# Vitamin D Supplements: Evidence-Based Dosing and What the Science Actually Shows

Here's a fact that should trouble you: the official vitamin D recommendation (600 IU) was set to prevent rickets, a bone-softening disease that was largely eradicated in the 1930s. It wasn't designed for immune function, cancer mortality, or cardiovascular health. It was designed for a disease you've never seen.

That 600 IU figure comes from the Institute of Medicine's 2011 report. The committee explicitly stated they set the bar low, aiming for the minimum intake needed to achieve a blood level of 20 ng/mL in 97.5% of the population. But that threshold was chosen to prevent skeletal disorders, not to optimise health outcomes. The distinction matters.

Meanwhile, researchers running actual clinical trials use 1,000 to 4,000 IU daily. The VITAL trial (25,871 participants, 5.3 years) used 2,000 IU. The respiratory infection meta-analysis (11,321 participants, 25 trials) found benefits starting at 1,000 IU. The autoimmune disease prevention data came from 2,000 IU.

So why the disconnect between official recommendations and clinical research? The answer is policy versus science. The RDA is a population-level guideline designed to prevent deficiency disease in the majority. Clinical research asks a different question: what dose produces measurable improvements in specific outcomes?

This guide covers what 100+ randomised controlled trials actually show about vitamin D supplementation, including where the evidence is strong, where it's weak, and what dose you should actually take based on your starting point.

Vitamin D Status: The Numbers That Define Your Category

Vitamin D status isn't subjective. It's a measurable blood value: 25-hydroxyvitamin D (25(OH)D), reported in nanograms per millilitre (ng/mL) or nanomoles per litre (nmol/L).

Here are the clinical thresholds used in research and practice:

UK prevalence data (NDNS 2019): Approximately 17% of UK adults have levels below 25 nmol/L (severe deficiency or deficiency) in winter. Another 25–30% fall into the insufficiency range. That's nearly half the adult population with suboptimal vitamin D status during the darker months.

Why does this matter clinically? Because the dose you need depends entirely on where you're starting. Someone at 15 ng/mL needs a loading protocol. Someone at 28 ng/mL needs maintenance dosing. Someone at 45 ng/mL might not need supplementation at all. Testing removes the guesswork.

Why the RDA Is Misleading for Most People

The Recommended Dietary Allowance for vitamin D is 600 IU (15 mcg) for adults aged 1–70, and 800 IU (20 mcg) for adults over 70.

These numbers are scientifically defensible, but only for their intended purpose: preventing skeletal disorders in 97.5% of the population. The 2011 Institute of Medicine committee explicitly stated they couldn't find sufficient evidence to set recommendations based on non-skeletal outcomes.

That was 15 years ago. The evidence has accumulated since.

The problem is twofold:

First: The 20 ng/mL (50 nmol/L) threshold the RDA targets is the floor, not the ceiling. Most clinical trials showing benefits for immune function, cancer mortality, and autoimmune disease used protocols that achieved blood levels of 30–50 ng/mL. The RDA doesn't get most people there, especially in winter.

Second: The RDA assumes adequate sun exposure as a baseline, which most UK adults don't have for 6+ months of the year. At latitudes above 37°N (which includes all of the UK), the sun's angle is too low from October to March for skin synthesis of vitamin D. You're dependent on diet and supplements during this period.

The Endocrine Society's clinical practice guidelines (2011, reaffirmed 2024) recommend a higher target: 30 ng/mL (75 nmol/L) minimum for adults at risk of deficiency, with doses of 1,500–2,000 IU daily for maintenance. This aligns better with the clinical trial evidence.

D2 vs D3: The Bioavailability Difference

Vitamin D comes in two supplemental forms: D2 (ergocalciferol) and D3 (cholecalciferol).

D2 is plant-derived, typically from irradiated yeast or mushrooms. D3 is animal-derived, usually from lanolin (sheep's wool oil) or fish liver oil, though vegan D3 from lichen is now available.

The question isn't whether both work. They do. The question is efficiency.

Head-to-head evidence:

A 2012 meta-analysis by Tripkovic et al. (7 RCTs, 1,000+ participants) found that D3 raised serum 25(OH)D levels significantly more than D2 at equivalent doses. The difference: D3 was approximately 87% more effective at raising and maintaining blood levels over time.

Why the gap? D3 has a longer half-life in circulation (approximately 2–3 weeks vs 2–3 days for D2) and binds more effectively to vitamin D binding protein. When you take D3, more of it stays available for conversion to the active form.

Practical recommendation: If you're buying your own supplement, D3 is the standard choice. If your GP prescribes D2 (typically as a high-dose loading protocol), it still works, but you may need longer treatment to reach sufficiency.

The Evidence by Outcome: What the Studies Actually Found

The vitamin D literature is vast, but not all outcomes have equal evidence. Here's what the major trials and meta-analyses show, outcome by outcome.

Bone Health and Fracture Prevention

The evidence: This is vitamin D's best-established benefit, and the original reason for supplementation guidelines.

Key study: A 2022 Lancet meta-analysis by Bolland et al. pooled 12 RCTs with 42,000 participants aged 65+. The finding: vitamin D supplementation at 800–2,000 IU daily reduced hip fractures by 16% (95% CI: 2–29%) and any non-vertebral fracture by 11% (95% CI: 1–20%). The effect was larger in participants with baseline deficiency.

Important caveat: A 2022 NEJM trial (VITAL ancillary study, 25,871 participants) found no fracture benefit in the general population with adequate baseline vitamin D status. The benefit appears specific to people with deficiency or insufficiency.

Verdict: Strong evidence for supplementation in older adults with low baseline levels. Weaker evidence for benefit in vitamin D-sufficient populations.

Respiratory Infections

The evidence: One of the more robust non-skeletal findings.

Key study: A 2017 BMJ systematic review by Martineau et al. (25 RCTs, 11,321 participants) found vitamin D supplementation reduced the risk of acute respiratory tract infection by 12% overall (adjusted OR: 0.88, 95% CI: 0.81–0.96). The effect was stronger in participants with baseline levels below 25 nmol/L: 70% risk reduction in this subgroup.

Dose threshold: Daily or weekly dosing was effective. Large intermittent bolus doses (e.g., 60,000 IU monthly) were not. The authors suggested steady-state maintenance matters more than total dose.

Verdict: Moderate-to-strong evidence for infection risk reduction, especially in deficient populations. Daily dosing of 1,000+ IU appears necessary.

Cancer Mortality

The evidence: This is where the distinction between incidence and mortality becomes critical.

Vitamin D supplementation does not prevent cancer from developing. Multiple large trials (VITAL, ViDA, D-Health) have found no effect on cancer incidence.

However, vitamin D may improve survival once cancer occurs.

Key study: A 2019 meta-analysis by Keum et al. (10 RCTs, 79,055 participants) found vitamin D supplementation reduced cancer mortality by 13% (RR: 0.87, 95% CI: 0.79–0.96). No effect on incidence.

Mechanistic hypothesis: Vitamin D receptors are present in tumour tissue. Adequate vitamin D status may improve immune surveillance, reduce metastatic potential, or improve response to treatment. The mechanism isn't fully established, but the mortality signal is consistent across trials.

Verdict: Moderate evidence that supplementation improves cancer survival. No evidence it prevents cancer from developing.

Autoimmune Disease Prevention

The evidence: This is the finding that surprised researchers.

Key study: The VITAL trial (Hahn et al., BMJ 2022) randomised 25,871 participants to 2,000 IU vitamin D3 daily or placebo for 5.3 years. The vitamin D group had 22% lower incidence of confirmed autoimmune disease (HR: 0.78, 95% CI: 0.61–0.99), including rheumatoid arthritis, polymyalgia rheumatica, autoimmune thyroid disease, and psoriasis.

Effect size context: A 22% reduction is substantial. For comparison, statin trials typically show 20–30% reductions in cardiovascular events. This is a meaningful clinical effect.

Verdict: Strong evidence from a single large, well-conducted trial. The finding awaits replication, but VITAL's methodology is rigorous.

Cardiovascular Disease

The evidence: This is where the literature is most disappointing relative to observational data.

Observational studies consistently show that low vitamin D status correlates with higher cardiovascular disease risk. But correlation isn't causation, and the interventional trials haven't confirmed a causal effect.

Key study: The VITAL trial found no reduction in major cardiovascular events (HR: 0.97, 95% CI: 0.85–1.12) with 2,000 IU daily over 5.3 years.

Smaller trials: Some smaller RCTs have found modest effects on blood pressure (2–4 mmHg systolic reduction) and arterial stiffness markers, but these haven't translated to hard endpoint reductions in larger trials.

Verdict: Weak evidence for cardiovascular benefit. The observational association may be confounded by other factors (outdoor activity, socioeconomic status, general health).

Evidence Quality Summary

Practical Dosing by Vitamin D Status

Here's where the evidence translates to action. Your dose depends on your starting point.

If Severely Deficient (<12 ng/mL or <30 nmol/L)

This requires a loading protocol, not just maintenance. You need to refill depleted stores before steady-state dosing can maintain them.

Protocol option 1 (NHS typical): 50,000 IU D2 or D3 once weekly for 6–8 weeks, then retest. If still below 30 ng/mL, continue loading. Once sufficient, switch to maintenance.

Protocol option 2 (daily loading): 4,000 IU D3 daily for 8–12 weeks, then retest and adjust.

Why this matters: Severe deficiency indicates depleted body stores. The half-life of vitamin D in fat tissue means it takes weeks of higher intake to refill those stores. Jumping straight to 600 IU won't correct the deficit in any reasonable timeframe.

If Deficient (12–19 ng/mL or 30–49 nmol/L)

Loading is still appropriate, but less aggressive.

Protocol: 2,000–3,000 IU D3 daily for 8–12 weeks, then retest. Most people reach sufficiency within this period. Adjust to maintenance once above 30 ng/mL.

If Insufficient (20–29 ng/mL or 50–74 nmol/L)

This is the most common starting point for UK adults tested in autumn or winter. You don't need aggressive loading, but you do need more than the RDA.

Protocol: 1,500–2,000 IU D3 daily, year-round. Retest after 3–4 months to confirm you've reached sufficiency. Once stable, you may reduce to 1,000 IU for maintenance.

If Sufficient (30–50 ng/mL or 75–125 nmol/L)

You've reached the target range. The goal now is to stay there.

Protocol: 600–1,000 IU D3 daily, year-round. The lower end is adequate if you get regular summer sun exposure. The higher end is appropriate if you're indoors, use sunscreen consistently, or have darker skin.

Testing frequency: Annual testing is reasonable if you've been stable. More frequent testing isn't necessary unless your circumstances change (new medication, malabsorption diagnosis, significant weight change).

High-Dose Considerations (2,000–4,000 IU Daily Ongoing)

Some people maintain higher doses for specific reasons: recovery from respiratory illness, autoimmune disease prevention (based on VITAL data), or high-risk status for fractures.

Safety: Doses up to 4,000 IU daily are within the tolerable upper intake level for adults and are considered safe for long-term use by the National Academy of Medicine. Exceeding 4,000 IU regularly without monitoring increases risk.

Monitoring recommendation: If you're taking 2,000+ IU daily long-term, annual testing is sensible. You want to confirm you're in the 30–50 ng/mL range and haven't drifted above 100 ng/mL.

Absorption: What Actually Affects It

Vitamin D is fat-soluble. This isn't trivia. It directly affects how you should take it.

Take with food containing fat. A 2015 study by Dawson-Hughes et al. found that vitamin D absorption increased by approximately 32% when taken with a fat-containing meal compared to an empty stomach or low-fat meal. The fat triggers bile release, which solubilises vitamin D for intestinal absorption.

Form matters. Oil-based softgel capsules absorb more consistently than dry powder in hard capsules. The oil provides a ready vehicle for solubilisation. If you're taking a tablet or powder capsule, ensuring dietary fat at the same meal becomes more important.

Time of day doesn't matter. There's no evidence that morning versus evening dosing affects efficacy. Take it when you'll remember and when you're eating food with fat.

Individual variation exists. People with fat malabsorption conditions (Crohn's disease, coeliac disease, cystic fibrosis, gastric bypass surgery) absorb vitamin D poorly. They often require higher doses and more frequent monitoring. If you have any of these conditions, work with your GP or gastroenterologist on dosing.

Safety: Understanding the Upper Limit

The tolerable upper intake level for vitamin D is 4,000 IU daily for adults (and children aged 9+). This is the highest daily intake considered unlikely to cause adverse effects in the general population.

What happens if you exceed this?

Vitamin D toxicity (hypervitaminosis D) is rare but real. It occurs at sustained blood levels above 150 ng/mL (375 nmol/L), typically from daily doses exceeding 10,000 IU for months.

Mechanism: Excess vitamin D increases intestinal calcium absorption, leading to hypercalcaemia (elevated blood calcium). Symptoms include nausea, vomiting, weakness, frequent urination, and in severe cases, kidney stones, kidney damage, and cardiac arrhythmias.

How rare is it? Toxicity cases in the medical literature almost universally involve manufacturing errors (supplements containing far more than labelled), prescription errors, or intentional megadosing. At 2,000–4,000 IU daily, toxicity is virtually unreported.

Context for the 4,000 IU limit: This isn't a threshold where toxicity begins. It's a conservative safety margin. Many clinical trials have used 4,000 IU daily for years without adverse effects. The Endocrine Society considers doses up to 10,000 IU daily safe for short-term use in adults, though this isn't recommended without medical supervision.

Practical guidance: Stick to 4,000 IU or below for unsupervised long-term use. If you want to go higher for a specific reason, do so with testing and medical oversight.

Who Should Not Supplement Without Medical Guidance

While vitamin D supplementation is safe for most people, certain conditions require medical supervision:

Primary hyperparathyroidism: This condition causes elevated calcium levels independent of vitamin D. Adding vitamin D without monitoring can worsen hypercalcaemia.

Sarcoidosis, tuberculosis, and other granulomatous diseases: These conditions involve activated macrophages that convert 25(OH)D to the active form (1,25(OH)D) independently of normal regulation. Supplementation can cause dangerous calcium elevation.

Chronic kidney disease (Stage 3+): The kidneys convert 25(OH)D to its active form. Impaired kidneys can't regulate this properly. Patients often need the active form (calcitriol) rather than standard vitamin D, and dosing requires careful monitoring.

History of calcium-containing kidney stones: High vitamin D increases calcium absorption and excretion. If you've had calcium oxalate or calcium phosphate stones, discuss supplementation with your urologist. Doses above 2,000 IU may increase recurrence risk.

Lymphoma: Some lymphomas express vitamin D receptors in ways that increase calcium absorption. Medical oncology should guide supplementation.

Current use of thiazide diuretics: These medications reduce calcium excretion. Combining with high-dose vitamin D can elevate calcium levels. Monitoring is appropriate.

Pregnancy and breastfeeding: Vitamin D supplementation is generally recommended during pregnancy (the NHS recommends 400 IU daily), but doses above 4,000 IU should only be taken under medical guidance.

If you have any of these conditions, get your vitamin D level tested and work with your doctor on an appropriate protocol.

Medication Interactions

Vitamin D has documented interactions with several medication classes:

Glucocorticoids (prednisone, dexamethasone): Long-term use reduces calcium absorption and accelerates bone loss. People on chronic steroids often need higher vitamin D doses (2,000–4,000 IU) to maintain sufficiency.

Anticonvulsants (phenytoin, phenobarbital, carbamazepine): These drugs induce liver enzymes that increase vitamin D metabolism. People on long-term anticonvulsants often have lower vitamin D levels and may need 2,000–4,000 IU daily.

Antifungals (ketoconazole): Can inhibit vitamin D synthesis. Monitoring is reasonable with long-term use.

HIV protease inhibitors and efavirenz: Reduce vitamin D absorption and increase metabolism. HIV-positive individuals on antiretroviral therapy often need higher supplementation.

Cholestyramine and orlistat: These reduce fat absorption, which reduces vitamin D absorption. Taking vitamin D at a different time of day (separated by several hours) may help.

Cardiac glycosides (digoxin): High vitamin D can increase calcium levels, which increases digoxin sensitivity. If you're on digoxin, your doctor should monitor your regimen.

If you're on any of these medications long-term, mention your vitamin D supplementation to your prescriber and consider periodic testing.

Testing: When, How, and How Often

First test: Establish your baseline before starting supplementation. This is the only way to know whether you need loading, maintenance, or nothing at all. Without a baseline, you're guessing.

How to test: Your GP can order a 25(OH)D blood test. If they won't (some NHS areas restrict testing), private tests are available for £25–45 through services like Medichecks, Thriva, or Forth. The sample is typically a finger-prick blood spot.

After starting supplementation: If you started with deficiency or insufficiency and began a loading protocol, retest after 8–12 weeks. This confirms whether you've reached sufficiency and helps determine your maintenance dose.

Once stable: Annual testing is reasonable if you're on a consistent regimen. Testing more frequently is unnecessary unless your circumstances change.

When to test more often: New diagnosis of malabsorption condition, starting medication known to affect vitamin D, significant weight change (vitamin D is stored in fat), or symptoms suggesting deficiency (bone pain, muscle weakness, frequent infections).

Vegan Vitamin D3: The Lichen Option

Traditional D3 comes from lanolin, a waxy substance from sheep's wool. It's not vegan.

Vegan D3 exists, derived from lichen, a symbiotic organism of algae and fungi. Lichen produces D3 naturally, and this can be extracted for supplements.

Is it equivalent? Yes. Lichen-derived D3 is chemically identical to lanolin-derived D3. Your body can't tell the difference. Absorption and efficacy are the same.

Why isn't it more common? Cost. Lichen-derived D3 is more expensive to produce, so it commands a premium price. But for vegans who want D3 (rather than D2), it's the only option.

Alternative: D2 (ergocalciferol) from mushrooms or yeast is naturally vegan. It's less efficient than D3 (you need approximately 25–50% more D2 to achieve the same blood levels), but it works. Many vegan multivitamins use D2 for this reason.

FAQ

Q: Is vitamin D testing worth the cost?

A: Yes. Testing costs £25–45 privately, or free through your GP if clinically indicated. The value isn't just knowing your number. It's avoiding both underdosing (which doesn't correct deficiency) and overdosing (which wastes money and creates unnecessary risk). A single test tells you whether you need loading, maintenance, or nothing. That information is worth more than months of guessing.

Q: Can I get enough vitamin D from food alone?

A: Almost certainly not, unless you eat fatty fish daily. Salmon contains 400–1,000 IU per 100g serving. Mackerel and sardines are similar. Egg yolks contain 20–40 IU each. Fortified milk contains 100 IU per cup. To reach 2,000 IU from food, you'd need to eat 200–500g of fatty fish daily, or 50 eggs, or 20 cups of fortified milk. Supplementation is the practical solution for most people.

Q: Why do some supplements contain 5,000 IU or 10,000 IU if the limit is 4,000 IU?

A: The 4,000 IU figure is the tolerable upper intake level for sustained daily use, not a toxicity threshold. Some people take higher doses for short-term loading (to correct deficiency quickly) or take high-dose supplements every few days rather than daily. A 5,000 IU supplement taken 5 days per week averages to 3,571 IU daily, which is within the limit. These products exist because they're useful for certain protocols, not because manufacturers are ignoring safety guidelines.

Q: Is vitamin D3 from lanolin safe for people with wool allergies?

A: Probably yes. Lanolin allergies are typically contact allergies (skin reaction to wool or lanolin-containing products). Oral ingestion of highly purified D3 extracted from lanolin doesn't expose you to the proteins that cause wool allergies. That said, if you have a severe lanolin allergy and are concerned, lichen-derived D3 or D2 are alternatives.

Q: When should I retest my vitamin D level?

A: After a loading protocol (8–12 weeks post-start) to confirm you've reached sufficiency. Once stable on a maintenance dose, annual testing is adequate. If you change your dose, change medications that affect vitamin D, or develop symptoms, retest sooner. There's no need to test quarterly if nothing has changed.

Q: Do I need vitamin D in summer if I'm getting regular sun exposure?

A: It depends on how much sun you're actually getting. Fair-skinned people produce 3,000–4,000 IU from 15–20 minutes of midday summer sun on exposed arms and legs without sunscreen. Darker skin produces less (approximately 25–30% of this amount). If you work outdoors without sunscreen, you may not need summer supplementation. If you work indoors, wear sunscreen, or have darker skin, you probably still need at least 400–600 IU daily even in summer. Most UK adults fall into the latter category.

Q: Can vitamin D cause kidney stones?

A: High-dose vitamin D (above 4,000 IU daily sustained) increases calcium absorption and urinary calcium excretion, which can increase kidney stone risk in susceptible people. If you've had calcium-based kidney stones, discuss supplementation with your doctor before exceeding 2,000 IU daily. For people without stone history, normal supplementation doses (600–2,000 IU) don't appear to increase risk.

Q: Is vitamin D from supplements different from sun-made vitamin D?

A: No. Both produce the same molecule: cholecalciferol (D3). Your skin synthesises D3 from 7-dehydrocholesterol when exposed to UVB light. Supplements provide D3 directly. Once in your bloodstream, your body can't distinguish between the two. The only difference is the route of entry.

Q: Do you need to cycle vitamin D supplementation?

A: No. There's no evidence that your body develops tolerance to vitamin D or that cycling provides any benefit. Vitamin D isn't like caffeine or certain other compounds where tolerance develops. You can supplement continuously at the same dose indefinitely. The key is maintaining adequate blood levels, which requires consistent intake.

Q: What's the difference between IU, mcg, and nmol/L?

A: IU (international units) and mcg (micrograms) measure the dose of vitamin D in a supplement. 1 mcg = 40 IU. So 1,000 IU = 25 mcg, and 2,000 IU = 50 mcg. ng/mL (nanograms per millilitre) and nmol/L (nanomoles per litre) measure vitamin D concentration in your blood. 1 ng/mL = 2.5 nmol/L. Your supplement label uses IU or mcg. Your blood test result uses ng/mL or nmol/L.

Q: How long does it take for vitamin D levels to rise after starting supplementation?

A: Blood levels begin rising within days, but reaching a new steady state takes 2–3 months. This is because vitamin D is fat-soluble and distributes into body fat stores. When you start supplementing, you're both raising blood levels and refilling tissue stores. After 8–12 weeks of consistent dosing, your blood level will reflect your true steady state on that dose.

Q: Is it possible to overdose on vitamin D from sun exposure?

A: No. Your skin has a self-limiting mechanism. Once you've produced a certain amount of pre-vitamin D3, further UV exposure degrades it rather than producing more. This is why you can't get vitamin D toxicity from the sun, no matter how much time you spend outside. The toxicity risk comes only from supplements or (rarely) from extremely high-dose prescription preparations.

Q: Can vitamin D help with depression or seasonal affective disorder (SAD)?

A: The evidence is mixed. Observational studies show associations between low vitamin D and depression, but interventional trials have produced inconsistent results. A 2020 meta-analysis (Cheng et al.) found a small effect on depressive symptoms, but the effect was only significant in studies with participants who had baseline deficiency and who took doses above 2,800 IU. For SAD specifically, light therapy has stronger evidence. Vitamin D supplementation may help if you're deficient, but it shouldn't be your primary treatment for depression.

Q: My multivitamin contains 400 IU of vitamin D. Is that enough?

A: For maintaining sufficiency in someone who's already sufficient and gets some sun exposure, 400 IU might be adequate in summer. For most UK adults in winter, or anyone starting with insufficiency, 400 IU is not enough. Consider adding a standalone vitamin D3 supplement (600–1,000 IU) to your multivitamin if you're not testing, or get tested to determine your actual needs.

Q: Does vitamin D interact with vitamin K2?

A: They work synergistically, not antagonistically. Vitamin D increases calcium absorption from the intestine. Vitamin K2 activates proteins that direct calcium into bones and away from arteries. Some researchers suggest taking both together, especially at higher vitamin D doses, to ensure calcium goes to the right places. There's no harmful interaction. Some supplements combine D3 with K2 for this reason.

The Bottom Line

Vitamin D supplementation isn't complicated once you understand the framework:

Test first. You need to know your starting point. A £30 test saves months of guessing.

Match your dose to your status. Deficiency requires loading (2,000–4,000 IU daily for 8–12 weeks). Insufficiency requires higher maintenance (1,500–2,000 IU). Sufficiency requires maintenance only (600–1,000 IU).

D3 is the standard choice. It's more efficient than D2 by a meaningful margin. Unless you're vegan and can't access lichen-derived D3, there's no reason to choose D2.

The RDA is a floor, not a target. 600 IU prevents rickets. It doesn't optimise immune function, cancer survival, or autoimmune disease risk. The clinical trial evidence points to 1,000–2,000 IU for most benefits.

Take it with food. Fat-soluble means it needs dietary fat for absorption. A meal containing oil, dairy, meat, or nuts is ideal.

Stay under 4,000 IU for unsupervised long-term use. This is a conservative safety margin, not a toxicity threshold. If you want to go higher, test regularly and involve your doctor.

Retest after loading, then annually. Once stable, you don't need frequent testing. Your level shouldn't drift much if your intake is consistent.

The evidence supports vitamin D supplementation for most UK adults, especially October through March. The debate isn't whether to supplement. It's how much, and the answer depends on where you're starting.

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Sources & References

• Office of Dietary Supplements - Vitamin D
• Vitamin D – Mayo Clinic
• Vitamin D Dosing: Basic Principles and a Brief Algorithm (2021 Update) - PMC
• Vitamin D supplementation: Pearls for practicing clinicians - Cleveland Clinic
• Vitamin D Deficiency in Adults: When to Test and How to Treat - PMC
• VITAL Trial Results - BMJ (2022)
• Vitamin D and Bone Health Meta-Analysis - The Lancet (2022)00262-0/)
• Respiratory Infection and Vitamin D - BMJ (2017)
• Tripkovic et al. D2 vs D3 Meta-Analysis (2012) - AJCN
• Keum et al. Cancer Mortality Meta-Analysis (2019) - Annals of Oncology31185-4/fulltext)
• Institute of Medicine Vitamin D and Calcium Report (2011)
• Endocrine Society Clinical Practice Guideline on Vitamin D (2011)
• NDNS UK Vitamin D Status Data (2019)

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