Vitamin D
vitamin D · cholecalciferol · vitamin D3 · 25-OH-D
The 'sunshine vitamin' widely supplemented for bone, immunity and longevity.
TypeSupplement / dietary
Grade
C
Limited
- Class
- Fat-soluble vitamin
- Primary use
- Bone, immune & deficiency correction
- Evidence strength
- medium
- Last reviewed
- 2026-07-01
Bottom line
Correcting a genuine deficiency clearly matters. Routine high-dose supplementation in people who are already replete has repeatedly failed to prevent disease in big trials — so the honest answer is 'test, don't guess'.
What the evidence says
Vitamin D is unusual because both the enthusiasm and the disappointment are evidence-based. Correcting real deficiency benefits bone health and falls/fractures in older, deficient people. But large randomized prevention trials (notably VITAL) found that supplementation in generally replete adults did not significantly reduce cardiovascular disease, cancer, or — in follow-up analyses — meaningfully move most 'longevity' endpoints. The result: value concentrated in the deficient, little added benefit in the already-sufficient. Grade C reflects that split, not that the vitamin is unimportant.
Key studies
- [1]
VITAL — vitamin D for cardiovascular disease and cancer prevention · RCT
No significant prevention benefit in a generally replete population.
PubMed ↗ - [2]
Vitamin D, falls and fractures in older adults · meta-analysis
Benefit concentrated in deficient/older populations.
PubMed ↗ - [3]
Vitamin D toxicity and upper limits (review) · review
Confirms a real ceiling — excess causes harm.
PubMed ↗
Mechanism
A pro-hormone: converted to 25-OH-D then active 1,25-(OH)₂-D, regulating calcium/phosphate handling, bone remodelling and immune modulation via the vitamin D receptor across many tissues.
Safety
Safe at sensible doses. Excess is genuinely harmful: chronic very high intake can cause hypercalcaemia (kidney, vascular and neurological effects). 'More is better' is false here — there is a real upper bound. Blood 25-OH-D testing is cheap and removes the guesswork.
Dosage context
Typical maintenance is ~800–2000 IU/day, higher for correcting documented deficiency under guidance. Mega-dosing without a measured deficiency adds risk, not benefit. Dosing is best guided by a blood level, not by habit.
From the field
The most over-supplemented vitamin in the longevity crowd. Our line is simple and unglamorous: test your level, correct a real deficiency, and don't mega-dose on faith. That's why it's a C, not an A.