Vitamin D is a fat-soluble secosteroid hormone produced in the skin on sun exposure. Deficiency is extremely common — estimates suggest 40%+ of Western adults have insufficient levels. Evidence is clearest for bone health, immune function and muscle function. The broad health claims made for vitamin D are more mixed than the supplement industry implies, but correcting a genuine deficiency has meaningful benefits.
Vitamin D deficiency is common and worth correcting. The strongest evidence is for bone health and immune function. Many extravagant health claims (cancer prevention, cardiovascular protection) from observational studies have not held up consistently in intervention trials.
Each row grades the claimed effect by strength of human evidence, not mechanism or marketing.
Supplementation recommended for deficient individuals and broadly recommended in Northern latitudes during winter. 600–800 IU/day is the official daily requirement; practical supplementation for deficiency often requires 1,000–4,000 IU under appropriate monitoring.
Often recommended at very high doses (5,000–10,000 IU/day or more) for a wide range of conditions. Large doses without monitoring carry toxicity risk.
The deficiency correction evidence is clear. The intervention trial record for non-skeletal outcomes (cancer, cardiovascular, cognitive) is more mixed than observational data suggested. The bar for supplementation should be correcting a known or likely shortfall, not chasing secondary prevention.
Vitamin D3 (cholecalciferol) is more effective than D2 (ergocalciferol) at raising and maintaining 25(OH)D levels.
The target blood level is generally considered 50–125 nmol/L (20–50 ng/mL); deficiency is typically defined as below 50 nmol/L.
Vitamin D is fat-soluble — take with a meal containing fat for best absorption.
There is debate about whether vitamin K2 should be co-supplemented to direct calcium to bones and away from arteries — biologically plausible, not yet proven in clinical trials.
Toxicity from vitamin D is possible at very high doses — above 10,000 IU/day chronically without monitoring. Hypercalcaemia is the main risk.
Mechanism is not outcome. Each mechanism is labelled by how far it has been validated in humans.
The active form of vitamin D (1,25-dihydroxyvitamin D) binds to the vitamin D receptor in nearly every cell type, influencing hundreds of genes involved in immune regulation, calcium handling and cell differentiation.
Vitamin D increases intestinal absorption of calcium and phosphorus — the primary mechanism for its role in bone mineralisation.
VDR is expressed on most immune cells. Vitamin D promotes innate immune responses (antimicrobial peptides) and modulates adaptive immunity — reducing autoimmune-type responses.
Vitamin D receptors are present in brain areas involved in mood regulation and VDR activity influences serotonin synthesis — a plausible route for its putative effects on depression.
Safe at 1,000–4,000 IU/day in healthy adults with normal calcium metabolism. Vitamin D toxicity (hypervitaminosis D) is rare at sensible doses but real at sustained very high doses.
This page is educational and not medical advice. Test your levels if possible before supplementing, and monitor if taking doses above 2,000 IU long-term.
A small, curated set — not a literature dump. Each reference comes with a single-line takeaway.
Vitamin D supplementation reduced risk of acute respiratory infections — effect was strongest in those with low baseline levels taking daily/weekly doses.
2,000 IU/day vitamin D3 over 5 years did not significantly reduce cancer incidence or cardiovascular events, but reduced autoimmune disease risk by 22%.
Reviews evidence for vitamin D + calcium in bone density and fracture prevention; strongest in elderly and deficient populations.