Vitamin D is the nutrient most worth supplementing for most people in northern latitudes. An estimated 35–40% of Europeans and North Americans are deficient (serum 25(OH)D below 50 nmol/L), and the health consequences — reduced immune function, poor bone density, muscle weakness, impaired mood regulation — are real and well-documented.
This guide explains what form to take, how much, whether you need D3+K2, and which products are actually worth buying.
D3 vs D2: This Matters
Vitamin D3 (cholecalciferol) is the form produced in human skin when exposed to UVB radiation. It raises serum 25(OH)D roughly 87% more effectively than D2 at equivalent doses and is the form found in nearly all high-quality supplements.
Vitamin D2 (ergocalciferol) is produced by fungi and plants (including UV-exposed mushrooms). It's less potent, metabolised differently, and clears from circulation faster. It's found in some plant-based supplements and is the form used in some prescription vitamin D products.
Take D3 unless you are strictly vegan — and even then, vegan D3 sourced from lichen is now widely available and equally effective. See our detailed comparison: vitamin D3 vs D2.
How Much Do You Need?
The answer depends on your current blood level. Without testing, you're guessing.
General guidance without testing:
- Maintenance for adults with reasonable sun exposure: 400–800 IU/day
- Maintenance for adults with limited sun exposure (UK/Northern Europe winters, office workers): 1,000–2,000 IU/day
- Correcting likely deficiency: 2,000–4,000 IU/day
With a blood test showing 25(OH)D below 50 nmol/L: corrective dosing of 3,000–5,000 IU/day for 8–12 weeks, then retest. Many doctors prescribe 800–1,600 IU/day (NHS standard) — evidence suggests this is often insufficient to correct deficiency efficiently.
Upper safe limit: 4,000 IU/day is widely considered safe for indefinite use. The EU Tolerable Upper Intake Level is 4,000 IU/day. Vitamin D toxicity (hypercalcaemia) can occur with sustained very high doses (typically 10,000+ IU/day for months), but is uncommon at 4,000 IU/day.
Should You Take D3 + K2?
Vitamin K2 (particularly the MK-7 form) is increasingly included in vitamin D supplements, and there's biological rationale for this combination:
- Vitamin D increases calcium absorption from the gut
- Without adequate K2, this calcium may deposit in arteries (calcification) rather than bone
- Vitamin K2 activates matrix Gla protein (MGP) and osteocalcin — proteins that direct calcium to bone and away from arteries
- Studies (including the Rotterdam Study) find K2 intake independently associated with reduced arterial calcification and improved bone density
The evidence for clinical benefit of the combination over D3 alone is suggestive but not definitive in RCTs. The safety rationale is more compelling: anyone at risk of arterial calcification (older adults, cardiovascular risk factors) has reason to include K2.
MK-7 is the preferred K2 form: longer half-life than MK-4, better sustained serum levels. Look for at least 90–100 mcg MK-7 paired with vitamin D.
Note: High-dose vitamin K2 may interact with anticoagulants (warfarin). If you're on blood thinners, consult your doctor before taking K2.
Take Vitamin D with Fat
Vitamin D is a fat-soluble vitamin. Taking it with a fat-containing meal significantly improves absorption — studies show 30–50% greater 25(OH)D response when vitamin D is taken with a meal versus fasted. If you take it in the morning with a fat-free coffee, you're losing a meaningful fraction of the dose.
What to Ignore on Vitamin D Labels
- "Pharmaceutical grade" — not a regulated term
- "Enhanced absorption formula" without detail — often just oil-based soft gels (which is fine, but not special)
- Proprietary blends hiding actual dosages — find products listing exact mcg or IU per serving
- D2 in anything labelled as premium — see above
Best Vitamin D Supplements 2026
Affiliate disclosure: NutritionPromise earns a commission on qualifying purchases at no extra cost to you. Rankings are based on evidence, form, dose, quality testing, and value.
1. Thorne Vitamin D/K2 — Best Overall
Form: D3 (cholecalciferol) + K2 (MK-4, 200 mcg) in MCT oil Dose: 1,000 IU D3 per drop (liquid, adjustable dosing) Third-party tested: NSF Certified for Sport Best for: Adults wanting combination D+K2 with dosing flexibility
Thorne's liquid D/K2 delivers vitamin D3 in MCT oil (enhances absorption), with high-dose K2 in MK-4 form. NSF certification means it passes third-party purity and potency testing. The liquid format allows easy dose adjustment — useful when correcting deficiency and tapering to maintenance. Thorne has one of the most credible quality assurance reputations in the supplement industry.
2. BetterYou Vitamin D4000 Oral Spray — Best for Absorption Optimisation
Form: D3 in MCT oil, oral spray delivering to buccal mucosa Dose: 4,000 IU per 4 sprays Certified by: Informed Sport Best for: Those who prefer spray administration, or have fat malabsorption concerns
BetterYou's oral spray delivers vitamin D3 directly to the mucous membrane in the mouth, bypassing the digestive absorption pathway that requires dietary fat. This makes it particularly useful for people with conditions affecting fat absorption (Crohn's, coeliac, gastric bypass). For healthy adults, absorption advantage over oil-based soft gels is modest but real. Informed Sport certified.
3. Doctor's Best Vitamin D3 5000 IU — Best Value D3 Only
Form: D3 in olive oil Dose: 5,000 IU per soft gel Third-party tested: Labdoor A+ rated Best for: Those correcting known deficiency, wanting cost-effective high-dose D3
Doctor's Best D3 consistently performs well in independent testing (Labdoor rated it A+), is affordable, delivers D3 in olive oil for absorption, and has no unnecessary additives. At 5,000 IU, it's suited to correcting deficiency rather than maintenance — take every other day or 3× per week once deficiency is corrected.
4. Vitabiotics Ultra Vitamin D3 + K2 — Best UK Budget Option
Form: D3 + K2 (MK-7, 45 mcg) Dose: 1,000 IU D3 Availability: Widely available in UK pharmacies (Boots, Superdrug, Amazon) Best for: UK adults wanting a combined D3+K2 at low cost
Vitabiotics is the UK's leading supplement brand and passes Medicines and Healthcare products Regulatory Agency (MHRA) manufacturing standards. The D3+K2 combination at 1,000 IU is well-suited to maintenance dosing for adults in the UK. Not third-party tested to the same standard as Thorne, but represents very strong value for a reputable manufacturer.
5. Nordic Naturals Zero Sugar Vitamin D3 Gummies — Best for Those Who Dislike Capsules
Form: D3 in coconut oil Dose: 1,000 IU per gummy Third-party tested: IFOS/Informed Sport Best for: People who won't consistently take capsules, children
Gummies are less bioavailable than oil-based soft gels, but better than not taking them. Nordic Naturals formulates without excessive sugar (2g per serving), uses real fruit pectin rather than gelatin, and passes independent testing. Good option for compliance in people who consistently forget or avoid capsules.
When to Test, Not Just Supplement
If you've been supplementing vitamin D for 3+ months and want to know whether your dosing is adequate, a 25(OH)D blood test is the answer. In the UK, this is available through the NHS (by doctor request) or privately through services like Thriva, Medichecks, or Randox for £20–40.
Target serum level: 75–125 nmol/L is the range associated with optimal health outcomes in most studies. Below 50 nmol/L is deficient; 50–75 nmol/L is insufficient.
For signs your vitamin D might be low, see our full guide to vitamin D deficiency symptoms, and for how vitamin D fits into your overall supplementation strategy, see what vitamins should I take daily.
Frequently Asked Questions
Can you get enough vitamin D from the sun? In summer, yes — 10–30 minutes of midday sun on arms and legs produces 10,000–25,000 IU in fair-skinned people. But UVB doesn't penetrate adequately at latitudes above 37° during autumn and winter, and factors like clothing, sunscreen (SPF 15 blocks ~93% of vitamin D synthesis), and skin tone dramatically reduce production year-round.
Should you take vitamin D every day or in larger doses less frequently? Daily dosing produces more stable serum levels. Weekly or bi-weekly megadoses (e.g., 50,000 IU weekly) are sometimes prescribed for rapid correction and are effective, but daily lower-dose supplementation is preferable for maintenance.
Does vitamin D help with depression? The relationship between vitamin D deficiency and depression is real — multiple meta-analyses show correlation, and supplementation in deficient individuals improves mood scores. Whether supplementation improves mood in non-deficient individuals is less clear.
Can you take too much vitamin D? Yes. Vitamin D toxicity (hypervitaminosis D) causes hypercalcaemia — symptoms include nausea, weakness, frequent urination, and in severe cases, kidney damage. This typically requires sustained very high doses (10,000+ IU/day for months). At 4,000 IU/day, the risk is extremely low in adults.
Does K2 interfere with vitamin D? No — they work synergistically. K2 helps direct the calcium that vitamin D facilitates absorption of, making the combination safer and potentially more effective for bone health than vitamin D alone.
Sources & References
- Tripkovic L, et al. "Comparison of vitamin D2 and D3 supplementation in raising serum 25(OH)D status." American Journal of Clinical Nutrition, 2012.
- Holick MF. "Vitamin D deficiency." New England Journal of Medicine, 2007.
- Martineau AR, et al. "Vitamin D supplementation to prevent acute respiratory tract infections." BMJ, 2017.
- Geleijnse JM, et al. "Dietary intake of menaquinone is associated with a reduced risk of coronary heart disease." Journal of Nutrition, 2004.
- Dawson-Hughes B, et al. "Meal conditions affect the absorption of supplemental vitamin D3." JBMR Plus, 2020.