Vitamin K: K1 vs K2, How Much You Need, and the One Warning (2026)
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.
The one warning that matters: vitamin K directly opposes warfarin and other vitamin K antagonist blood thinners — that's literally how those drugs work. Anyone on one must not change their vitamin K intake without their clinician. Outside of that, vitamin K is one of the safest nutrients around: the Adequate Intake is 120 mcg/day for men, 90 mcg/day for women, and there's no established upper limit. There are two families — K1 (leafy greens, mostly used for clotting) and K2 (MK-7/MK-4, the form sold for bone and artery support) — and they are not interchangeable by the microgram.
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Vitamin K at a glance
- What it is
- A fat-soluble vitamin with two families: K1 (phylloquinone) and K2 (menaquinone: MK-7, MK-4)
- What it does
- Activates clotting factors (mainly K1) and calcium-handling proteins osteocalcin/MGP (mainly K2)
- Adequate Intake
- 120 mcg/day men, 90 mcg/day women (AI, not RDA — see why below)
- Upper limit
- None established for either form
- Most people's status
- K1: usually adequate from food. K2: commonly low — rare in the Western diet outside natto
- Critical caution
- Opposes warfarin/VKA anticoagulants — clinician + INR monitoring required, every time
- Deeper K2 dive
- See our dedicated K2 cluster for MK-7 vs MK-4 depth
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.
| Item | Value ( mcg) |
|---|---|
| Adult AI (women) | 90 mcg |
| Retail MK-4 (nutritional) | 100 mcg |
| MK-4 (Japan drug dose) | 45,000 mcg |
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.
Standalone phylloquinone, near the adult AI — food-gap insurance.
The everyday bone/artery-mechanism form, at the trial-anchored dose.
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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
Frequently Asked Questions
What is the difference between vitamin K1 and K2?
K1 (phylloquinone) is the form in leafy greens; your liver uses most of it for blood clotting, and most Americans get close to enough from food. K2 (menaquinone — mainly MK-7 and MK-4) is the form sold as a targeted bone/artery supplement; it is rare in a typical Western diet. K1 and K2 are not interchangeable milligram-for-milligram — they have different half-lives and different evidence bases. Genuine dietary K1 deficiency is uncommon outside malabsorption, liver disease, or newborns.
How much vitamin K do I need per day?
The Adequate Intake (AI) — set because the evidence base was not sufficient for a full RDA — is 120 mcg/day for men and 90 mcg/day for women (as phylloquinone-equivalent, NAS/FNB). There is no established Tolerable Upper Intake Level for vitamin K in either form. Supplement doses vary a great deal by form: standalone K1 products run 45-120 mcg, MK-7 trials used 90-180 mcg, and MK-4 is a completely different story (see the dosage guide).
Is it safe to take vitamin K with warfarin or other blood thinners?
Not without your prescribing clinician. Vitamin K directly antagonizes warfarin and other vitamin K antagonist (VKA) anticoagulants (acenocoumarol, phenprocoumon) — that is how those drugs work. Anyone on a VKA must not start, stop, or meaningfully change their vitamin K intake (food or supplement) without their clinician and INR monitoring. Direct oral anticoagulants (apixaban, rivaroxaban, dabigatran, edoxaban) do not work through the vitamin K pathway, so this specific interaction does not apply to them — but any medication change still warrants telling a physician.
Does vitamin K2 actually prevent osteoporosis or heart disease?
The honest answer is mixed. K2 mechanistically activates proteins that route calcium into bone and away from arteries, and that mechanism is well established in humans. But the hard-outcome trials are inconsistent: the two landmark 3-year MK-7 trials (Knapen 2013, 2015) were mixed on their own primary endpoints, the K1 ECKO trial and Shea 2009 CAC trial were both null on their primary endpoints, and the strongest artery-related result (Rotterdam Study) is observational, not a randomized trial. Vitamin K should be described as mechanistically plausible with mixed, surrogate-heavy outcome evidence — not as a proven treatment for either condition.
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
On this page
Vitamin K Deficiency: Signs, Who\'s At Risk, Why Newborns Get a Shot
Signs of vitamin K deficiency in adults
Vitamin K's job in the body is activating clotting factors, so a real deficiency shows up as impaired clotting — not the vague, overlapping symptoms (fatigue, brain fog) that make some other nutrient deficiencies hard to pin down. The signs are specific:
- Easy bruising — bruises that appear with minimal or no trauma.
- Bleeding gums — especially with brushing or flossing, without other dental disease explaining it.
- Prolonged bleeding from cuts — small wounds that take unusually long to stop bleeding.
- Blood in urine or stool — a sign to take seriously and get evaluated, since it has many possible causes beyond vitamin K.
- Heavy menstrual bleeding — can be a sign in some cases, though it has many more common causes.
- Severe cases: internal bleeding, which is a medical emergency.
These same signs show up when someone is over-anticoagulated on warfarin — both situations involve impaired activation of vitamin-K-dependent clotting factors, just from different directions (too little vitamin K vs. a drug blocking its recycling).
Who's actually at risk?
This is the honest, narrow list. Outside of these groups, dietary K1 deficiency is genuinely uncommon in the developed world:
| Risk factor | Why it matters |
|---|---|
| Fat malabsorption conditions | Celiac disease, inflammatory bowel disease (Crohn's, ulcerative colitis), cystic fibrosis, and bariatric/weight-loss surgery all reduce absorption of fat-soluble vitamins, including K. |
| Chronic liver disease | The liver both stores vitamin K and uses it to produce clotting factors — significant liver disease can impair both sides of that process. |
| Long-term broad-spectrum antibiotic use | Can reduce gut bacteria that contribute to vitamin K production and, in some cases, directly interfere with vitamin K metabolism. |
| Very low dietary fat or leafy-green intake | Vitamin K is fat-soluble and concentrated in green vegetables — diets very low in both reduce intake and absorption simultaneously. |
| Vitamin K antagonist medications | Warfarin and similar drugs (acenocoumarol, phenprocoumon) work by functionally inducing a vitamin-K-deficient state — that's the mechanism, not a side effect. This is a medically managed situation, not something to self-correct with supplements. |
Source: StatPearls (Daley & Sina, PMID 30725668), a clinical review covering adult deficiency causes.
Newborns: why they get a vitamin K shot at birth
This is the other major vitamin K deficiency risk group, and it's a genuinely different situation from adult deficiency. Newborns are born with very low vitamin K stores for several compounding reasons: vitamin K crosses the placenta poorly, breast milk is naturally low in it, and a newborn's gut hasn't yet developed the bacterial population that contributes to vitamin K production later in infancy.
Without intervention, this puts infants at risk for Vitamin K Deficiency Bleeding (VKDB), which is classified by when it occurs:
- Early VKDB (within 24 hours of birth) — often linked to maternal medications that interfere with vitamin K.
- Classic VKDB (day 1-7) — the most common presentation without prophylaxis.
- Late VKDB (week 2 to 6 months) — less common but more likely to involve serious bleeding, including intracranial hemorrhage.
The standard prevention is a single intramuscular vitamin K injection shortly after birth, which is highly effective. Oral alternatives exist but are less effective and require multiple doses spread over weeks, with more opportunities for a missed dose. StatPearls explicitly documents a genuine public-health concern here: VKDB incidence has been rising in step with parental refusal of the birth-dose injection, alongside the lower real-world efficacy of oral alternatives when families choose that route. This is not a close call clinically — it's one of the most effective, well-established newborn preventive interventions in medicine, and it's worth being direct about that given how much misinformation circulates around it.
Vitamin K deficiency vs. "low K2 status" — a real distinction
It's worth separating two things people often conflate. Clinically diagnosed vitamin K deficiency — the bleeding-risk condition described above — is almost always about K1 (phylloquinone) status and its effect on clotting factors. Having suboptimal K2 status for the bone/artery mechanism (elevated undercarboxylated osteocalcin or dp-ucMGP, the research biomarkers used in the K2 trials) is a much softer, non-diagnostic concept — it doesn't cause bleeding, isn't a recognized clinical deficiency, and isn't something a blood test in a routine physical would flag. If you're wondering whether you might benefit from more K2 for bone/artery support specifically, that's a different question from deficiency — see our hub page for the honest evidence on what K2 supplementation actually does.
What to do if you're in a risk group
- Talk to your clinician first if you have a malabsorption condition, liver disease, or are on long-term antibiotics — deficiency in these contexts is a medical question, not a self-diagnosis.
- If you're on warfarin, do not attempt to self-correct anything related to vitamin K — see our warfarin and blood thinners guide. Consistency of intake, managed by your prescriber, is what matters.
- Otherwise, a standalone K1 supplement near the 90-120 mcg AI is reasonable, low-stakes food-gap insurance — see our dosage guide for the full picture.
If you want K1 food-gap insurance: Bluebonnet Vitamin K1 100 mcg at $0.10/day is a simple, near-AI standalone pick. Compare every form on our best vitamin K guide.
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More vitamin k 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.
Vitamin K and Warfarin
The single highest-stakes page in this cluster: why consistency beats avoidance, and what DOACs change.
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
- Knapen MHJ, et al. "Three-year low-dose menaquinone-7 supplementation helps decrease bone loss in healthy postmenopausal women." Osteoporos Int. 2013;24(9):2499-507. PMID: 23525894
- Knapen MHJ, Braam LAJLM, Drummen NE, Bekers O, Hoeks APG, Vermeer C. "Menaquinone-7 supplementation improves arterial stiffness in healthy postmenopausal women. A double-blind randomised clinical trial." Thromb Haemost. 2015;113(5):1135-44. PMID: 25694037
- Braam LAJLM, et al. "Vitamin K1 supplementation retards bone loss in postmenopausal women between 50 and 60 years of age." Calcif Tissue Int. 2003;73(1):21-6. PMID: 14506950
- Cheung AM, et al. "Vitamin K supplementation in postmenopausal women with osteopenia (ECKO trial): a randomized controlled trial." PLoS Med. 2008;5(10):e196. PMID: 18922041
- Geleijnse JM, et al. "Dietary intake of menaquinone is associated with a reduced risk of coronary heart disease: the Rotterdam Study." J Nutr. 2004;134(11):3100-5. PMID: 15514282
- Shea MK, et al. "Vitamin K supplementation and progression of coronary artery calcium in older men and women." Am J Clin Nutr. 2009;89(6):1799-807. PMID: 19386744
- Sconce E, et al. "Vitamin K supplementation can improve stability of anticoagulation for patients with unexplained variability in response to warfarin." Blood. 2007;109(6):2419-23. PMID: 17110451
- Schurgers LJ, et al. "Vitamin K-containing dietary supplements: comparison of synthetic vitamin K1 and natto-derived menaquinone-7." Blood. 2007;109(8):3279-83. PMID: 17158229
- Sato T, Schurgers LJ, Uenishi K. "Comparison of menaquinone-4 and menaquinone-7 bioavailability in healthy women." Nutr J. 2012;11:93. PMID: 23140417
- Theuwissen E, Cranenburg EC, Knapen MH, et al. "Low-dose menaquinone-7 supplementation improved extra-hepatic vitamin K status, but had no effect on thrombin generation in healthy subjects." Br J Nutr. 2012;108(9):1652-7. PMID: 22289649
- Miki T, Nakatsuka K, Naka H, et al. "Vitamin K(2) (menaquinone 4) reduces serum undercarboxylated osteocalcin level as early as 2 weeks in elderly women with established osteoporosis." J Bone Miner Metab. 2003;21(3):161-5. PMID: 12720050
- Daley SF, Sina RE. "Vitamin K Deficiency in Neonates and Adults." StatPearls (NCBI Bookshelf). 2026. PMID: 30725668