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Vitamin D3 (Cholecalciferol)
Hormone-like vitamin that regulates 1,000+ genes — deficiency is widespread and linked to poor immunity, depression, and bone loss.
What the evidence says
Vitamin D3 helped in about half (7/12) of the studies that measured an effect — promising, but not unanimous.
Most evidence is from high-quality meta-analyses and randomised trials published 1981–2026 with a typical study size of 3,919 participants.
Based on 986 studies · 66 meta-analyses · 847 RCTs · 501,145 total participants
Confidence
High confidenceWhat the studies found
By outcome
A deep meta-analytic base supports vitamin D for respiratory infection and, with calcium, fracture outcomes, with a small, very-low-certainty effect on depressive symptoms, but large trials show little benefit in already-replete people, capping the score below the top tier.
868 randomised trials
88% were about Vitamin D3 in an audit of 50 of them
Every randomised or controlled trial of Vitamin D3 indexed in PubMed, not a hand-picked subset. Meta-analyses and systematic reviews are deliberately excluded from this count: most of them cover many compounds at once, so counting them would inflate it.
PubMed · as of Sep 2026
356 trials ongoing or recruiting · NaN completed on ClinicalTrials.gov
399 of the completed trials have posted results
Registered trials show research momentum for Vitamin D3, not proof of effect — a registration is a plan, and posted results are sponsor-reported, not peer-reviewed. They are never counted toward the evidence rating above.
Browse these trials on ClinicalTrials.govClinicalTrials.gov · as of Oct 2026
Under EU law (Reg. 1924/2006), EFSA reviews whether a specific health claim for Vitamin D3 meets the evidence standard. These are independent regulatory decisions on claims — not studies, and never counted toward the evidence score above.
Authorised claims
“Vitamin D contributes to normal absorption/utilisation of calcium and phosphorus”
“Vitamin D contributes to normal blood calcium levels”
“Vitamin D contributes to the maintenance of normal bones”
“Vitamin D contributes to the maintenance of normal muscle function”
“Vitamin D contributes to the maintenance of normal teeth”
“Vitamin D contributes to the normal function of the immune system”
“Vitamin D has a role in the process of cell division”
Not authorised by EFSA
“Vitamin D is needed for the normal / proper function of the thyroid. Vitamin D is needed for the proper regulation of blood calcium levels”
“Not authorised” means the specific claim wording did not meet the EU’s evidence standard — often a matter of dossier or phrasing, not proof of no effect. EFSA judges marketing claims and is kept separate from our study-based evidence score.
Source: EU Register of nutrition and health claims (European Commission / EFSA) · register snapshot Feb 2013
Showing the 991 most recent of 993 studies.
Vitamin D3 is a fat-soluble vitamin that functions as a hormone in the body. Deficiency is extremely common, especially in northern latitudes, and is linked to poor immune function, depression, muscle weakness, and bone loss. Optimal blood levels are 40-60 ng/mL.
Modulates both innate and adaptive immune responses
Regulates over 1,000 genes throughout the body
Essential for calcium absorption and bone health
How Vitamin D3 works — from molecular targets to health outcomes. Click an edge to see supporting research.This visualization is in beta — pathways are being refined and expanded.
Generally recommended; 1000-4000 IU is considered safe
Often need higher doses due to reduced skin synthesis
May need higher doses; melanin reduces vitamin D synthesis
May increase calcium levels
Vitamin D can raise blood calcium, and high blood calcium can increase susceptibility to digoxin toxicity. Avoid excessive supplementation and have calcium and renal function assessed when indicated.
Steroids may reduce vitamin D absorption
Both metabolized by liver; monitor if using high doses — both affect lipid metabolism; may increase risk of myopathy through additive effects on muscle tissue
Tip: Don't exceed 10,000 IU daily without monitoring
Tip: Take with food
Vitamin D3 has an evidence score of 7.5/10 — strong evidence based on 988 indexed studies, including 25 meta-analyses. Hormone-like vitamin that regulates 1,000+ genes — deficiency is widespread and linked to poor immunity, depression, and bone loss. Representative study: PMID 35842938.
The commonly studied dose of Vitamin D3 is 2000-4000 IU daily. Individual needs vary — start at the lower end of the range and adjust based on how you respond.
The best time to take Vitamin D3 is in the morning. Take it with food. Vitamin D3 (cholecalciferol) is a fat-soluble secosteroid hormone precursor that requires bile acid emulsification and dietary fat for intestinal absorption via passive diffusion and cholesterol transporters (NPC1L1, SR-B1).
Vitamin D3 is generally well-tolerated and considered safe for most healthy adults at recommended doses. Reported side effects are uncommon and include hypercalcemia (at very high doses), GI upset. Use caution if any of these apply to you: Hypercalcemia; Kidney disease (consult doctor); Sarcoidosis or other granulomatous diseases.
Zinc
Likely helpsInvolved in 300+ enzymatic reactions — supports immune defense, testosterone production, wound healing, and sleep quality.
Magnesium
Mixed evidenceSupports 300+ enzymatic reactions — critical for sleep, stress response, muscle function, and cognitive health.
Omega-3
Probably helpsEssential fatty acids critical for brain health, mood regulation, and reducing inflammation throughout the body.
Ashwagandha
Likely helpsReduces cortisol and anxiety while improving sleep quality and physical recovery in stressed adults.
What Actually Works vs What’s Hype
The honest tier list — proven staples vs situational vs mostly marketing. The hub the other guides feed into.
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Berberine, inositol, magnesium vs the cinnamon/chromium hype — and the med-interaction warning.
Migraine Prevention
Magnesium, riboflavin, CoQ10 — the supplements with real migraine-prevention evidence (and doses).
Thyroid Support
Selenium helps; iodine can WORSEN Hashimoto’s. The honest take on "thyroid support".
Male Fertility & Sperm Health
CoQ10, zinc, selenium, carnitine modestly help sperm parameters — but live-birth evidence is weak.
Female Fertility & TTC
Folate is essential; inositol + CoQ10 are the best bets — but conception evidence is limited.
Mood & Depression
Saffron, EPA omega-3, SAMe have real adjunct evidence — but these aren’t a substitute for care, and some interact dangerously.
Joint Pain & Osteoarthritis
Curcumin, omega-3, UC-II collagen, Boswellia vs the old glucosamine default — and OA vs RA.
Immune Support
Zinc timing, vitamin D, the real vitamin C effect — and the elderberry caution.
Perimenopause & Menopause
Hot flashes, bone, sleep, muscle — the basics that hold up vs the phytoestrogen hype.
Perimenopause
The transition years (40s): cycle changes, mood, sleep, brain fog — foundations, vitex, and starting muscle work early.
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Inositol (40:1), and the supporting cast for insulin resistance, cycles & fertility.
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Worth it if you’re deficient (and many are) — dosing, testing, D3 vs D2, and the K2 question.
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Build Your First Supplement Stack
Start with 2–3 foundations, add one at a time, build by goal — without wasting money.
Supplement Timing & Combining
When to take what, with or without food, and which supplements compete vs pair well.
Hair Loss: What Actually Works
The proven options (minoxidil, finasteride) vs adjuncts and weak naturals — tiered for pattern hair loss.
Supplements in Pregnancy
Commonly recommended vs ask-your-clinician vs avoid — a general, safety-first overview.
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Reviewed by Dr. Baher Al Hakim · Last reviewed May 2026 · evidence from 993 studies · how we score · editorial policy
This information is for educational purposes only. It is not a substitute for professional medical advice. Always consult a qualified healthcare provider before starting, stopping, or changing any supplement or medication.
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