Vitamin D
For informational purposes only — not medical advice. Always consult a qualified healthcare provider before making changes to your health regimen. Full disclaimer →
- Vitamin D acts as a hormone, not just a nutrient. It regulates hundreds of genes across virtually every organ system — immune function, cancer suppression, cardiovascular health, brain function, and inflammation. Standard 'bone health' framing dramatically undersells its importance.
- Longevity-optimal target: 50–80 ng/mL. Standard labs call anything above 20–30 ng/mL sufficient. But the research on all-cause mortality, cancer risk, and immune function consistently points to the 50–80 ng/mL range as optimal.
- Over 40% of American adults are deficient. Sun avoidance, sunscreen use, indoor lifestyles, darker skin, obesity, and aging all reduce vitamin D production. Most people eating a Western diet cannot achieve optimal levels through diet alone.
- Supplementation is safe and effective — but dose matters. Most adults need 2,000–5,000 IU/day to maintain levels in the optimal range. Testing before and after supplementation is the only way to know your dose.
- Always test 25(OH)D specifically — not 1,25-dihydroxyvitamin D (calcitriol), which is a different form and not useful for assessing vitamin D status.
Why Vitamin D Is One of the Most Consequential Deficiencies in Modern Life
For most of human history, significant sun exposure was unavoidable. Humans spent the majority of their lives outdoors, producing vitamin D through their skin continuously throughout the day. The modern lifestyle — offices, cars, sunscreen, clothing — has changed this so thoroughly that over 40% of American adults now have insufficient vitamin D levels, and a substantial fraction are frankly deficient.
The stakes are higher than most people appreciate. Standard medicine frames vitamin D primarily as a bone health nutrient — important for preventing rickets and osteoporosis, but largely a secondary concern. This framing has persisted despite decades of research demonstrating that vitamin D receptors are expressed in virtually every tissue in the human body, that it regulates hundreds of genes, and that deficiency is consistently associated with increased risk of cardiovascular disease, multiple cancers, autoimmune conditions, dementia, and all-cause mortality.
A landmark meta-analysis published in the BMJ in 2014, analyzing data from over 500,000 individuals across multiple longitudinal studies, found that low vitamin D status was associated with increased mortality from cardiovascular disease, cancer, and all-cause mortality, with the relationship following a clear dose-response pattern. 1
The practical implication: vitamin D status belongs in every longevity assessment, and the standard clinical thresholds — designed to prevent the most severe deficiency diseases — are inadequate benchmarks for anyone interested in optimizing long-term health.
Standard Reference Ranges vs. Longevity-Optimal Ranges
The clinical thresholds for vitamin D were established primarily around bone health endpoints — specifically, the level required to maintain calcium absorption and prevent secondary hyperparathyroidism. They do not reflect the emerging research on vitamin D's broader role in immune regulation, cancer prevention, and cardiovascular health.
| Category | 25(OH)D Level | Standard Interpretation | Longevity Assessment |
|---|---|---|---|
| Longevity-optimal | 50–80 ng/mL | Sufficient / high normal | Optimal — associated with lowest all-cause mortality |
| Good | 40–50 ng/mL | Sufficient | Good — meaningful improvement over deficiency |
| Monitor | 30–40 ng/mL | Sufficient (bone health) | Below longevity target — supplement to reach 50+ |
| Insufficient | 20–30 ng/mL | Borderline / insufficient | Insufficient — significant supplementation needed |
| Deficient | Below 20 ng/mL | Deficient | Deficient — high-dose supplementation required |
An important note on the upper end: levels above 100 ng/mL may be associated with toxicity — primarily through hypercalcemia and calcification of soft tissues. Vitamin D toxicity from supplementation is rare but real at very high doses. The 50–80 ng/mL range reflects both the evidence for benefit and a meaningful safety margin below concerning levels.
Research by Garland et al. analyzing pooled data from multiple studies found that serum 25(OH)D levels of 40–60 ng/mL were associated with approximately 50% lower risk of breast cancer compared to levels below 20 ng/mL. 2
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Analyze My Biomarkers →Vitamin D as a Hormone: Why the Biology Matters
The most important thing to understand about vitamin D is that calling it a "vitamin" is, in a technical sense, misleading. A vitamin is a compound the body cannot synthesize and must obtain from diet. Vitamin D is synthesized in the skin from cholesterol under UVB radiation, and then activated into its hormonal form (1,25-dihydroxyvitamin D, also called calcitriol) in the liver and kidneys.
Calcitriol acts as a steroid hormone, binding to vitamin D receptors (VDRs) found in the nucleus of virtually every cell type in the body — including immune cells, neurons, cardiac muscle, pancreatic beta cells, and cancer cells. Once bound, it regulates the transcription of over 1,000 genes.
This genomic reach is why vitamin D deficiency has such broad consequences:
- Immune regulation: Vitamin D modulates both innate and adaptive immunity. It enhances the production of antimicrobial peptides (including cathelicidin, which is active against bacteria, viruses, and fungi) while simultaneously suppressing the excessive inflammatory responses that drive autoimmune disease. Low vitamin D is associated with increased susceptibility to respiratory infections, and with higher rates of multiple sclerosis, rheumatoid arthritis, and type 1 diabetes.
- Cancer suppression: Vitamin D promotes cellular differentiation (the process by which cells mature into their intended function), inhibits excessive cell proliferation, induces apoptosis in malignant cells, and inhibits tumor angiogenesis (the formation of blood vessels that feed tumors). These mechanisms are active in many tissue types, which explains the associations between low vitamin D and multiple cancer types.
- Cardiovascular health: VDRs are expressed in cardiomyocytes (heart muscle cells) and vascular smooth muscle cells. Vitamin D regulates the renin-angiotensin system, modulates blood pressure, reduces arterial inflammation, and improves endothelial function. Deficiency is independently associated with increased risk of hypertension, heart failure, and coronary artery disease.
- Metabolic function: Vitamin D receptors are expressed in pancreatic beta cells, and vitamin D supports insulin secretion and insulin sensitivity. Deficiency is associated with increased risk of type 2 diabetes and metabolic syndrome — a relationship that is not fully explained by confounding with obesity.
- Brain health: VDRs are expressed throughout the central nervous system. Vitamin D promotes nerve growth factor synthesis, protects neurons from oxidative stress, and modulates neurotransmitter synthesis. Low levels are consistently associated with increased risk of depression, cognitive decline, and dementia.
What Drives Vitamin D Down — and Up
Unlike most biomarkers covered in this library, vitamin D status is driven primarily by a single input — sun exposure — with supplementation as the principal intervention when that input is inadequate. Understanding why modern life makes deficiency so common is essential context.
Factors that lower vitamin D
- Living at high latitudes — insufficient UVB radiation reaches the skin at latitudes above approximately 35°N or below 35°S for months at a time; residents of northern US cities, Canada, and northern Europe cannot produce meaningful vitamin D from sunlight in winter regardless of time spent outside
- Sunscreen use — SPF 30 reduces vitamin D skin synthesis by approximately 95%; any meaningful sun protection essentially eliminates cutaneous vitamin D production
- Indoor lifestyle — the average American spends over 90% of their time indoors; window glass blocks UVB entirely
- Darker skin pigmentation — melanin acts as a natural sunscreen; people with darker skin require significantly more sun exposure to produce the same amount of vitamin D as those with lighter skin
- Obesity — vitamin D is fat-soluble and is sequestered in adipose tissue; people with higher body fat have lower circulating vitamin D for a given level of sun exposure or supplementation
- Aging — vitamin D synthesis in the skin declines significantly with age; adults over 70 may produce only 25% as much vitamin D from sun exposure as younger adults
- Gut malabsorption — conditions like Crohn's disease, celiac disease, and bariatric surgery reduce absorption of fat-soluble vitamins including D
- Certain medications — including anticonvulsants, glucocorticoids, and some HIV medications, which accelerate vitamin D metabolism
Factors that raise vitamin D
- UVB sun exposure — midday sun exposure to bare skin (without sunscreen) is the most efficient source; 15–30 minutes of full-body exposure in summer for light-skinned individuals can produce 10,000–20,000 IU; however, this must be balanced against skin cancer risk
- Vitamin D3 supplementation — the most practical and controllable way to achieve and maintain optimal levels; dose must be calibrated to the individual through testing
- Fatty fish (salmon, mackerel, sardines) — among the few meaningful dietary sources; a serving of wild salmon contains roughly 600–1,000 IU; dietary sources alone are generally insufficient to reach optimal levels
- Cod liver oil — one of the most concentrated dietary sources, providing 1,000–2,000 IU per tablespoon
- Weight loss — reducing adipose tissue releases sequestered vitamin D back into circulation
- Taking vitamin D with the largest meal of the day — vitamin D is fat-soluble; absorption improves significantly when taken with dietary fat
Vitamin D3 vs. D2 — and the Case for K2 Co-supplementation
When supplementing vitamin D, two practical questions arise: which form, and what else to take with it.
D3 vs. D2: Vitamin D3 (cholecalciferol) is clearly superior to vitamin D2 (ergocalciferol) for maintaining blood levels. A systematic review published in the American Journal of Clinical Nutrition found that D3 was approximately 87% more potent than D2 in raising serum 25(OH)D levels and maintained those levels for significantly longer. 3 Always choose D3 for supplementation.
Vitamin K2 co-supplementation: As vitamin D increases calcium absorption from the gut, there is a theoretical concern — and growing evidence — that without adequate vitamin K2, absorbed calcium may be deposited in soft tissues (arteries, kidneys) rather than routed to bone. Vitamin K2 activates proteins (including osteocalcin and matrix Gla protein) that direct calcium into bone and away from arteries. Many longevity physicians recommend taking vitamin K2 (as MK-7, 100–200 mcg/day) alongside vitamin D supplements, particularly at higher doses. This is especially relevant for people with cardiovascular risk factors.
Magnesium is also required for vitamin D metabolism — it is a cofactor for the enzymes that activate vitamin D in the liver and kidneys. People with low magnesium status may not fully benefit from vitamin D supplementation, and magnesium supplementation has been shown to improve vitamin D activation. Given that magnesium deficiency is itself prevalent in Western populations, this is worth noting.
How to Test Vitamin D
Vitamin D is measured as 25-hydroxyvitamin D (25(OH)D) in a standard blood draw. This is the storage form of vitamin D and the appropriate marker for assessing overall vitamin D status. It should not be confused with 1,25-dihydroxyvitamin D (calcitriol), which is the active hormonal form — calcitriol is tightly regulated and does not reflect vitamin D stores or deficiency.
Always confirm that the test being ordered is 25(OH)D total — which includes both D2 and D3 fractions. This is standard in most commercial labs but worth verifying.
Through a longevity testing service: Function Health, InsideTracker, and Marek Health all include 25(OH)D as part of their comprehensive panels. The advantage of a full panel is seeing vitamin D in context with other markers — including hormones, metabolic function, and inflammation — which provides a more complete picture of what deficiency may be affecting.
À la carte through Ulta Lab Tests: A 25(OH)D test can be ordered without a doctor's visit for $30–45 at most US locations, with results in 24–48 hours. This is the most practical option for retesting after supplementation to verify you've reached your target range.
Through your physician: Any primary care physician can order 25(OH)D. It is increasingly included in annual blood panels, but you may need to request it specifically. It is commonly covered by insurance when ordered for patients with risk factors for deficiency.
How Often Should You Test?
If you are starting supplementation to correct a deficiency, retest after 8–12 weeks to assess your response and adjust dose. Vitamin D levels take time to stabilize — the half-life of 25(OH)D is approximately 2–3 weeks, so allow at least two months for a new dose to reach steady state before drawing conclusions.
Once you've achieved and maintained a level in the 50–80 ng/mL range for two consecutive tests, testing every 6–12 months is sufficient. Seasonal variation is meaningful — levels often drop in winter even with consistent supplementation, so testing in late winter gives you the most clinically relevant baseline.
If you are supplementing at higher doses (above 5,000 IU/day), annual testing is important to ensure you remain in the safe and optimal range and have not drifted above 100 ng/mL.
Sources
- Schöttker B, et al. "Strong Associations of 25-Hydroxyvitamin D Concentrations with All-Cause, Cardiovascular, Cancer, and Respiratory Disease Mortality in a Large Cohort Study." BMJ, 2014. PubMed →
- Garland CF, et al. "Vitamin D and Prevention of Breast Cancer: Pooled Analysis." Journal of Steroid Biochemistry and Molecular Biology, 2007. PubMed →
- Trang HM, et al. "Evidence That Vitamin D3 Increases Serum 25-Hydroxyvitamin D More Efficiently Than Does Vitamin D2." American Journal of Clinical Nutrition, 1998. PubMed →
| Range Type | Value (ng/mL) | Notes |
|---|---|---|
| Standard Clinical Range | Above 20 ng/mL | Designed to identify disease risk — not longevity optimisation. |
| Longevity-Optimal Target | 50–80 ng/mL |
Associated with reduced all-cause mortality and extended healthspan.
Standard medicine defines deficiency below 20 ng/mL and sufficiency above 30 ng/mL — thresholds based primarily on bone health endpoints. Longevity medicine targets 50–80 ng/mL based on observational data showing the lowest all-cause mortality in this range, and research demonstrating pleiotropic benefits beyond skeletal health at higher levels.
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What is the optimal vitamin D level for longevity?
For longevity optimization, the target 25(OH)D level is 50–80 ng/mL. Standard clinical guidelines consider anything above 20 ng/mL sufficient and anything above 30 ng/mL optimal for bone health. But research on all-cause mortality, cancer incidence, cardiovascular disease, and immune function consistently identifies the 50–80 ng/mL range as associated with the best outcomes in population studies. Levels above 100 ng/mL may carry risk of toxicity, so the upper bound is meaningful — more is not necessarily better.
How much vitamin D should I take?
There is no universal dose because individual response to supplementation varies significantly based on body weight, gut absorption, baseline levels, sun exposure, and genetics. Most adults with low or deficient levels need 2,000–5,000 IU of vitamin D3 per day to achieve and maintain the optimal range. The only reliable way to determine the right dose is to test your baseline level, supplement for 8–12 weeks, then retest. Adjusting dose based on measured blood levels is far more reliable than following a generic recommendation.
Why is vitamin D called a hormone?
Unlike true vitamins that must be obtained from diet because the body cannot make them, vitamin D is synthesized by the body (in the skin, via sunlight) and then activated into a form (1,25-dihydroxyvitamin D, or calcitriol) that acts as a steroid hormone. It binds to vitamin D receptors (VDRs) found in virtually every cell type in the body, regulating the expression of hundreds of genes. This is qualitatively different from how most vitamins work and is why researchers increasingly refer to vitamin D as a hormone system.
Can you get enough vitamin D from sunlight?
In theory, yes — but in practice, most people living in modern societies cannot. Adequate UVB exposure to bare skin is only available at sufficient latitudes during limited hours (roughly 10am–3pm) and only for certain months of the year. Sunscreen use, indoor lifestyles, darker skin pigmentation (which reduces UVB penetration), obesity (which sequesters vitamin D in fat tissue), and aging (which reduces skin synthesis efficiency) all dramatically reduce vitamin D production from sunlight. For most adults in developed countries, supplementation is necessary to maintain optimal levels year-round.
What is the difference between vitamin D2 and vitamin D3?
Vitamin D3 (cholecalciferol) is the form produced in human skin and found in animal-derived foods. Vitamin D2 (ergocalciferol) is derived from plant sources and fungi. Multiple studies have shown that vitamin D3 is more effective at raising and sustaining blood levels of 25(OH)D than vitamin D2 at equivalent doses — D3 has a longer half-life and greater potency. For supplementation purposes, always choose vitamin D3. D2 is still used in some prescription formulations but is considered inferior for maintaining optimal status.