Vitamin D affects many biological systems, but the evidence base is uneven. The list below groups eight claimed benefits by the current strength of the evidence, drawing on the largest randomised trials (VITAL, D2d, DO-HEALTH) and recent meta-analyses.
Vitamin D's canonical role: it drives intestinal calcium absorption and enables normal bone mineralisation. Adequate 25(OH)D reduces the risk of osteomalacia in adults and rickets in children. Meta-analyses of large trials show ~15% reduction in hip fracture risk in older adults supplemented with 800+ IU/day plus calcium.
Vitamin D receptors are expressed on immune cells (T cells, macrophages, dendritic cells). It upregulates cathelicidin and defensins, tightens epithelial barriers, and modulates T-regulatory response. A 2017 BMJ meta-analysis found supplementation cut acute respiratory infections by ~12%, with the largest effect in deficient individuals.
Meta-analyses of trials in adults >65 show that daily doses of 700–1,000 IU cholecalciferol reduce the risk of falling by ~19%. The effect appears to be mediated through improved lower-limb strength and postural sway.
Observational studies show a robust inverse association between 25(OH)D and cardiovascular events. However, large randomised trials (VITAL, D2d) have not shown clear benefit from supplementation, suggesting the association may reflect confounding.
VITAL (25,000 participants, 5 years) showed no reduction in total cancer incidence with 2,000 IU/day. However, subgroup and post-hoc analyses suggested a small reduction in cancer mortality, particularly in normal-weight individuals — hypothesis-generating rather than definitive.
D2d randomised 2,423 adults with prediabetes to 4,000 IU/day cholecalciferol or placebo. Primary outcome (progression to diabetes) was not significant overall (HR 0.88, p=0.12), but in participants who reached ≥ 40 ng/mL 25(OH)D, risk fell substantially.
Observational associations are consistent; randomised trials in general populations are largely null. Supplementation may benefit patients with baseline deficiency and diagnosed major depression, but is unlikely to help mildly-low mood in people with sufficient 25(OH)D.
Deficiency in pregnancy is associated with preeclampsia, gestational diabetes, and small-for-gestational-age infants. Cochrane reviews conclude supplementation probably reduces preeclampsia and low birth weight; the current expert consensus is 600 IU/day in pregnancy, with higher doses (1,500–2,000 IU/day) for known deficiency.