Strongest evidence: The most robust findings are for increasing 25-hydroxyvitamin D levels (high evidence strength, 14 of 16 studies beneficial, doses 240–4000 IU/day). High-strength evidence also supports a small effect on reducing inflammation (6 of 6 studies beneficial, especially in older adults and inflammatory conditions). Moderate-strength evidence shows beneficial effects for improved insulin sensitivity (6 of 9 studies, dose 1000–4000 IU/day), reduced C-reactive protein (5 of 9 studies, moderate effect in clinical populations), reduced triglycerides (5 of 8 studies, small effect in metabolic disorders), improved quality of life (7 of 8 studies, 4000 IU/day or 60,000 IU/week), and reduced parathyroid hormone (3 of 4 studies, up to 5,000 IU/day).
Mixed or weaker evidence: Effects on blood cholesterol are mixed (3 of 8 studies beneficial, small effect), with most neutral results in clinical populations. Interleukin-6 reduction shows moderate evidence (5 of 8 studies beneficial), and HOMA-IR reduction is supported by 3 of 5 meta-analyses but with small effect sizes. Outcomes with lower study counts, such as tumor necrosis factor alpha (3 of 5 beneficial) and hemoglobin levels (4 of 5 beneficial), show promise but require more research.
Effective dose patterns: Across multiple outcomes, doses of 1000–4000 IU/day appear commonly in studies for insulin sensitivity, quality of life, and PTH reduction. Weekly bolus doses up to 60,000 IU were also reported. No single dominant dose emerged, and effects often required at least 8–12 weeks to manifest.
Population insights: Benefits are most consistently observed in clinical populations with metabolic conditions (e.g., type 2 diabetes, PCOS, MASLD), older adults, and individuals with baseline vitamin D deficiency (25(OH)D <20 ng/mL). Effects in healthy, replete individuals are often less pronounced or absent.
Notable caveats: Publication bias is a concern in several outcomes (null-result studies may be underreported). Many studies lacked specification of supplement form or dose, and effects may not generalize to healthy populations. Benefit was often conditional on baseline deficiency or specific clinical conditions.