Vitamin D3
Secosteroid hormone essential for bone health, immune function, and increasingly recognized for systemic health effects.
Evidence Summary
Vitamin D is a secosteroid hormone with clear deficiency-related pathology and strong evidence for bone health and immune function. Randomized data from the 25,871-participant VITAL trial (2,000 IU/day D3) showed a 22% lower incidence of autoimmune disease over ~5 years (hazard ratio 0.78, 95% CI 0.61-0.99), a 17% lower incidence of advanced (metastatic or fatal) cancer, and a 38% lower rate among adults of normal body weight (BMI <25). A 2025 VITAL telomere sub-study (n=1,054) found D3 reduced leukocyte telomere attrition by ~0.14 kilobase pairs over 4 years versus placebo — roughly three fewer years of cellular aging — while marine omega-3 showed no telomere effect. Notably, the autoimmune benefit dissipated within about two years of stopping supplementation, underscoring that vitamin D acts as ongoing maintenance rather than a one-time fix. Benefits are most consistent when correcting a true deficiency; testing 25(OH)D and titrating to labs is preferred over fixed high doses. Educational information, not medical advice.
Evidence Scale
Mechanism of Action
Functions as hormone affecting gene expression in most tissues. Regulates calcium metabolism, immune function, cell growth, and neuromuscular function.
Who Is This For?
Widespread deficiency (50%+ of population). Immune support, bone health, mood, chronic disease prevention.
Protocol & Dosing
Dose
2000-10000 IU daily depending on levels. Target 25(OH)D 50-80 ng/mL.
Frequency
Daily with meals
Duration
Ongoing
Protocol Summary
Oral D3 (cholecalciferol) with fat-containing meal. K2 often combined for calcium direction. Test and titrate to optimal levels.
Latest Evidence
2025: VITAL trial links D3 to slower cellular aging
A 2025 sub-study of the 25,871-person VITAL randomized trial (2,000 IU/day vitamin D3) measured leukocyte telomere length in 1,054 participants and found that D3 reduced telomere attrition by about 0.14 kilobase pairs over four years compared with placebo — an effect the investigators equated to roughly three fewer years of cellular aging. Marine omega-3 showed no telomere effect. This builds on earlier VITAL findings: 22% lower incidence of autoimmune disease over about five years (hazard ratio 0.78), and 17% lower incidence of advanced, metastatic or fatal cancer — rising to 38% among adults of normal body weight. The autoimmune benefit faded within roughly two years of stopping, which is why physicians frame vitamin D as ongoing maintenance rather than a one-time course.
These are randomized-trial and secondary-analysis results; benefits are most consistent when a true deficiency is corrected, and vitamin D is not a substitute for cancer screening or disease-specific treatment. Test 25(OH)D and titrate to labs rather than mega-dosing; avoid in hypercalcemia, sarcoidosis, or granulomatous disease. Educational information, not medical advice.
VITAL RCT · n=25,871 · 2,000 IU/day · ~5-yr follow-up
Vitamin D3 relative risk reductions in the VITAL trial
Source: VITAL randomized controlled trial (2,000 IU/day vitamin D3). Autoimmune disease −22% (HR 0.78, 95% CI 0.61–0.99; Hahn et al., BMJ 2022; n=25,871, 5.3-yr). Advanced (metastatic or fatal) cancer −17% overall and −38% among adults with BMI <25 (Chandler et al., JAMA Network Open 2020). The autoimmune benefit dissipated within ~2 years of stopping supplementation. Educational information, not medical advice.
Related on Peak Human
Vitamin D3 + K2
The paired supplement — K2-MK7 directs calcium to bone.
Vitamin K2 (MK-7) & Heart Health
Why D3 is best partnered with K2.
Diagnostic Testing
Measure 25(OH)D before and during supplementation.
Magnesium
A required cofactor for vitamin D activation.
Peak Human Shop
Practitioner-grade D3/K2 and cofactors.
Longevity
Physician-guided healthy-aging protocols.
Key references: Vitamin D3 & leukocyte telomere length: 4-year VITAL findings — Am. J. Clin. Nutr. (2025) · Vitamin D & incident autoimmune disease: VITAL 2-year post-trial follow-up — PubMed (2024) · Vitamin D3 & advanced cancer: secondary analysis of VITAL — JAMA Netw. Open (2020)
Interactions & Precautions
Contraindications
- Hypercalcemia
- Sarcoidosis
- Some lymphomas
Potential Risks
- •Toxicity at very high doses
- •Hypercalcemia
Potential Side Effects
Practitioner Notes
Clinical annotations from Dr. Goel
Test 25(OH)D before and during supplementation; target roughly 50-80 ng/mL and titrate to labs rather than fixed high doses. Pair with K2-MK7 to direct calcium and with magnesium, a cofactor for vitamin D activation. VITAL data suggest the autoimmune and advanced-cancer benefits concentrate in deficiency correction and normal-weight individuals and wane after stopping, so treat supplementation as ongoing maintenance. Avoid in hypercalcemia, sarcoidosis, and other granulomatous disease.
Dr. Sanjeev Goel
Chief Medical Officer, Peak Human
Cost & Access
Cost Range
$
Accessibility
Availability varies by location
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