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Evidence-Based Supplement Research
Evidence-Based Supplement Research
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Vitamin D and Reduced Hemoglobin A1c

Research synthesisLow evidenceModerate effect7 studies · 2 beneficial · 5 neutral · 0 harmful

Across 7 studies, 2 reported beneficial moderate-sized effects on reducing Hemoglobin A1c, while 5 found neutral effects. The predominant effect size is moderate among beneficial studies, but the overall evidence is mixed, with most studies showing no significant impact. The median study duration was 98 days, and the most studied populations were clinical (e.g., type 2 diabetes patients).

  • Studied populations: adult patients with type 2 diabetes mellitus (T2DM), hypertensive obese obstructive sleep apnea patients

Caveats: Many of the included studies did not reach statistical significance — effect may be smaller than the predominant direction suggests. Evidence base is small (only 7 studies) — conclusions should be considered preliminary. The two beneficial studies were meta-analyses, but neutral findings from systematic reviews and RCTs temper confidence. Doses were often not specified or varied widely, limiting dose-response conclusions.

Generated Jul 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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