Vitamin K2 (Menaquinone)
Vitamin K is the umbrella name for a family of fat-soluble compounds needed for the normal synthesis of clotting factors and for the activation (γ-carboxylation) of several bone and vascular proteins. Vitamin K1 (phylloquinone, found mostly in green vegetables) and vitamin K2 (menaquinones, found in fermented foods and animal products) are the two main subgroups. The UK-authorised health claims apply to "vitamin K" generically; the K2 supplement category is positioned around tissue-distribution and half-life differences between the forms, but UK regulation does not distinguish them at label level.
Camden Medicals editorial · Last reviewed 11 June 2026 · Next review June 2027
- Cross-checked against
- NHS
- NICE
- BNF
- EFSA
- FSA
Camden's own editorial team graded each health claim below on the strength of the published evidence — trials weighed with Cochrane RoB 2, systematic reviews with AMSTAR 2, under the CEGA method. See the grade beside every condition.
- Class
- Vitamin
- Typical daily dose
- K2 supplements typically deliver 75–180 µg of MK-7 per day. K2 + D3 combination products often pair 100 µg MK-7 with 1000–4000 IU vitamin D3.
- Top use evidence
- Strong
On this page
- What it is
- At a glance
- What people use it for
- How it works
- Common myths
- Common online questions
- The official position
- Authorised UK claims
- What the guidance says
- Camden's evidence review
- Safety, interactions & who should avoid it
- How to take it
- How to spot quality
- Commonly combined with
- Related ingredients
- Sources
What it is
Vitamin K is a family of 2-methyl-1,4-naphthoquinone derivatives. Vitamin K1 (phylloquinone) is the form synthesised by plants, found in highest amounts in green leafy vegetables (kale, spinach, broccoli, rocket). Vitamin K2 is a family of menaquinones (MK-4 through MK-13), designated by the number of isoprene units in the side chain.
The two K2 forms most relevant to supplementation are MK-4 (the form found in chicken, beef liver, and egg yolk; also synthesised endogenously from K1 in some tissues) and MK-7 (found in natto, a Japanese fermented soybean food, and produced commercially by bacterial fermentation). MK-7 has a substantially longer plasma half-life (around 3 days) compared with K1 and MK-4 (hours), which is the basis for many K2 marketing claims about sustained tissue coverage.
All K vitamers act as cofactors for γ-glutamyl carboxylase, which activates the K-dependent proteins: clotting factors II, VII, IX, X in the liver; osteocalcin in bone; matrix Gla-protein (MGP) in vascular tissue. Carboxylation of MGP is implicated in the inhibition of vascular calcification — the rationale behind the K2 + D3 combination supplement category.
At a glance
- Vitamin K is essential for normal blood clotting and bone metabolism. Deficiency in adults is uncommon outside specific clinical conditions.
- Two subgroups — K1 (phylloquinone, from green leafy vegetables) and K2 (menaquinones, from fermented foods and animal products). The K2 supplement forms are MK-4 (animal-source) and MK-7 (bacterial fermentation, longer half-life); all meet the "vitamin K" classification on labels.
- UK-authorised health claims apply to vitamin K generally — not specifically to K2 or MK-7. Marketing that implies a K1-vs-K2 superiority goes beyond what the GB register authorises.
- CRITICAL DRUG INTERACTION — vitamin K interacts directly with warfarin. Anyone on warfarin must NOT start, stop, or change their vitamin K intake (food or supplement) without speaking to their anticoagulation team; the INR will swing.
What people use it for
Adults eating a typical UK diet with regular green vegetables
Vitamin K1 intake from leafy greens is generally adequate. Authorised UK claims for vitamin K (blood clotting, bone maintenance) apply at NRV-level intakes. Most adults do not need a supplement for general adequacy. [4,1]
Some evidenceStrongAdults supplementing vitamin D3 at higher doses (1000 IU+)
A K2 + D3 combination is the most common supplement context for vitamin K2. The mechanistic story (D3 absorbs calcium, K2 directs it) is supported by laboratory work; outcome trials are suggestive but UK-authorised claims do not differentiate K1 from K2 or specify the "directs calcium to bone" framing.
Some evidenceLimitedOlder adults concerned about bone health
Vitamin K (K1 or K2) at NRV intake meets the authorised "contributes to maintenance of normal bones" claim. Trial evidence on K2 specifically reducing fracture risk in osteoporosis is mixed and not at NICE-guideline level. NHS bone-health pathways apply.
Some evidenceLimitedPeople taking warfarin
DO NOT start, stop, or change vitamin K supplementation without speaking to your anticoagulation team. Vitamin K is the direct antagonist of warfarin; INR will swing. Consistent dietary K is the goal — not zero, just stable. [3,2]
Some evidenceStrongAdults with malabsorption (cystic fibrosis, coeliac, post-bariatric surgery, chronic liver / pancreatic disease)
Higher risk of vitamin K deficiency — talk to your GP and specialist team about appropriate testing and supplementation; this is a clinical management question.
Some evidenceLimited
How it works
Activated K-dependent proteins regulate distinct processes by location. In the liver, fully carboxylated clotting factors maintain haemostasis. In bone, fully carboxylated osteocalcin binds calcium and integrates into the bone mineral matrix. In vasculature, fully carboxylated matrix Gla-protein inhibits soft-tissue calcium deposition. The "K2 + D3" supplement category rests on the logic that vitamin D promotes calcium absorption and K2 directs that calcium toward bone rather than vasculature. The mechanistic story is biologically plausible; clinical trial outcomes for cardiovascular or fracture endpoints are suggestive but not at the level of an authorised UK health claim distinguishing K2 from K1.
Common myths
Myth"K2 is "the bone vitamin" and K1 only does clotting"
RealityK1 contributes to normal bone maintenance under UK-authorised claims; K2 contributes to normal blood clotting under the same authorised wording. The regulation does not split the claims by form. The "K2 = bone, K1 = clotting" framing is marketing positioning, not UK-authorised regulatory language. [4]
Myth"Higher K2 dose prevents heart disease"
RealitySome observational and short trial data has suggested associations between higher K2 intake and lower vascular calcification markers. Outcome trials with cardiovascular endpoints are limited. UK NICE cardiovascular prevention pathways do not list K2. Marketing that implies cardiovascular disease prevention is operating outside both the evidence base and authorised UK supplement claims.
Myth"You can't overdose on vitamin K"
RealityAt typical supplemental doses for healthy adults, vitamin K has a wide safety margin and EFSA has not set a tolerable upper limit. The exception is anyone on warfarin, where even small changes in vitamin K intake meaningfully alter INR. For them, the issue is not toxicity — it is treatment effect. [3]
Common online questions
Synthesised from the questions UK shoppers most often ask online about Vitamin K2 (Menaquinone). Each answer is editorial and links to its evidence in the Sources list below.
K1 vs K2 vs MK-4 vs MK-7 — which should I buy?
For UK-authorised health claims (clotting, bone maintenance) the regulation does not differentiate K1, K2, MK-4 or MK-7 — vitamin K NRV intake is what carries the claim. The K2 supplement category is built around tissue-distribution and half-life differences. MK-7 has a longer plasma half-life than MK-4 (and K1), which translates into more sustained tissue exposure with once-daily dosing. If a supplement specifies K2, MK-7 is the form most studied in modern trials and most commonly used in K2 + D3 combinations. [4]
Can I just eat green vegetables instead?
Yes — for K1, regular intake of kale, spinach, broccoli, rocket and other leafy greens easily meets the UK adequate intake. K2 from food sits in a smaller list (natto, certain cheeses, liver, egg yolks). Most people in the UK obtain modest K2 intakes from animal products and fermented dairy without specifically targeting it. [1]
Why is K2 always combined with vitamin D3?
The combination is built on the mechanistic story that D3 increases calcium absorption and K2 supports the carboxylation of proteins that direct calcium into bone (osteocalcin) and away from vasculature (matrix Gla-protein). Regulation does not authorise the marketing framing of "K2 directs calcium to bone" — the authorised claims for D3 (calcium absorption) and K (bone maintenance) are individually permitted; the combined narrative is mechanistic, not regulatory. [4]
Is K2 safe to take long-term?
Vitamin K has a wide safety margin in healthy adults. EFSA has not set a tolerable upper intake level for vitamin K from food or food supplements at typical supplemental doses (90–180 µg/day for K2 supplements). The single most important safety consideration is the warfarin interaction — discussed in the safety section.
I take warfarin — can I take a K2 supplement?
Speak to your anticoagulation team before any change. Vitamin K is the direct antagonist of warfarin. If you take warfarin and are starting, stopping, or changing the dose of any vitamin K supplement, your INR will change and may move out of the therapeutic range. The standard advice is to keep dietary vitamin K stable rather than to avoid it — a supplement that adds significant K above your usual baseline is a meaningful change. [3,2]
⚖️ The official position
What may lawfully be claimed about Vitamin K2 (Menaquinone) in Great Britain. This is a regulatory position, not an evidence grade.
A health claim is authorised in Great Britain.
“Vitamin K contributes to normal blood clotting”
“Vitamin K contributes to the maintenance of normal bones”
This claim is authorised for use in Great Britain under the GB Nutrition and Health Claims regulation. A product may carry it when it provides at least 15% of the UK NRV per recommended daily portion.
Authorised UK health claims
Verbatim from the GB Nutrition and Health Claims Register (Reg 432/2012 as assimilated in GB). A product can carry these claims when it provides at least 15% of the UK NRV per recommended daily portion.
2 authorised claims — show / hide
- "Vitamin K contributes to normal blood clotting"
- "Vitamin K contributes to the maintenance of normal bones"
UK regulatory landscape
UK regulatory tier: Food supplement
EFSA claim status
Claims relating specifically to vitamin K2, to vascular calcification, or to osteoporosis are not authorised.
UK regulatory rules evolve. This summary is editorial — businesses should consult regulatory counsel; consumers should consult their pharmacist or GP.
🩺 What the guidance says
What UK and international health bodies say. Guidance leads; Camden’s own evidence review follows below.
NHSConditional
Vitamins and minerals — Vitamin K
The NHS sets the adult vitamin K requirement at roughly 1 microgram per kilogram of body weight a day — about 65–75 µg for a typical adult — and states that most people can get all the vitamin K they need from a varied, balanced diet, with green leafy vegetables and vegetable oils given as the main sources. It notes that the body stores vitamin K in the liver, so it is not needed in the diet every day, and that taking too much from supplements is unlikely to cause harm at the amounts most products provide. The NHS does not distinguish K1 from K2 or recommend a particular form.
NHSConditional
Warfarin — how and when to take it, side effects
The NHS advises people taking warfarin to keep their diet consistent rather than to avoid vitamin K, because warfarin works by blocking vitamin K and large or sudden changes in intake — from food or supplements — can move the INR out of the safe range. It tells patients to speak to their doctor or anticoagulant clinic before making big changes to what they eat or before starting supplements. This is the single most important practical point for anyone considering a vitamin K product while on warfarin.
EFSANot assessed
Dietary Reference Values for vitamin K
EFSA's 2017 opinion sets EU dietary reference values for vitamin K, expressed as an Adequate Intake based on observed intakes in healthy populations because the data were insufficient to derive an Average Requirement. The opinion treats phylloquinone (K1) as the basis for the reference value and notes that the contribution of the menaquinones (K2) to requirements could not be quantified. It is a nutrient- requirement opinion — it establishes how much vitamin K the body needs, not a recommendation to take any particular product, and it does not assess health claims for vitamin K or K2.
GB-NHCConditional
Great Britain nutrition and health claims (NHC) register
Only claims on the Great Britain NHC register may be used in commercial communications in Great Britain. The register authorises two claims for vitamin K — that it contributes to normal blood clotting and to the maintenance of normal bones — usable when a product delivers at least 15% of the vitamin K reference intake per daily portion. The register key covers vitamin K generally and does not separate K1 from K2; there is no authorised claim specific to K2, to vascular calcification, or to osteoporosis. Absence of those claims reflects regulatory status, not a finding that any effect has been disproven.
SOCIETY-OTHERNot assessed
Nutrition for bones (Royal Osteoporosis Society)
The Royal Osteoporosis Society's bone-nutrition guidance centres on calcium and vitamin D and on getting nutrients from a healthy, balanced diet rather than from individual supplements. It does not single out vitamin K2 as a recommended supplement for bone health, which is consistent with vitamin K2 not appearing in UK osteoporosis treatment pathways. The Society frames supplements as relevant in specific circumstances and suggests discussing them with a GP or pharmacist.
BNFNot assessed
Warfarin sodium (BNF)
🔬 Camden’s evidence review
The research Camden reviewed, graded on its strength. This is our own appraisal — it sits beneath the official guidance above, never above it.
Bone health (fracture / BMD)
EvidencelimitedTrials of K2 (mostly MK-4 at high pharmacological doses, and some MK-7) on bone mineral density and fracture endpoints are mixed. The largest meta-analyses report small effects on BMD markers; fracture-endpoint evidence is heterogeneous. UK NICE pathways for osteoporosis prioritise bisphosphonates, denosumab, calcium and vitamin D adequacy — vitamin K2 is not first-line in those pathways.
Vascular calcification / cardiovascular outcomes
EvidencelimitedSome observational and short trial data link higher K2 intake with reduced vascular calcification markers. Outcome trials with cardiovascular endpoints are limited. UK NICE cardiovascular prevention pathways do not list K2 supplementation as a recommended intervention.
Bleeding / clotting in healthy adults
EvidenceinsufficientAt NRV intakes vitamin K supports normal clotting. There is no evidence supplementation in healthy adults beyond NRV meaningfully alters baseline clotting beyond restoring adequacy in deficient states.
Safety
Vitamin K at NRV intake is safe for almost all adults. The single critical safety consideration is the warfarin interaction — anyone on warfarin must speak to their anticoagulation team before changing vitamin K intake.
Talk to your pharmacist or GP first if you:
- You take warfarin (Coumadin, Marevan) — vitamin K directly antagonises warfarin and your INR will change. DO NOT start, stop, or change a vitamin K supplement without speaking to your anticoagulation team.
- You take any other vitamin-K-antagonist anticoagulant (acenocoumarol, phenindione).
- You have a malabsorption condition (cystic fibrosis, coeliac, chronic pancreatitis, short bowel) — vitamin K absorption may be impaired; talk to your specialist.
- You have liver disease — vitamin K metabolism and clotting-factor synthesis are hepatic.
- You are pregnant or breastfeeding — standard NRV applies; high-dose supplementation needs discussion.
- You are taking high-dose vitamin D3 alongside K2 — discuss the combination at the dose you are taking.
Common side effects: Vitamin K at supplemental doses in healthy adults rarely causes side effects. Allergic reactions are uncommon.
Pregnancy and breastfeeding
Vitamin K at NRV intake is part of normal nutrition during pregnancy. High-dose supplementation in pregnancy beyond NRV is not established as needed; talk to your pharmacist or midwife if considering it.
NRV intake is standard. Newborns receive a separate vitamin K injection / oral dose under NHS guidance to prevent vitamin K deficiency bleeding.
Talk to your GP, midwife, or pharmacist before starting any supplement during pregnancy or breastfeeding.
More clinical detail (for clinicians and informed readers)
Contraindications
- Active warfarin therapy without anticoagulation-team supervision.
- Known hypersensitivity to vitamin K or supplement constituents.
Drug interactions
- WARFARIN and other vitamin-K-antagonist anticoagulants — direct, clinically significant interaction. INR-affecting.
- DOACs (apixaban, rivaroxaban, dabigatran, edoxaban) — vitamin K does NOT antagonise these directly. Supplementation can be discussed with your prescriber.
- Cholestyramine, colestipol, orlistat — fat-soluble vitamin absorption may be reduced; separate dosing.
- Long-term broad-spectrum antibiotics — gut bacteria contribute to K2 production; prolonged antibiotic use may marginally lower K2 status.
This is not an exhaustive list. Always tell your prescriber and pharmacist about every supplement you take.
Common side effects
- Generally well-tolerated at supplement-label doses; no common side-effect profile beyond rare allergic reactions.
Rare side effects
- Allergic / hypersensitivity reactions — uncommon.
How to take it
- Typical supplemental range
- K2 supplements typically deliver 75–180 µg of MK-7 per day. K2 + D3 combination products often pair 100 µg MK-7 with 1000–4000 IU vitamin D3.
- Timing
- Take with a meal containing some fat — vitamin K is fat-soluble.
How to spot quality
Look for
- Form named clearly: vitamin K2 as menaquinone-7 (MK-7) is the most-studied modern supplement form.
- µg per serving stated, ideally with %NRV (NRV = 75 µg).
- For MK-7 specifically: trans-MK-7 isomer >99% (the cis isomer is biologically inactive — the trans purity is a quality marker).
- GMP-certified manufacture; ideally licensed material (MenaQ7® and similar branded MK-7 preparations are common).
- Combination with vitamin D3: clear per-form dose declaration on the label.
Red flags
- Label says only "vitamin K2" without specifying MK-4 or MK-7 — these have different half-lives and dose contexts.
- No trans-isomer purity stated for MK-7 supplements.
- Marketing claims that K2 "directs calcium to bones" — this is mechanistic language, not a UK-authorised claim.
- Claims of cardiovascular disease prevention or osteoporosis treatment.
- Combination products that bury vitamin K dose in a "proprietary blend".
- Marketing that targets people on warfarin without prominent interaction warnings.
Commonly combined with
Other ingredients that share a biological pathway, cofactor relationship, or evidence-backed protocol with this one. Mechanisms below cite primary physiology — not folk pairing. Doses are literature ranges, not recommendations. Talk to your pharmacist or GP before changing your stack, especially if you take prescription medication.
Vitamin D3
Limited evidenceD3 raises calcium absorption; K2 directs that calcium into bones, away from arteries.
Vitamin D3 (cholecalciferol) increases intestinal calcium absorption and bone remodelling via the active 1,25-dihydroxy form. Without adequate vitamin K2, the resulting calcium can deposit in soft tissues including arterial walls. K2 is the cofactor that gamma-carboxylates osteocalcin (in bone matrix) and matrix-Gla-protein (in vascular tissue) — only carboxylated forms bind calcium effectively. The combination is theorised to maximise the bone-building effect of D3 while reducing the cardiovascular concern around high-dose D3 in K-insufficient adults. Long-term cardiovascular outcome trials of the combined stack remain limited.
Evidence: Schwalfenberg 2017 review (J Nutr Metab) and Maresz 2015 (Integr Med) summarise the mechanistic and observational case. EFSA has authorised individual claims for both vitamins; no combined-stack claim exists. [7,8]
Doses studied: 1000–2000 IU D3 with 90–180 µg K2 (MK-7 form preferred for half-life)
Sources
Numbered references cited above plus general authoritative reading. Citations in the body link to the matching number here.
How to read these sources:
- Tier 1 (UK authoritative): NHS, NICE, BNF, EFSA, FSA, GB NHC Register, SACN, MHRA.
- Tier 2 (primary literature): peer-reviewed RCTs cited by PMID.
- Tier 3 (mechanistic): in-vitro / animal-model literature — interpret with caveat.
- NHS — Vitamin K
- NHS — Anticoagulant medicines (warfarin)
- BNF — Warfarin sodium (interaction warning)
- GB Nutrition and Health Claims Register (gov.uk)
- EFSA — Scientific Opinion on Dietary Reference Values for vitamin K (2017)
- NIH ODS — Vitamin K Fact Sheet for Health Professionals
- PubMed PMID 28698808
- PubMed PMID 26770129