Vitamin K (menaquinone)

vitaminmenaquinone

What is it

Vitamin K2, or menaquinone, is a fat-soluble vitamin that activates proteins involved in blood clotting, bone metabolism, and the regulation of calcium deposition in arteries and soft tissues. It is found mainly in fermented foods and animal products.

How it works

Vitamin K serves as a cofactor for the enzyme gamma-glutamyl carboxylase, which activates several vitamin K-dependent proteins by adding carboxyl groups. These include clotting factors made in the liver, osteocalcin (which helps bind calcium to bone), and matrix Gla protein (which prevents calcium from accumulating in arteries and soft tissue). Menaquinone (K2) differs from phylloquinone (K1) in its side chain length, indicated by the number after MK (e.g., MK-4, MK-7). MK-7 has a longer half-life and reaches non-hepatic tissues like bone and arteries more effectively than K1. Bacteria in the gut and during fermentation produce most dietary K2; K1 comes mainly from leafy greens.

Evidence for 4 uses

AI-assisted evidence assessment — talk to your doctor before relying on any single supplement.

Blood clotting

Grade A

Strong evidence

Vitamin K (both K1 and K2) is required for normal blood clotting. Deficiency causes bleeding; this is the foundational, undisputed role.

Bone density support

Grade B

Good evidence

Several trials, mostly in postmenopausal women, suggest vitamin K2 (particularly MK-7) modestly improves bone density or reduces fracture markers. Effect sizes are smaller than for established osteoporosis drugs.

Arterial calcification prevention

Grade C

Moderate evidence

Observational studies link higher K2 intake to less arterial calcification and lower cardiovascular event rates. Randomized trials are limited but suggest possible benefit at higher doses.

Prevention of warfarin-induced calcification

Grade C

Moderate evidence

Warfarin blocks vitamin K and may contribute to arterial and valvular calcification. Whether K2 supplementation reverses or prevents this is an active research area.

3 commercial forms

MK-4 (menaquinone-4)

shorter half-life, higher doses needed

The form used in much of the bone-health research from Japan, typically at 45 mg per day. Short half-life requires multiple doses, but it has the strongest evidence base for bone outcomes.

MK-7 (menaquinone-7)

long half-life, low-dose effective

Sourced from natto (fermented soybeans) or produced commercially. Long half-life (about 72 hours) makes once-daily dosing at 45 to 180 mcg sufficient.

MK-9

found in some fermented dairy

A longer-chain form found in fermented cheese. Less commonly available as a supplement.

Dosage

The Adequate Intake (AI) for total vitamin K is 120 mcg per day for adult men and 90 mcg for women. There is no separate RDA for K2 specifically. Common K2 supplements provide 45 to 180 mcg of MK-7 or 1.5 to 5 mg of MK-4. No tolerable upper intake level has been established because toxicity is not seen at high oral doses.

When and how to take it

Vitamin K2 is fat-soluble and absorbs best with a meal containing some dietary fat. MK-7 has a long half-life (about 72 hours) so daily dosing maintains steady levels easily. MK-4 has a much shorter half-life and may benefit from twice-daily dosing if used at the higher pharmacological doses studied for bone health. If you take vitamin K2 alongside vitamin D, take them together at the same meal — they are commonly paired for bone and cardiovascular support.

Food sources

Food	Amount	%DV
Natto (fermented soybeans), 3 oz	850 mcg K2	—
Hard cheese (gouda), 1 oz	20 to 25 mcg K2	—
Soft cheese (brie), 1 oz	15 mcg K2	—
Egg yolk, 1 large	32 mcg K2	—
Chicken thigh, 3 oz cooked	10 mcg K2	—
Butter, 1 Tbsp	1 mcg K2	—
Sauerkraut, 1/2 cup	8 mcg K2	—

Safety

Vitamin K2 is generally well tolerated with no established toxicity. The main safety consideration is interaction with anticoagulant medications, since vitamin K opposes warfarin's effect on blood clotting. Allergic reactions are rare. MK-7 in supplements has been used for years in osteoporosis research at doses up to 360 mcg without significant safety problems.

Who should be cautious

People on warfarin should not start vitamin K supplements without consulting the prescriber managing their anticoagulation. Pregnant and breastfeeding women have similar requirements as other adults. Newborns receive a vitamin K injection at birth to prevent bleeding because their gut bacteria have not yet developed.

Interactions

Vitamin K substantially interacts with warfarin (Coumadin) and other vitamin K antagonist anticoagulants. People on warfarin need consistent vitamin K intake — sudden changes in dietary or supplemental K destabilize INR control. Newer anticoagulants like apixaban and rivaroxaban are not affected. High-dose vitamin E may interfere with vitamin K function in some people.

Frequently asked questions

What is the difference between K1 and K2?⌄

K1 (phylloquinone) comes mainly from leafy greens and is used by the liver for clotting factors. K2 (menaquinone) comes from fermented foods and animal products and is preferentially used in bone and arteries.

Which is better, MK-4 or MK-7?⌄

MK-7 has a longer half-life and is easier to dose once daily. MK-4 has more research at high doses for bone health but requires multiple daily doses.

Can I take K2 with warfarin?⌄

Only with your prescriber's guidance. Vitamin K opposes warfarin's effect, and any change in intake — including supplements — can destabilize anticoagulation.

Should I take K2 with vitamin D?⌄

Many people pair them. Vitamin D promotes calcium absorption; K2 helps direct calcium to bone rather than arteries. The combination is reasonable but not strictly necessary.

How much K2 do I need?⌄

There is no separate RDA for K2. Supplemental doses of 90 to 180 mcg of MK-7 daily are typical for bone and cardiovascular support.

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Disclaimer: These statements have not been evaluated by the FDA. This page is educational, not a substitute for personalized medical advice. Evidence grades are AI-assisted assessments — talk to your doctor before starting any new supplement, especially if you're pregnant, breastfeeding, on medications, or managing a chronic condition.