Should I take D3 with K2?

There is a plausible mechanistic case for combined supplementation — D3 increases calcium absorption, K2 activates matrix Gla-protein and osteocalcin which direct calcium to bone rather than soft tissue — but the head-to-head RCT data versus D3 alone is limited. The strongest single trial supporting K2 supplementation is Knapen et al. 2013 in postmenopausal women on bone mineral density, not specifically a combined-stack trial. If you are taking D3 and considering adding K2, it is a defensible choice for general bone health in postmenopausal women, and a more speculative one in younger adults. Speak to your GP or pharmacist first, particularly if you are on any anticoagulant.

Is the K2/D3 calcium paradox real?

The mechanism is real and supported by animal models and observational data — vitamin K-dependent matrix Gla-protein is a key inhibitor of arterial calcification. The clinical leap — that K2 supplementation prevents arterial disease in humans — is more speculative. Cardiovascular surrogate-marker trials (e.g. Knapen 2015 on arterial stiffness) show modest effects; hard-endpoint trials with mortality and event reductions do not yet exist. A 2020 Cochrane review concluded the evidence was insufficient to recommend K2 for cardiovascular disease prevention. Calling the calcium paradox 'proven' overstates the data.

MK-4 vs MK-7 — which is better?

For UK supplementation, MK-7 is the more practical choice. It has a longer half-life (around 72 hours vs a few hours for MK-4), produces sustained tissue levels at much lower daily doses (90–180 micrograms vs 45 mg for MK-4 trials), and matches the dose used in the strongest bone density RCT (Knapen 2013). MK-4 has its own evidence base, primarily Japanese osteoporosis trials at 45 mg/day, but the dose is impractical and the trials had methodological concerns. Most evidence-led UK K2 products use MK-7 specifically.

Will K2 affect my warfarin INR?

Yes. Warfarin acts by inhibiting vitamin K recycling in the body. Adding K2 from any source — including supplements — reduces warfarin's effect and lowers your INR, which increases clotting risk. If you are on warfarin or any vitamin K antagonist, do not start K2 without first consulting your anticoagulant clinic. Direct oral anticoagulants (apixaban, rivaroxaban, edoxaban, dabigatran) do not work through vitamin K, but you should still check with your prescriber before starting any new supplement.

How much K2 do I actually need?

The UK does not have a specific Reference Nutrient Intake for K2 separately from total vitamin K. The Knapen 2013 bone density trial used 180 micrograms/day of MK-7 for three years. Cardiovascular surrogate-marker trials have used 180–360 micrograms/day. Population observational studies suggesting cardiovascular benefit have estimated dietary K2 intake of 30–45 micrograms/day in northern European cohorts. A reasonable evidence-aligned supplemental dose is 90–180 micrograms/day of MK-7 if you choose to supplement.

Can I get enough K2 from food?

Difficult, in a typical UK diet. The richest source by a wide margin is natto (fermented soybeans), which is rarely eaten in the UK. Aged cheeses like Gouda and Edam, egg yolk from pastured hens, butter from grass-fed cattle, and liver provide modest amounts. UK average dietary K2 intake is estimated at 10–20 micrograms/day, well below the 180 micrograms/day used in the strongest bone trial. Increasing food intake is sensible; matching trial doses without supplementation is essentially impractical.

Vitamin K2 + Vitamin D3: Should You Stack Them? UK Evidence Review

Nutripedia Research Team15 April 2026

Updated 3 May 2026

Combining vitamin K2 with vitamin D3 has become one of the most popular supplement stacks in the UK. The mechanistic story is plausible. The clinical evidence is thinner than the marketing suggests, and the warfarin interaction makes it a stack you should not start without speaking to a clinician first.

Not medical advice

Nutripedia summarises published peer-reviewed research. This content is for informational purposes only and is not a substitute for professional medical advice. Product mentions are not endorsements.

Before You Read This

Nutripedia is a research librarian, not a doctor. Nothing on this page is medical advice. Consult a UK GP, NHS pharmacist, or registered dietitian before starting any supplement, especially if you are pregnant, breastfeeding, on medication, or managing a chronic condition. Two specific points before you read further. Vitamin K2 antagonises warfarin and other vitamin K antagonist anticoagulants. If you take warfarin, do not start vitamin K2 without consulting your anticoagulant clinic. Second, the NHS recommends vitamin D supplementation for most UK adults during the autumn and winter months but does not specifically recommend vitamin K2 — that is your decision to make on the available evidence.

Our research is based on 43 peer-reviewed studies. View the full evidence database

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What to Look for on a UK Label

If you decide to use a K2 + D3 stack, here is what an evidence-aligned product looks like in 2026. K2 form: MK-7 specifically, ideally from a named raw material (MenaQ7, K2VITAL) with disclosed all-trans isomer content of 99%+. K2 dose: 90–180 micrograms/day for general use, scaling up to 180 micrograms/day to mirror the Knapen trials. D3 dose: 1,000–2,000 IU/day for general use; the NHS recommends 400 IU as the population minimum, but most current UK clinical guidance accepts up to 4,000 IU/day as safe in non-deficient adults. Carrier oil: K2 and D3 are fat-soluble; oil-based capsules (MCT, olive, or sunflower) provide better and more consistent absorption than dry tablet forms. Third-party testing: heavy metals, identity, and potency testing on a per-batch basis is the gold standard. Avoid products that bundle K2 and D3 with a long list of other unnecessary ingredients. Realistic 2026 UK pricing: a 60–90 day supply of an evidence-aligned MK-7 + D3 product runs £15–£30 from independent UK retailers; budget products run £8–£15 but more often skip raw material disclosure. **Track this evidence on Nutripedia.** Create a free account to follow new K2 and D3 trials as they publish, compare verified UK retailers across MK-7 raw materials and D3 dosing options, and keep your stack decisions anchored to the latest evidence rather than the marketing cycle. Sign up to keep this research at your fingertips.

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Sources

Nutripedia is an educational resource. Content is sourced from peer-reviewed studies and does not constitute medical advice. Product mentions are not endorsements. Consult a healthcare professional before starting any supplement.

Reviewed by

Archie Roberts

Founder, Nutripedia — ALDR Ltd

This page summarises published research from PubMed, NHS, EFSA, and SACN. It does not constitute medical advice; consult a qualified healthcare professional before changing any supplement regimen.

Last reviewed: 03 May 2026Methodology