Vitamin D

Reviewed

Reviewed on 2026-09-19 against the evidence rules: every claim carries a grade, every claim and every safety flag carries a source, and every citation was re-resolved against the record it names. The read was delegated by the owner on 2026-09-18 and signed by the Lead on his behalf (D-295) — it is a delegated review, not the owner's own reading of the page. Every claim on it carries an evidence grade and a named weakest link. It is educational and is not medical advice.

Last reviewed 2026-08-23

Two systematic reviews reach different answers about older adults, and neither of them is about a well-nourished thirty-year-old topping up. The useful question here is whether you are low, and that needs a blood test rather than a page.

Before anything else

Safety

Adverse effects
The upper intake level the US National Academy of Medicine sets for adults is 4,000 IU a day. A three-year trial gave exactly that dose to 2,423 adults and found adverse events slightly less frequent than on placebo, with kidney stones, high blood calcium, high urinary calcium and reduced kidney function all rare and no more common in the supplemented group. At the ceiling, in that population, over three years, the harm did not appear.
Source: Safety and tolerability of high-dose daily vitamin D(3) supplementation in the vitamin D and type 2 diabetes (D2d) study — a randomized trial in persons with prediabetes
Interaction
Thiazide diuretics reduce the calcium your kidneys excrete. In a trial of 328 adults taking up to 4,000 IU for three winter months, the 84 who were also on hydrochlorothiazide ended with serum calcium 0.2 mg/dL higher than those who were not, and one of them crossed into frank hypercalcaemia. Small, real, and a reason to have calcium checked rather than to guess.
Source: Risk of hypercalcemia in blacks taking hydrochlorothiazide and vitamin D
No data in your group
Almost the entire trial literature on vitamin D and cognition is in people aged 60 and over. Anyone younger is extrapolating.
Source: Effects of Vitamin D Supplementation on Cognitive Function in Older Adults: A Grading of Recommendations Assessment, Development, and Evaluation (GRADE) Methodology-Assessed Systematic Review and Meta-Analysis of Randomized Clinical Trials; Vitamin and mineral supplementation for maintaining cognitive function in cognitively healthy people in mid and late life
Product quality
Doses and durations differ enormously between the trials pooled above — 400 IU daily with calcium for eight years is not the same intervention as a high-dose bolus for twelve weeks, and pooling them is why two careful reviews of the same question disagree.
Source: Vitamin and mineral supplementation for maintaining cognitive function in cognitively healthy people in mid and late life; Effects of Vitamin D Supplementation on Cognitive Function in Older Adults: A Grading of Recommendations Assessment, Development, and Evaluation (GRADE) Methodology-Assessed Systematic Review and Meta-Analysis of Randomized Clinical Trials

Every claim, with its grade

What the evidence says, and how strong it is

A grade belongs to one claim, in one population, for one preparation — never to a compound, a food or a page. Open a grade to see the four links between the evidence and the thing you would actually do, and which of them is doing the least work.

A GRADE-assessed meta-analysis of nine randomised trials in adults aged 60 and over found global cognition improved by a standardised mean difference of 0.66 (95% confidence interval 0.02 to 1.30), with larger effects on language and working memory. The authors ask for further trials before treating this as settled.
In whom: Adults aged 60 and over
What form: Vitamin D3 supplementation, doses and durations as in nine randomised trials
Measured as: Global cognition, language and working memory
Evidence grade B: May support
Why this grade
Weakest link
Population — the people studied are not the people reading. All nine trials are in adults aged 60 and over, and the authors themselves ask for further trials before this is treated as settled.
What was actually studied
Adults aged 60 and over, 9 randomised trials
Detail worth knowing
Global cognition SMD 0.66 (95% CI 0.02–1.30, p = .04); language and working memory larger; executive function 0.10 (−0.04 to 0.25), attention 0.02 (−0.33 to 0.38) and perceptual speed not significant. The authors ask for further trials to confirm.
In the same nine trials, executive function (0.10, 95% confidence interval −0.04 to 0.25), attention (0.02, −0.33 to 0.38) and perceptual speed did not move. The domains that did not respond are in the same paper as the domains that did.
In whom: Adults aged 60 and over
What form: Vitamin D3 supplementation across the same nine trials
Measured as: Executive function, attention and perceptual speed
Evidence grade B: May support
Evidence does not support
Why this grade
Weakest link
Population — the people studied are not the people reading. The same nine trials in the over-60s: the domains that moved and the domains that did not are in one paper about one age group.
What was actually studied
Adults aged 60 and over, 9 randomised trials
Detail worth knowing
Global cognition SMD 0.66 (95% CI 0.02–1.30, p = .04); language and working memory larger; executive function 0.10 (−0.04 to 0.25), attention 0.02 (−0.33 to 0.38) and perceptual speed not significant. The authors ask for further trials to confirm.
In a Cochrane review, one trial of 4,143 cognitively healthy participants taking 400 IU of vitamin D3 a day with calcium showed a difference of −0.1 MMSE points after a mean of 7.8 years (95% confidence interval −0.81 to 0.61) and no difference in dementia incidence. A pilot at 4,000 IU on alternate days found probably no effect over six months.
In whom: Cognitively healthy adults aged 40 and over
What form: Vitamin D3 400 IU/day with calcium, mean 7.8 years
Measured as: Overall cognitive function and incidence of dementia
Evidence grade C: Early research suggests
Evidence does not support
Why this grade
Weakest link
Preparation — what was tested is not what is sold. 400 IU a day taken with calcium is not the dose or the form a supplement buyer takes, and it is the intervention this null describes.
What was actually studied
Cognitively healthy people aged 40 and over, 28 studies (n = 83000)
Detail worth knowing
B vitamins: little or no effect on global cognition to 5 years (SMD −0.03 to 0.06) and probably none at 5–10 years. Vitamin D3 400 IU/day with calcium in 4,143 participants: −0.1 MMSE points (95% CI −0.81 to 0.61) after a mean 7.8 years. Only two of the 28 studies had a mean baseline age under 60.
Only two of the 28 studies in the Cochrane review had participants with a mean baseline age under 60, and the 2026 meta-analysis excluded anyone under 60 by design. For a replete adult in their twenties or thirties there is no adequate trial to grade, searched on 2026-08-23.
In whom: Healthy adults under 60 who are not deficient
What form: Vitamin D supplementation at any dose
Measured as: Any cognitive outcome
Evidence grade D: Human evidence insufficient
Why this grade
Weakest link
Population — the people studied are not the people reading. The 2026 meta-analysis excluded anyone under 60 by design and only two of the 28 Cochrane studies had a mean baseline age below it, so for a replete adult under sixty there is no group to reason from.
What was actually studied
Cognitively healthy people aged 40 and over, 28 studies (n = 83000); Adults aged 60 and over, 9 randomised trials
Detail worth knowing
B vitamins: little or no effect on global cognition to 5 years (SMD −0.03 to 0.06) and probably none at 5–10 years. Vitamin D3 400 IU/day with calcium in 4,143 participants: −0.1 MMSE points (95% CI −0.81 to 0.61) after a mean 7.8 years. Only two of the 28 studies had a mean baseline age under 60.
Global cognition SMD 0.66 (95% CI 0.02–1.30, p = .04); language and working memory larger; executive function 0.10 (−0.04 to 0.25), attention 0.02 (−0.33 to 0.38) and perceptual speed not significant. The authors ask for further trials to confirm.

Sources

Everything above, and where it comes from

Behrouzi R, Shahinfar H, Jazayeri S (2026). Effects of Vitamin D Supplementation on Cognitive Function in Older Adults: A Grading of Recommendations Assessment, Development, and Evaluation (GRADE) Methodology-Assessed Systematic Review and Meta-Analysis of Randomized Clinical Trials. Nutrition Reviews (online ahead of print)meta-analysis · Adults aged 60 and over, 9 randomised trials · checked 2026-08-23Global cognition SMD 0.66 (95% CI 0.02–1.30, p = .04); language and working memory larger; executive function 0.10 (−0.04 to 0.25), attention 0.02 (−0.33 to 0.38) and perceptual speed not significant. The authors ask for further trials to confirm.
Rutjes AW, Denton DA, Di Nisio M, et al. (2018). Vitamin and mineral supplementation for maintaining cognitive function in cognitively healthy people in mid and late life. Cochrane Database of Systematic Reviews 2018(12):CD011906.pub2meta-analysis · Cognitively healthy people aged 40 and over, 28 studies · checked 2026-08-23B vitamins: little or no effect on global cognition to 5 years (SMD −0.03 to 0.06) and probably none at 5–10 years. Vitamin D3 400 IU/day with calcium in 4,143 participants: −0.1 MMSE points (95% CI −0.81 to 0.61) after a mean 7.8 years. Only two of the 28 studies had a mean baseline age under 60.
Johnson KC, Pittas AG, Margolis KL, et al. (D2d research group) (2022). Safety and tolerability of high-dose daily vitamin D(3) supplementation in the vitamin D and type 2 diabetes (D2d) study — a randomized trial in persons with prediabetes. European Journal of Clinical Nutrition 76(8):1117–1124rct · Overweight or obese adults with prediabetes, 3 years of follow-up · checked 2026-08-23The trial dose was 4,000 IU/day, described in the paper as "the tolerable upper intake level for adults by the National Academy of Medicine". Adverse events were slightly less frequent on vitamin D than placebo (IRR 0.94, 95% CI 0.90–0.98); nephrolithiasis, hypercalcaemia, hypercalciuria and low eGFR were rare and did not differ by group.Erratum: Eur J Clin Nutr. 2022 Oct;76(10):1491 (doi:10.1038/s41430-022-01130-5) — Crossref records it as "Correction to: Safety and tolerability of high-dose daily vitamin D3 supplementation in the vitamin D and type 2 diabetes (D2d) study". What it corrects could not be read on 2026-08-29 — the publisher's page refuses an unauthenticated fetch — so this register records that a correction exists and does not guess at its contents. Erratum register checked 2026-08-29.
Chandler PD, Scott JB, Drake BF, et al. (2014). Risk of hypercalcemia in blacks taking hydrochlorothiazide and vitamin D. American Journal of Medicine 127(8):772–778rct · Post hoc analysis of a dose-finding trial in 328 Black adults, median age 51; 84 of them taking hydrochlorothiazide · checked 2026-08-23Placebo or 1,000, 2,000 or 4,000 IU of cholecalciferol daily for 3 winter months. At 3 months the hydrochlorothiazide users had serum calcium 0.2 mg/dL higher (p < .001), and one of the 84 had frank hypercalcaemia against none of the 44 comparison participants.

And it costs nothing

The one thing worth doing

Ask for a 25(OH)D result at your next blood test before you buy anything. Knowing whether you are low is the part that changes what to do, and it is free where a test is already being taken.

Regulatory note — this note has not yet been reviewed by a lawyer. In the United States, supplements are sold under DSHEA: a structure/function claim is not reviewed by the FDA before it appears, and the label must carry the words "This statement has not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease." In the EU and the UK, only health claims on the authorised register may be used in advertising, and a food or supplement advertisement may not say a product prevents, treats or cures a disease. Whether an evidence page that carried a purchase link would itself count as advertising under those rules is an open question that counsel, not this page, will answer; there are no purchase links here. Nothing on this page is medical advice.