneficial results derive from meta-analyses, but individual trials, including a high-quality RCT, found no significant effect. Evidence is inconsistent and clinical significance remains unclear. Doses and forms were largely unspecified, limiting comparability.

Generated Jun 11, 2026
Doses used in studies
  • IU/day: 10,000 (median 10,000, IQR 10,000–10,000) — 1 study
Time to effect
Median: 3.3 months · IQR 3 months–3.5 months · Range 2.8 months–3.7 months — Reported in 2 of 7 studies
Safety in these studies
7 of 7 papers
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