Fat-Soluble Vitamin
Vitamin D3 (Cholecalciferol)
Moderate EvidenceStrong evidence that deficiency is harmful and worth correcting; observational links to cognition. But supplementation has not improved cognition in RCTs of replete/mildly-deficient adults. The pattern holds across outcomes. In 1,658 older adults, severe deficiency (below 25 nmol/L) was associated with about twice the risk of dementia over 5.6 years (Littlejohns 2014) — an association, not proof of cause. Yet in 620 adults aged 50 or over with mild-to-moderate deficiency and early cognitive impairment, two years of daily supplementation did not improve executive function or any other cognitive outcome (VitaMIND 2025), and in 18,353 adults aged 50 or over, 2,000 IU a day for a median 5.3 years did not change depression risk or mood scores (Okereke 2020). Where a benefit does appear, it is in people who were clearly deficient: young adults with fatigue and 25(OH)D below 20 µg/L reported improvement more often after a single 100,000 IU dose than after placebo, 72% against 50% (Nowak 2016). Compared with omega-3, which rests on the same correct-a-shortfall logic, vitamin D has a simple blood test, so there is less reason to guess — and note that a 5,000 IU capsule on its own is above the 4,000 IU adult upper intake level. Verdict: test 25(OH)D, correct a deficiency with your doctor, and do not expect a cognitive or mood benefit if your level is already adequate.
Recommended Dose
Up to 4,000 IU daily, guided by a 25(OH)D blood test (higher only under medical supervision)
Timing
Morning with a fatty meal
Mechanism of Action
Vitamin D3 (cholecalciferol) is the form most supplements use, and vitamin D status is judged by the blood marker 25-hydroxyvitamin D, 25(OH)D. In head-to-head trials D3 raised 25(OH)D more effectively than D2, although the advantage was clear with bolus doses and was lost with daily dosing (Tripkovic 2012). Binds to VDR receptors in the brain, modulating a broad range of genes involved in neuroplasticity, immune regulation, and calcium signaling. Laboratory work links it to serotonin synthesis through the TPH2 gene (Patrick & Ames 2014) — a proposed mechanism, not a demonstrated clinical effect.
Side Effects & Safety
Safe at recommended doses. Toxicity possible above 10,000 IU/day chronically. May raise calcium levels — K2 is often paired with it, but it is a precaution, not an antidote to excess vitamin D. Test 25(OH)D levels. In a three-year trial, hypercalcaemia occurred in 0%, 3% and 9% of participants taking 400, 4,000 and 10,000 IU a day respectively (see references).
Do not use / talk to a doctor first
- Hypercalcaemia or hyperparathyroidism
- Sarcoidosis / granulomatous disease
- Kidney stones or significant kidney disease
Interactions
Thiazide diuretics
Raised hypercalcaemia risk — monitor serum calcium
Source: NIH ODS; PMC5623087
Digoxin / cardiac glycosides
Hypercalcaemia can increase arrhythmia risk
Source: NIH ODS
Vitamin K2
Commonly paired to direct calcium to bone — a precaution, not an antidote
Source: Common practice
Pregnancy & breastfeeding
Vitamin D is recommended in pregnancy at standard doses, but high-dose supplementation should be doctor-supervised; test 25(OH)D.
Who should avoid / see a doctor
Do not exceed 4,000 IU/day without medical monitoring. Test 25(OH)D. Special caution on thiazide diuretics or cardiac glycosides, or with hypercalcaemia, sarcoidosis, kidney stones or kidney disease.
Synergies
Used in These Stacks
See Vitamin D3 (Cholecalciferol) inside a full protocol — the stack pages cover when and why to combine it.
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References
- 1.Littlejohns 2014, Neurology — Severe vitamin D deficiency was associated with roughly double the risk of all-cause dementia and Alzheimer disease (n=1,658, Cardiovascular Health Study). (observational — association, not causation)
- 2.Patrick & Ames 2014, FASEB J — Vitamin D activates brain TPH2 transcription (serotonin synthesis). (molecular mechanism, not a clinical outcome)
- 3.VitaMIND RCT 2025, Alzheimer's & Dementia — Supplementation gave no significant benefit to cognition or executive function in adults with mild-to-moderate deficiency — the basis for the Moderate rating. (randomised controlled trial)
- 4.Institute of Medicine / NIH ODS — Vitamin D fact sheet — Adult tolerable Upper Intake Level of 4,000 IU/day; thiazide-diuretic interaction (hypercalcaemia); toxicity is a hypercalcaemia phenomenon. (authoritative body)
- 5.Aging Clin Exp Res 2021 — Hypercalcaemia occurred in 0% / 3% / 9% of participants at 400 / 4,000 / 10,000 IU/day over 3 years; toxicity generally above 25(OH)D ~150 ng/mL.
- 6.Drug–vitamin D interactions: a systematic review — Thiazide diuretics + vitamin D increase hypercalcaemia risk — monitor serum calcium.
- 7.Tripkovic 2012, Am J Clin Nutr — Meta-analysis of RCTs directly comparing vitamin D3 with D2 in adults: D3 raised serum 25(OH)D more effectively (p=0.001); the advantage was significant for bolus dosing and lost with daily supplementation.
- 8.Okereke 2020, JAMA — VITAL-DEP: 18,353 adults aged 50+ randomised to vitamin D3 2,000 IU/day or placebo, median 5.3 years: no difference in risk of depression or clinically relevant depressive symptoms (HR 0.97, 95% CI 0.87–1.09) or in PHQ-8 mood scores. (randomised controlled trial)
- 9.Nowak 2016, Medicine (Baltimore) — Otherwise healthy adults (mean age 29) with fatigue and serum 25(OH)D below 20 µg/L, randomised double-blind: a single oral 100,000 IU dose of vitamin D3 reduced fatigue-scale scores more than placebo at 4 weeks; improvement reported by 72% vs 50% (p=0.01).
