Vitamin K2

Vitamin K2 activates the proteins that route calcium into bone and keep it out of artery walls — a mechanism that is well established and a biomarker story that is genuinely strong, but a hard-outcome evidence base that rests on one good surrogate-endpoint trial and a pile of observational data. It is a cheap, very safe pairing with vitamin D3 for bone and vascular health, not the proven life-extender the supplement market implies.

Vitamin K comes in two families. K1 (phylloquinone), from leafy greens, is what the liver uses to make clotting factors, and ordinary diets supply plenty of it. K2 (the menaquinones), from fermented foods and animal fats, is the form that reaches tissues outside the liver — the vasculature, the skeleton — and it is chronically thin in modern Western diets. Almost everything interesting about K2 for longevity sits in that extrahepatic role, and almost none of it is captured by the official intake numbers, which were set for blood clotting alone.

What K2 actually does: the calcium traffic problem

Vitamin K is the required cofactor for one specific chemical step — carboxylation — that switches on a small family of "Gla proteins." Until they are carboxylated, these proteins are inert; they cannot bind calcium. Two of them carry the whole longevity argument:

Osteocalcin, made by bone cells, pulls calcium into the skeletal matrix.
Matrix Gla Protein (MGP), made in artery walls, sweeps calcium out of soft tissue and is the most potent natural brake on vascular calcification known.

When K2 status is low, both proteins circulate in their uncarboxylated, switched-off form. You can absorb calcium perfectly well (vitamin D handles that) and still lack the machinery to route it — the setup behind the so-called "calcium paradox," where the same person loses calcium from bone while depositing it in arteries.^[1] The uncarboxylated form of MGP (dp-ucMGP) is measurable in blood and predicts arterial stiffness and cardiovascular events — which is why K2 has a strong biomarker case even where the hard-outcome trials are missing.^[2]

This is the same routing logic that makes K2 part of the calcium triad and the bone-density stack — it doesn't add calcium, it decides where calcium goes.

MK-4 versus MK-7: why the form matters

The two K2 forms worth knowing behave very differently in the body.

Form	Half-life	Dosing	Evidence profile
MK-4 (menatetrenone)	1–2 hours	Needs large, multiple daily doses	The only form with fracture-endpoint RCT data — but at pharmacological doses (~45 mg/day) in Japanese osteoporosis cohorts. Nutritional doses don't raise blood levels at all.
MK-7 (menaquinone-7)	~3 days	Once daily, 90–180 µg	Standard nutritional doses produce sustained, measurable blood levels and full protein carboxylation. Strong mechanism and biomarker data; the better-studied surrogate-endpoint trials use this form.

The practical upshot: a small daily MK-7 dose maintains continuous carboxylation in a way that nutritional MK-4 cannot, because MK-4 is cleared within hours and absorbed poorly at food-level doses.^[3] MK-4's fracture data is real but comes from doses roughly 250× higher than a supplement capsule — a Japanese prescription protocol, not a longevity dose. For a healthy adult, MK-7 at 90–180 µg/day is the sensible default.

The cardiovascular evidence: strong mechanism, thinner outcomes

This is where honesty matters most, because the marketing runs well ahead of the trials.

The observational signal is real but observational. The landmark population data is the Rotterdam Study, which followed 4,807 adults for up to a decade: those in the highest third of dietary menaquinone (K2) intake had roughly half the coronary-heart-disease mortality and severe aortic calcification of the lowest third — while K1 intake showed no association at all, neatly fitting the extrahepatic mechanism.^[4] But this is a dietary cohort: people eating more K2 (from cheese and fermented foods) differ in many ways, and no observational study can prove the K2 itself did the work.

The one good interventional trial uses a surrogate endpoint. A three-year, double-blind, placebo-controlled trial gave 244 healthy postmenopausal women 180 µg/day of MK-7 and found it not only halted the expected age-related arterial stiffening but modestly reversed it, measured by carotid stiffness and pulse-wave velocity.^[5] That is the strongest randomized result K2 has on the vascular side — and arterial stiffness is a validated risk marker, not a hard endpoint like heart attacks or death.

Where hard endpoints have been tested, the results are mixed-to-null. Chronic kidney disease patients have the worst vascular calcification and the highest dp-ucMGP, so they are the obvious test case. Pooled trials show K2 reliably lowers the dp-ucMGP biomarker — but has not consistently reduced actual calcification scores or cardiovascular events in advanced disease.^[6] The biomarker moves; the outcome, so far, often doesn't. That gap is the honest centre of the K2 story.

The bone evidence: maintenance, not dramatic gains

The same Knapen cohort showed that 180 µg/day of MK-7 over three years significantly slowed the loss of bone mineral density and content at the lumbar spine and femoral neck versus placebo.^[7] Meta-analytic data confirm K2 shifts bone-turnover markers in a favourable direction — chiefly by converting uncarboxylated osteocalcin to its active form.^[8] Combined vitamin D3 + K2 outperforms either alone on density and osteocalcin in pooled trials.^[9]

The realistic framing: K2 is a supporting nutrient for bone, not a primary driver. The dominant osteogenic stimulus is mechanical — heavy resistance training and impact loading do far more for bone than any micronutrient. K2's job is to make sure the calcium you do absorb is directed correctly. See Bone density.

Food sources: why supplementation even comes up

K2 is genuinely scarce in Western diets, which is the real argument for a supplement.

Natto (fermented soybeans) is in a class of its own — roughly 1,000 µg of MK-7 per 100 g, an order of magnitude beyond any other common food. Habitual natto eaters show higher carboxylated osteocalcin, modestly higher bone density, and — in a 15-year cohort of elderly Japanese men — lower all-cause and cardiovascular mortality, an effect that does not appear with unfermented soy.^[10] See Fermented foods.
Aged and fermented cheeses are the main Western source, varying widely by culture and ripening — Dutch and Swiss hard cheeses and French soft cheeses (Munster, Camembert, Brie) carry meaningful amounts; Mediterranean cheeses (mozzarella, feta, parmesan) carry almost none.^[11] This K2 content is part of why fermented dairy reads as cardiovascular-neutral-to-favourable despite its saturated fat. See Dietary fats.
Egg yolk, butter, and dark poultry meat supply MK-4, but in amounts far below what nutritional studies suggest is needed to shift the proteins.

If you eat natto regularly, you don't need a K2 supplement. Almost no one outside Japan does.

Dose, safety, and the one real caveat

Dose. 90–180 µg/day of MK-7 is the band used in the three-year trials and is what the rest of this site recommends when pairing with D3. There is no clear added benefit demonstrated above that for healthy adults; specialist osteoporosis protocols sometimes go higher.

Safety is excellent. Natural vitamin K has no established toxicity, and neither the NIH nor EFSA set a tolerable upper limit because there's no signal to base one on.^[12] An industry-independent review put the highest observed safe intake of MK-7 at 375 µg/day with no effect on clotting parameters.^[13] (Synthetic vitamin K3, menadione, is a different molecule — cytotoxic and not used in human supplements.)

The one caveat that matters: warfarin and other vitamin-K-antagonist anticoagulants. These drugs work by blocking vitamin K recycling, so supplemental K2 directly opposes them — doses as low as 10–20 µg/day can destabilise INR. If you take warfarin (or acenocoumarol, phenprocoumon), do not start K2 without your prescriber and INR monitoring. The newer direct oral anticoagulants (apixaban, rivaroxaban, dabigatran) don't touch the vitamin K cycle, so K2 doesn't interfere with them.^[14]

What is not established

The source literature for K2 is full of dramatic longevity claims that don't survive scrutiny, and it's worth naming them so you can discount the marketing:

Telomeres. A single cross-sectional analysis found higher dietary vitamin K associated with marginally longer telomeres (about 0.2 base pairs per µg).^[15] This is observational, tiny, and not a basis for any claim that K2 slows cellular aging.
Sirtuins / lifespan extension. K2 extends lifespan in nematode (C. elegans) models via a stress-response pathway.^[16] Worm data is hypothesis-generating, not evidence for humans.
Brain and cognition. MK-4 is the dominant K vitamer in brain tissue, and post-mortem cohorts link higher brain MK-4 to less Alzheimer pathology — but this is observational, and any cognitive benefit in living humans is unproven.^[17]

Treat these as mechanism and curiosity, not as reasons to supplement.

The practical short version

If you take vitamin D3 for bone or already supplement calcium, add MK-7 at 90–180 µg/day. It's cheap, very safe, and biologically coherent — it directs the calcium D helps you absorb. See Vitamin D and Calcium.
If you eat natto regularly, skip the supplement — you're already well above any studied dose.
Take it with a fat-containing meal (K2 is fat-soluble) and pair it with adequate magnesium for the full bone stack. See Magnesium.
Don't expect a longevity miracle. The honest evidence supports K2 as a sensible adjunct for bone density and arterial flexibility, with one good surrogate-endpoint trial behind it — not as a proven extender of lifespan.
On warfarin: coordinate with your prescriber first. On a direct oral anticoagulant: no interaction.