Research synthesisModerate evidenceSmall effect9 studies · 6 beneficial · 3 neutral · 0 harmful
Across 9 studies, 6 reported beneficial effects on insulin sensitivity, though effect sizes were predominantly small (3 of 6 beneficial studies) and the highest-quality evidence (a systematic review of 1063 women with PCOS) found only modest, non-significant improvements. Median study duration was 63 days (9 weeks), and the most commonly reported dose range was 1000–4000 IU/day. Effects appeared more consistent in populations with vitamin D deficiency or metabolic conditions (e.g., PCOS, gestational diabetes, obesity, diabetes/prediabetes).
- Effective dose range: 1000–4000 IU/day
- Studied populations: Clinical populations with metabolic conditions (PCOS, gestational diabetes, obesity, diabetes/prediabetes) and individuals with vitamin D deficiency (baseline 25(OH)D <20 ng/mL).
Caveats: The highest-quality evidence (systematic review in PCOS, evidence score 8) and two meta-analyses (obese children, epilepsy patients) showed neutral effects, while beneficial findings came from lower-quality studies or specific subgroups. Benefit may be conditional on baseline vitamin D deficiency; most trials lasted 8–12 weeks, and long-term effects or effects in replete individuals are unclear. Only 2 of 9 studies reported duration, limiting generalizability.
Generated Jul 14, 2026