Vitamin K exists in two major forms with fundamentally different biology. Vitamin K1 (phylloquinone) is involved primarily in blood clotting. Vitamin K2 (menaquinone) is the form that directs calcium to the right places — into bones and teeth, and away from arteries and soft tissues.
The distinction matters clinically. Vitamin K2 deficiency is linked to arterial calcification, osteoporosis, and poor bone quality — and most people consuming a Western diet are deficient in K2 while getting adequate K1.
What vitamin K2 does
K2's primary role is activating vitamin K-dependent proteins outside the liver:
Osteocalcin: A protein produced by osteoblasts (bone-forming cells). Osteocalcin must be carboxylated (activated) by K2 to bind calcium and incorporate it into bone matrix. Undercarboxylated osteocalcin is a biomarker of K2 insufficiency.
Matrix Gla protein (MGP): The most potent known inhibitor of arterial calcification. MGP must be activated by K2 to function. Uncarboxylated MGP (ucMGP) accumulates in arteries without adequate K2, allowing calcium to deposit in vessel walls.
Protein S and protein C: Anticoagulant proteins that also require K2 for activation.
The result: adequate K2 → activated MGP → calcium stays out of arteries; activated osteocalcin → calcium goes into bone.
MK-4 vs MK-7: the critical difference
Vitamin K2 comes in several menaquinone forms, numbered by their isoprenoid side chain length (MK-4 through MK-13). The two relevant for supplementation are MK-4 and MK-7.
MK-4 (menaquinone-4)
- Found in: animal products — butter, eggs, meat, liver (animals convert K1 to MK-4)
- Half-life in plasma: 1–2 hours
- Requires multiple daily doses to maintain stable plasma levels
- Tissue-specific conversion from K1 occurs in certain organs
- Typical effective dose: 45mg/day (high-dose, used in Japanese osteoporosis treatment)
- Low-dose MK-4 (1–5mg) has limited evidence for extrahepatic (bone/arterial) effects
MK-7 (menaquinone-7)
- Found in: natto (fermented soybeans) — highest dietary source (1,000+ mcg per 100g)
- Half-life in plasma: 3–4 days (dramatically longer)
- Single daily dose maintains stable, sustained plasma levels
- Reaches extrahepatic tissues more effectively at lower doses
- Typical effective dose: 90–200mcg/day
- Bacterially synthesized in natto via Bacillus subtilis var. natto
The long half-life of MK-7 is the key pharmacological advantage. It persists in circulation long enough to activate MGP and osteocalcin in peripheral tissues, which MK-4 at physiological doses cannot do reliably due to rapid clearance.
Bone evidence
Knapen et al. (2013, Osteoporosis International): 180mcg/day MK-7 for 3 years significantly reduced age-related bone loss in postmenopausal women. Osteocalcin carboxylation improved; bone mineral density loss was attenuated at the femoral neck.
Japanese high-dose MK-4 trials: 45mg/day MK-4 is approved in Japan for osteoporosis treatment. Multiple RCTs show fracture risk reduction comparable to some bisphosphonates in specific populations. This is a pharmacological dose — not a supplement dose.
Meta-analysis (Huang et al., 2015): MK-7 supplementation significantly improved carboxylated osteocalcin and bone turnover markers vs. placebo.
Bottom line on bone: MK-7 at 90–200mcg/day has meaningful evidence for improving bone quality markers. The clinical translation to fracture prevention is plausible but less definitively established than the Japanese MK-4 data at 45mg.
Cardiovascular evidence
Rotterdam Study (Geleijnse et al., 2004): A landmark prospective cohort study of 4,807 subjects. Higher dietary K2 intake was associated with 57% reduced risk of aortic calcification, 52% reduced risk of coronary heart disease death, and 26% reduced all-cause mortality. K1 intake showed no such association.
Prospect-EPIC cohort: Higher K2 intake associated with reduced coronary events; K1 again not associated.
Intervention trials: Less definitive than observational data, but mechanistically consistent. Shea et al. (2009): MK-7 supplementation reduced uncarboxylated MGP (ucMGP) levels, indicating improved MGP activation and reduced theoretical calcification risk.
The limitation: Most cardiovascular evidence for K2 is observational. Large RCTs with hard cardiovascular endpoints (heart attacks, strokes) are lacking. The mechanistic data (ucMGP reduction) is compelling; the clinical trial evidence hasn't caught up.
K2 and vitamin D3 synergy
Vitamin D3 increases intestinal calcium absorption — which requires K2 to direct that calcium appropriately. The combination of D3 without K2 has raised theoretical concerns about increased soft tissue calcification in high-dose D3 supplementation.
Practical guidance: Anyone supplementing vitamin D3 at >2,000 IU/day should consider co-supplementing K2 (MK-7, 100–200mcg). This is standard practice in integrative medicine and has strong mechanistic rationale.
The combination is sold as D3+K2 in many formulations; this is pharmacologically sensible, not just marketing.
Dietary sources and Western dietary gap
K1 is abundant in leafy greens. K2 requires specific dietary sources:
| Food | K2 content |
|---|---|
| Natto | 1,000+ mcg/100g |
| Hard cheese (Gouda, Brie) | 40–75 mcg/100g |
| Soft cheese | 30–50 mcg/100g |
| Egg yolk | 15–30 mcg/100g |
| Butter | 10–20 mcg/100g |
| Liver (chicken) | 13 mcg/100g |
| Meat | 5–10 mcg/100g |
Natto consumption is common in Japan and correlates with dramatically lower hip fracture rates compared to Western Europe despite comparable calcium intakes. The K2 explanation is one contributing factor.
Western diets — low in fermented foods, grass-fed dairy, and organ meats — are typically low in K2. Estimated adequate intake for K2 has not been officially established in most countries; current research suggests 90–200mcg/day is the target range.
Safety and drug interactions
Anticoagulant warning: K2 (and K1) directly opposes the mechanism of warfarin (Coumadin). Anyone on warfarin must not supplement K2 without physician oversight and INR monitoring. Even consistent dietary K2 can affect warfarin dosing.
MK-7 vs. warfarin: MK-7's long half-life makes it more pharmacologically active and more likely to interfere with warfarin than MK-4. This is both a reason it works better for bone/arteries and why it's more clinically significant with anticoagulants.
New anticoagulants (DOACs): Apixaban, rivaroxaban, dabigatran do not work via the vitamin K cycle. K2 supplementation does not interfere with DOACs.
General safety: No toxicity reported with K2 supplementation at standard doses. No established upper limit (unlike K1 at high synthetic doses).
Practical protocol
For bone health and arterial calcification prevention:
- MK-7: 100–200mcg/day
- Take with fat-containing meal (fat-soluble vitamin)
- Take alongside vitamin D3 if supplementing D3 >2,000 IU
What to avoid:
- High-dose MK-4 (45mg) without medical supervision — this is a prescription dose in Japan
- K2 supplementation with warfarin without INR monitoring
- Confusing K1 (blood clotting, leafy greens) and K2 (calcium regulation, fermented foods)
The framework applied
For any vitamin K2 study:
- Which form? MK-4 vs. MK-7 results are not interchangeable — half-life and dosing completely differ
- What dose? 45mg MK-4 (Japanese pharmaceutical) ≠ 5mg MK-4 supplement ≠ 180mcg MK-7
- What biomarker? Carboxylated osteocalcin and ucMGP are the mechanistic markers; BMD takes years to change
- Observational vs. intervention? Most cardiovascular data is cohort-based
We automated this at Q-SCI. Any study — paste it, get a quality score.
Bottom line
- Vitamin K2 activates osteocalcin (bone calcium incorporation) and MGP (arterial calcification inhibitor) — distinct and critical roles from K1
- MK-7 is the preferred supplement form: 100–200mcg/day, single dose, long half-life, adequate extrahepatic tissue distribution
- MK-4 at 45mg/day (Japanese pharmaceutical dose) has strong fracture prevention evidence; low-dose MK-4 supplements are not equivalent
- Cardiovascular evidence is primarily observational but mechanistically strong and consistent
- Always pair with D3 supplementation — D3 increases calcium absorption, K2 directs it correctly
- Contraindicated with warfarin — check INR with physician
- Eat natto, hard cheese, egg yolks; supplement MK-7 if dietary intake is low
More evidence-based analyses at q-sci.org/blog. Score studies free at q-sci.org.
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