Vitamin D

vitamin D · cholecalciferol · vitamin D3 · 25-OH-D

The 'sunshine vitamin' widely supplemented for bone, immunity and longevity.

TypeSupplement / dietary

The grade answers: What does the human evidence support for: Bone, immune & deficiency correction?

Grade

Limited

The grade rates evidence quality — it is not advice to take or buy.

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.
Open on PubMed ↗
[2]
Vitamin D, falls and fractures in older adults · meta-analysis
Benefit concentrated in deficient/older populations.
Open on PubMed ↗
[3]
Vitamin D toxicity and upper limits (review) · review
Confirms a real ceiling — excess causes harm.
Open on PubMed ↗

See all studies on PubMed ↗

Scoped PubMed search, not a curated bibliography: the individual PMIDs for this entry have not yet been verified one by one. For that reason it is not used to support a high grade and is awaiting review.

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.

Examples of application

Dosed to a measured blood level rather than by guesswork.
Typically ~800–2000 IU/day for maintenance, taken with a meal.
Not mega-dosed on faith — excess is genuinely harmful.

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.

Follow this entry

We'll tell you if the grade for Vitamin D changes.

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How we assign grades → Method