Vitamin D Optimal Levels: 2026 Guide Including the 2024 Endocrine Society Update

Introduction: What Is Vitamin D and Why Is "Optimal" Contested?

Vitamin D is a fat-soluble hormone precursor — technically a secosteroid — that the body synthesises in the skin on exposure to UVB radiation, and also obtains in smaller amounts from certain foods. Once in the bloodstream, vitamin D undergoes two hydroxylation steps: first in the liver to 25-hydroxyvitamin D [25(OH)D], and then in the kidney to 1,25-dihydroxyvitamin D (calcitriol), its biologically active form.[1] Vitamin D receptors are found in nearly every tissue in the body, which has prompted decades of research into its role beyond bone health — including immune function, cardiovascular risk, cancer, and mood.

The question of what blood level constitutes "optimal" has generated genuine scientific controversy. Different bodies have drawn their thresholds from different evidence bases, different populations, and different definitions of health outcomes. That disagreement is not a failure of science — it reflects real complexity in the data. This guide explains where each major guideline stands in 2026, what the evidence shows for specific health outcomes, and how to interpret your own 25-hydroxyvitamin D result.

ng/mL vs. nmol/L: Units and Conversion

Vitamin D levels are reported in two units depending on where your lab is based. US laboratories almost universally use nanograms per milliliter (ng/mL). Most European, Canadian, and Australian labs report in nanomoles per liter (nmol/L). The conversion is straightforward: multiply ng/mL by 2.5 to get nmol/L, or divide nmol/L by 2.5 to get ng/mL.

Use our blood test unit converter to switch between ng/mL and nmol/L for any vitamin D result. Not sure where your result sits clinically? Try the vitamin D level interpreter for a personalised breakdown.

Major Guidelines Compared

No single body has definitive authority over vitamin D targets, and the guidelines that exist were written for different purposes and audiences. Here is a structured comparison of the major positions.

IOM / National Academy of Medicine 2010

The Institute of Medicine (now National Academy of Medicine) published its Dietary Reference Intakes for vitamin D in 2010, setting the Recommended Dietary Allowance at 600 IU/day for adults (800 IU for those over 70) and defining sufficiency as 25(OH)D ≥20 ng/mL.[2] This threshold was selected to cover the bone health needs of 97.5% of the population under minimal sun exposure. The IOM explicitly stated that benefits beyond bone health lacked sufficient randomised evidence and should not drive RDA setting. This conservative, population-level framing is the source of much downstream disagreement — it was never intended to define an individual patient's optimal level.

Endocrine Society 2011 (Holick et al.)

The Endocrine Society's 2011 clinical practice guideline, led by Michael Holick, took a more individual, clinical-patient lens.[3] It classified levels below 20 ng/mL as deficient, 21–29 ng/mL as insufficient, and recommended that clinicians target ≥30 ng/mL for adequacy, with a preferred range of 40–60 ng/mL. The guideline cited evidence from observational studies suggesting broader health benefits at higher levels, and acknowledged that many people with "normal" levels by IOM standards may still have sub-optimal vitamin D activity in bone and other tissues. It recommended testing in all at-risk patients and supplementing to achieve the higher target.

Endocrine Society 2024 Update (Demay et al.) — Key Change

The 2024 revision by Demay MB et al., published in The Journal of Clinical Endocrinology & Metabolism, represents a significant course correction.[4] After reviewing the substantial body of large randomised controlled trial data that accumulated after 2011 — including the VITAL trial (Manson 2019), D-HEALTH, and ViDA — the committee concluded that:

• There is insufficient evidence to endorse a single optimal 25(OH)D target for healthy adults.
• Routine vitamin D screening in healthy adults without risk factors is not recommended.
• Empirical supplementation is appropriate in select high-risk populations (see risk factors section below) without needing to first establish baseline levels.
• Evidence for benefits beyond bone health from supplementation in people who are not deficient remains inconsistent.

This does not mean the 2024 guideline endorses ignoring vitamin D — rather, it reflects an honest appraisal of what large RCTs have shown: that supplementing people who already have adequate levels does not reliably produce the disease-prevention benefits that observational studies had suggested.

NHS / NICE (UK)

UK guidance takes a practical, public health approach. NICE recommends that all adults in the UK consider taking 10 micrograms (400 IU) of vitamin D daily through autumn and winter, when UVB intensity is insufficient for skin synthesis at UK latitudes.[5] People at higher risk — those who are housebound, wear covering clothing, or have darker skin — are advised to supplement year-round. The deficiency threshold used in NHS clinical practice is typically below 25 nmol/L (10 ng/mL), with 25–50 nmol/L considered sub-optimal.

GrassrootsHealth — The Higher-Range Position

For completeness, GrassrootsHealth — a non-profit consortium of vitamin D researchers — advocates for population-wide measurement and targets of 40–60 ng/mL based on analysis of observational data and mechanistic arguments.[6] This position represents the upper end of the functional medicine spectrum. While some of the researchers associated with GrassrootsHealth have made important contributions to vitamin D science, the 40–60 ng/mL target for general health has not been confirmed by large RCTs and is not endorsed by any major national health body.

Why Do the Guidelines Disagree?

The core tension is between two legitimate but different frameworks. The IOM and the 2024 Endocrine Society update primarily ask: "What level of vitamin D prevents disease in a population, as demonstrated by RCTs?" The 2011 Endocrine Society guideline asked: "What level is associated with optimal bone and broader health in clinical patients?" These questions have different answers because observational associations frequently do not survive randomised testing.

A key issue is reverse causation: sicker people tend to have lower vitamin D, which makes low vitamin D appear causative in cross-sectional studies. When Mendelian randomisation studies — which use genetic variants affecting vitamin D synthesis to test causality — are applied to many of the outcomes proposed in observational research, the causal links are often weak or absent. This is why the 2024 guideline, informed by Mendelian randomisation analyses and large null RCTs, struck a far more cautious tone than its predecessor.

This does not mean vitamin D is unimportant — preventing frank deficiency is universally agreed to matter. But "not deficient" and "optimised for every health outcome" are not the same claim, and the evidence supports the former far more robustly than the latter.

Outcome-Specific Evidence

Bone Health

The link between vitamin D and bone health is the best-established and least controversial. Vitamin D promotes intestinal calcium absorption; when 25(OH)D falls below 20 ng/mL, calcium absorption drops from roughly 30–40% to 10–15% of dietary intake, stimulating parathyroid hormone (PTH) release and bone resorption.[1] Severe deficiency causes rickets in children and osteomalacia in adults — both characterised by defective bone mineralisation. All major guidelines agree that maintaining 25(OH)D above 20 ng/mL is essential for skeletal integrity. For people focused on bone health specifically, this is the most evidence-grounded target.

Falls and Fractures in Older Adults

Earlier meta-analyses suggested that vitamin D supplementation reduced falls and fractures in older adults, but more recent large RCTs have complicated this picture. A 2022 Cochrane review and the USPSTF concluded that supplementation alone — without confirmed deficiency — does not reliably prevent falls or fractures in community-dwelling adults over 50.[7] The benefit appears concentrated in people who are genuinely deficient or institutionalised. For older adults tracking their vitamin D levels over time, maintaining adequacy (≥20 ng/mL) is still recommended, but supplement-dose escalation beyond deficiency correction is not well-supported.

Cardiovascular Outcomes

The VITAL trial — a landmark double-blind RCT of 25,871 US adults randomly assigned to 2,000 IU/day vitamin D3 or placebo — found no significant reduction in major cardiovascular events over a median 5.3 years follow-up (Manson JE et al., N Engl J Med, 2019).[8] This was despite the fact that supplemented participants achieved meaningfully higher 25(OH)D levels. Mendelian randomisation studies have similarly failed to find consistent causal links between genetically higher vitamin D levels and reduced cardiovascular risk. Observational associations — which are strong — appear largely explained by confounding rather than causation.

Cancer

The VITAL trial's pre-specified secondary cancer analyses (Manson 2019) showed no statistically significant reduction in total cancer incidence with vitamin D supplementation. However, a secondary analysis published by Manson et al. in 2020 found a significant reduction in cancer mortality (not incidence) among participants who were not obese, with a hazard ratio of 0.83 — a finding that has generated interest but requires replication.[8] Separately, some meta-analyses suggest vitamin D may modestly reduce colorectal cancer risk, but effect sizes are small and inconsistent across populations. Current cancer bodies do not recommend vitamin D supplementation specifically for cancer prevention beyond avoiding deficiency.

Autoimmune Conditions

A notable post-hoc finding from VITAL was a statistically significant ~22% reduction in incident autoimmune disease (rheumatoid arthritis, psoriasis, polymyalgia rheumatica, and thyroid disease) in the vitamin D group versus placebo after 5 years (Hahn J et al., BMJ, 2022).[9] This is among the more promising RCT-level findings for vitamin D beyond bone health, though post-hoc analyses from a single trial require cautious interpretation. People managing conditions like lupus often have vitamin D insufficiency as a comorbidity, and maintaining adequate levels is broadly recommended by rheumatology guidelines.

Mood and Depression

Multiple observational studies associate low vitamin D with higher rates of depression, and vitamin D receptors are found in brain regions involved in mood regulation. However, randomised trials of supplementation for depression have produced mixed results — some showing modest benefit in deficient populations, others showing no effect in people with adequate baseline levels.[10] A 2020 Cochrane review found no clear evidence that vitamin D supplementation reduces depression in adults. Research suggests that correcting frank deficiency is worthwhile, but supplementing beyond adequacy as a mood intervention is not currently supported.

Pregnancy

Low vitamin D in pregnancy has been associated with gestational diabetes, pre-eclampsia, preterm birth, and impaired neonatal bone development.[11] Most obstetric guidelines recommend that pregnant women ensure adequate vitamin D intake — typically 400–600 IU/day in prenatal vitamins — with higher doses in confirmed deficiency. Women tracking biomarkers during pregnancy are often advised to test 25(OH)D and supplement accordingly. Evidence for benefit from supplementation in deficient pregnant women is stronger than in the general population. Similarly, women in perimenopause should maintain adequate levels given the increasing importance of bone protection in that life stage.

Risk Factors for Vitamin D Deficiency

Understanding who is at risk is central to the 2024 Endocrine Society guidance, which pivoted from universal screening to targeted supplementation in high-risk groups. Key risk factors include:

• Northern latitude and seasonal variation: Above approximately 37°N (and below 37°S), UVB intensity is insufficient for skin synthesis for several months each year. People in northern Europe, Canada, and northern US states are particularly affected. Office and tech workers who spend extended hours indoors face compounded risk regardless of latitude.
• Darker skin pigmentation: Melanin absorbs UVB and reduces cutaneous vitamin D synthesis. People of African, South Asian, and Middle Eastern descent typically require longer sun exposure to produce equivalent amounts of vitamin D3 compared to those with lighter skin.
• Obesity: Vitamin D is fat-soluble and is sequestered in adipose tissue, effectively reducing its bioavailability in the circulation. People with BMI above 30 generally need higher supplement doses to achieve the same serum response.
• Malabsorption conditions: Coeliac disease, Crohn's disease and IBD, cystic fibrosis, and prior gastric bypass surgery all impair fat-soluble vitamin absorption, making supplementation and monitoring particularly important.
• Kidney disease: The kidney performs the final activation step (1-alpha hydroxylation) converting 25(OH)D to calcitriol. Chronic kidney disease impairs this conversion, and patients may need activated vitamin D (calcitriol or alfacalcidol) rather than cholecalciferol.
• Liver disease: Severe hepatic disease impairs 25-hydroxylation, reducing conversion of dietary and skin-derived vitamin D to the storage form.
• Certain medications: Anticonvulsants (phenytoin, carbamazepine), glucocorticoids (chronic prednisolone), and some antiretrovirals accelerate vitamin D catabolism or reduce its absorption. Regular monitoring and higher supplementation doses are often needed in these patients.
• Older age: Both the skin's capacity for vitamin D synthesis and renal activation decline with age. Adults over 70 are routinely considered a supplementation priority by most guidelines.
• Pilots and flight crew: Frequent high-altitude flight at latitudes with seasonal UVB deficiency means aviation professionals often have low sun exposure despite spending time outdoors at destinations. Routine monitoring can be appropriate.

Testing: When and How

The standard test for vitamin D status is serum 25-hydroxyvitamin D [25(OH)D], also called calcidiol or calcifediol. This is the correct test for assessing whether you have adequate stores. It is distinct from 1,25-dihydroxyvitamin D (calcitriol), which is the active form but reflects kidney activity rather than body stores — that test is reserved for specific conditions such as chronic kidney disease, granulomatous diseases (sarcoidosis, TB), and some lymphomas.[1]

Fasting is not required before a 25(OH)D blood draw. The test is reported in ng/mL or nmol/L depending on your laboratory. Results can vary slightly between laboratories using different assay methods (immunoassay vs. LC-MS/MS), so tracking trends with the same lab is most informative. The 25-hydroxyvitamin D biomarker page provides detailed reference ranges and assay notes. You can also review your results against why "normal" on a lab report does not always mean optimal.

Per the 2024 Endocrine Society guidance, routine testing is not recommended in healthy adults without risk factors. Testing is appropriate when:

• You have one or more of the risk factors listed above
• You have symptoms potentially consistent with deficiency (bone pain, muscle weakness, frequent infections)
• You have osteoporosis, malabsorption, kidney or liver disease, or are taking interacting medications
• You are pregnant or planning pregnancy
• Your clinician needs a baseline before starting supplementation to guide dosing

If you start supplementation after a deficient result, retest after 8–12 weeks to assess response. This interval allows levels to reach a new steady state. Read our complete blood test guide for broader context on interpreting lab results.

Supplementation: Doing It Safely

D2 vs. D3

Vitamin D3 (cholecalciferol) — the form produced by human skin and found in fatty fish, egg yolks, and fortified foods — is generally preferred for supplementation over vitamin D2 (ergocalciferol, found in fungi and plant-based foods). Multiple head-to-head studies have shown that vitamin D3 raises 25(OH)D more effectively and sustains levels longer.[12] At standard maintenance doses, D3 is the recommended choice. At high therapeutic doses, the clinical difference narrows, and prescription-grade D2 is sometimes used.

Dosing

For maintenance in healthy adults with adequate levels, typical supplemental doses range from 400–2,000 IU/day. For repletion from deficiency (<20 ng/mL), clinicians may use:

• A loading phase: e.g., 50,000 IU weekly for 6–8 weeks (prescription-grade D2 or D3), then maintenance
• Daily repletion: e.g., 3,000–6,000 IU/day of D3 for 8–12 weeks, then retest

The response to supplementation varies substantially between individuals based on body weight, baseline level, co-administered calcium, and intestinal absorption. With standard maintenance doses of 1,000–2,000 IU/day, serum 25(OH)D typically rises by 6–10 ng/mL over 8–12 weeks. Dose escalation should be guided by monitoring rather than guesswork.

Toxicity

Vitamin D toxicity (hypervitaminosis D) is uncommon but real. It does not occur from sun exposure — skin synthesis is self-limiting because excess previtamin D is photodegraded. Toxicity from supplements requires sustained excessive intake, generally above 10,000 IU/day over months, driving serum 25(OH)D above 150 ng/mL (375 nmol/L) and causing hypercalcaemia.[13] Symptoms of hypercalcaemia include nausea, vomiting, polyuria, nephrolithiasis (kidney stones), and in severe cases, cardiac arrhythmia and renal failure. The US Tolerable Upper Intake Level is 4,000 IU/day for adults; some clinicians consider up to 10,000 IU/day safe for short-term use in monitored patients. Levels above 100 ng/mL warrant review even before toxicity symptoms appear. Do not mega-dose without medical supervision and periodic monitoring.

What "Optimal" Means in 2026

Synthesising the evidence in 2026, the following positions represent a reasonable reading of the science:

• Below 20 ng/mL (50 nmol/L): Universally considered deficient. Associated with impaired bone mineralisation, elevated PTH, and increased fracture risk. Supplementation is appropriate and evidence-backed.
• 20–29 ng/mL (50–74 nmol/L): A grey zone. Sufficient per IOM for bone health; insufficient per Endocrine Society 2011. Whether supplementation to raise levels above 30 ng/mL provides clinical benefit in an otherwise healthy person is not clearly established by RCTs.
• 30–50 ng/mL (75–125 nmol/L): Broadly accepted as sufficient across most clinical guidelines. Most observational associations with better health outcomes sit in this range. The 2024 Endocrine Society update does not endorse a specific number here but does support ensuring adequacy in at-risk populations.
• 50–100 ng/mL (125–250 nmol/L): The range advocated by some researchers and functional medicine clinicians. Large RCTs have not consistently shown additional benefit at this range compared to 30–50 ng/mL. Not contraindicated in the absence of toxicity, but not supported as a universal target.
• Above 100 ng/mL (>250 nmol/L): Warrants clinical review. While toxicity requires even higher levels, there is no evidence of health benefit above this range and emerging evidence of potential harm.

Track Your Vitamin D Over Time With Health3

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