Vitamin K: K1 vs K2, How Much You Need, and the One Warning (2026)

By Erin Rose · Updated July 19, 2026 · Methodology · About Us

Informational summary of published research — not medical advice. If you take warfarin or another vitamin K antagonist, do not start, stop, or change your vitamin K intake (food or supplement) without your prescribing clinician and INR monitoring. This statement has not been evaluated by the FDA. This product is not intended to diagnose, treat, cure, or prevent any disease.

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K1 vs K2: two nutrients wearing one name

"Vitamin K" isn't one thing. K1 (phylloquinone) is the form in leafy greens — kale, spinach, broccoli — and your liver uses most of it to activate clotting factors. It's a genuine food nutrient, and most Americans who eat any green vegetables get reasonably close to adequate; real dietary K1 deficiency is uncommon outside fat malabsorption, chronic liver disease, or newborns (more on that on our deficiency signs page).

K2 (menaquinone) is a family of related compounds — mainly MK-7 and MK-4 for supplement purposes — and it's the form marketed for bone and artery support, because it activates two calcium-handling proteins: osteocalcin (binds calcium into bone) and matrix Gla protein (helps keep calcium out of artery walls). K2 is genuinely rare in a typical Western diet outside fermented foods like natto, which is why it's the form people actually supplement. K1 and K2 are not interchangeable milligram-for-milligram: MK-7 has a multi-day half-life and works at microgram doses, while MK-4 clears in hours and needs a pharmacologic milligram dose to do the same job (Sato 2012). For the full MK-7-vs-MK-4 mechanics, see our dedicated comparison — we won't re-derive it here.

How much do you actually need?

The Adequate Intake (AI) — set because the evidence base wasn't strong enough for a full RDA — is 120 mcg/day for adult men and 90 mcg/day for adult women, per the National Academy of Sciences Food & Nutrition Board (StatPearls, PMID 30725668). There is no established Tolerable Upper Intake Level for vitamin K in either form — unlike some other fat-soluble nutrients, there's no defined toxicity ceiling. Supplement doses in practice vary a lot by form and goal; our full dosage guide breaks down the AI-vs-RDA nuance and the roughly 500-1,000x range between a retail MK-4 capsule and the pharmacologic MK-4 dose used in Japanese osteoporosis trials.

Does it actually work? The honest picture

Vitamin K's role in blood clotting is not in question — that's basic, settled physiology, and it's the reason warfarin works by blocking it. The debate is entirely about K2 and bone/artery health, and here the marketing runs well ahead of the trials.

For bone: the 3-year MK-7 trial by Knapen 2013 (n=244, healthy postmenopausal women only — the effect isn't established in men or younger adults; industry-funded, backed by VitaK/MenaQ7 vitamin-K commercial interests) hit its primary endpoint at the lumbar spine and femoral neck but missed at the total hip. A K1 trial at a much higher dose, Braam 2003 (1 mg/day), improved femoral neck BMD but not lumbar spine. The high-dose K1 ECKO trial (Cheung 2008, n=440, 5 mg/day K1) was a clean null on bone density at every site and timepoint, despite a 10-fold rise in serum K1 — proof that vitamin K status markers can improve while bone density itself doesn't move.

For arteries: the companion 3-year MK-7 trial (Knapen 2015) met its primary endpoint for two stiffness measures in the full cohort, but several other stiffness measures improved only in a subgroup with above-median baseline stiffness. The K1 trial that measured actual coronary calcium (Shea 2009, n=388) was null in its primary intention-to-treat analysis; a positive signal showed up only in a post-hoc adherent subgroup, which the authors themselves called "hypothesis-generating." The often-cited Rotterdam Study finding — higher dietary K2 linked to lower coronary heart disease — is a dietary-cohort association (Geleijnse 2004), not a randomized trial, and dietary K1 showed no such association in the same data.

Bottom line: the mechanism (K2 activates osteocalcin and matrix Gla protein) is well established in human PK studies (Theuwissen 2012), but the leap from "the biomarker moved" to "this prevents osteoporosis or heart disease" is not supported by the outcome-level trial evidence. Treat vitamin K2 as mechanistically plausible with mixed, surrogate-heavy outcomes — not a proven treatment for either condition.

The warfarin warning, in one paragraph

This is the single most important fact in the category, and there's currently no other page on this site dedicated to it. Warfarin and other vitamin K antagonists (acenocoumarol, phenprocoumon) work by blocking vitamin K recycling — so any dietary or supplemental vitamin K directly opposes the drug. Even a single MK-7 dose of 50 mcg or more can be clinically relevant (Schurgers 2007). Counterintuitively, an RCT in patients with unstable INR control found that consistent, medically-supervised low-dose vitamin K supplementation improved anticoagulation stability, because erratic day-to-day dietary swings — not vitamin K itself — were destabilizing control (Sconce 2007). The message is consistency, not blanket avoidance — and it is always a clinician's call, never a self-directed one. Direct oral anticoagulants (apixaban, rivaroxaban, dabigatran, edoxaban) don't work through the vitamin K pathway, so this specific interaction doesn't apply to them. Read the full warfarin guide →

K1, or K2, or both?

If you eat leafy greens regularly, you're probably close to adequate on K1 already — a standalone K1 supplement is mostly food-gap insurance, not a targeted intervention. If you're interested in the bone/artery mechanism, K2 (specifically MK-7, at the trial-anchored 90-200 mcg range) is the form actually studied for that purpose; MK-4 needs a pharmacologic dose most retail bottles don't provide (see our dosage guide for the full dose-gap breakdown). Blended K1+K2 products and D3+K2 combos exist mostly for convenience — see our best vitamin K guide for how to pick a form based on what you're actually trying to fix, and vitamin D with K2 for the D-pairing question specifically.

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Common myths

• "K1 and K2 are the same vitamin, so any dose works." No — different half-lives, different jobs, different evidence bases. See MK-7 vs MK-4.
• "A 100 mcg MK-4 bottle replicates the Japanese osteoporosis trials." Those trials used 45 mg/day (45,000 mcg) — about 450x the typical retail dose. See our dosage guide.
• "Vitamin K2 reverses arterial calcification." Not supported — the trials that measured actual calcification were negative; K2 supports the calcium-routing system, it doesn't dissolve existing plaque.
• "If I'm on warfarin, I should avoid all vitamin K." The evidence actually favors consistency under medical supervision, not blanket avoidance — see our warfarin guide.

The full guides

• Best Vitamin K Supplement

  K1, MK-7, MK-4, D3+K2 combos, and a multi-form blend, ranked by which form fits your goal — then cost per day.

• Dosage Guide

  Why it's an AI, not an RDA, and the ~500-1,000x dose gap between an MK-7 supplement and the pharmacologic MK-4 dose.

• Deficiency Signs

  Who's actually at risk as an adult, and why newborns get a vitamin K shot at birth.

• Vitamin K and Warfarin

  The single highest-stakes page in this cluster: why consistency beats avoidance, and what DOACs change.

Frequently Asked Questions

Related guides on this site

• Vitamin K2: MK-7 vs MK-4 & how much — the deeper K2-specific cluster this hub routes into
• MK-7 vs MK-4 — the full form comparison
• Best Vitamin K2 (MK-7) Supplement
• Vitamin D With or Without K2 — the D3+K2 pairing question
• Vitamin D Guides
• Calcium for Bones & Osteoporosis
• Calcium for Women Over 50
• Magnesium With Vitamin D — the adjacent bone/mineral stack

About our data

Every guide draws on clinical evidence from PubMed randomized trials and cohort studies (each citation verified against the primary source, with primary endpoints stated honestly), with product picks ranked by form-fit and cost per day. See our methodology and editorial standards.

Sources