Vitamin K1 (phylloquinone) comes from plants and mainly supports blood clotting. Vitamin K2 (menaquinones) comes from bacteria, fermented foods and animal products. It stays in the blood longer and is studied for bone and artery health. Both are "vitamin K," but they are not interchangeable, and the form you choose matters most if you take warfarin.
Below, we break down where each form comes from, how the body handles it, how MK-4 and MK-7 differ, and what the evidence does and does not show.
TL;DR
- K1 is the leafy-green form (kale, spinach, broccoli). It is taken up by the liver quickly, where it activates clotting proteins.
- K2 is a family of compounds (MK-4 to MK-13) found in natto, hard cheese and animal foods. Long-chain forms circulate for days and reach bone and blood vessels.
- MK-7 vs MK-4: in human studies MK-7 is absorbed well and stays in the blood. MK-4 at nutritional doses often fails to raise serum levels.
- Evidence for K2 is promising, not settled. Observational data look good. Trials have not yet proven it prevents osteoporosis or heart disease.
- On warfarin? Do not start, stop or change any vitamin K source, K1 or K2, without your prescriber.
Table of Contents
- What vitamin K1 and K2 actually are
- Food sources: where each form comes from
- What each form does in your body
- MK-4 vs MK-7: the supplement question
- What the evidence says about bones and arteries
- Vitamin K, K2 and warfarin
- So which should you take?
- Our perspective
- Compare K2 ingredients on NutraSmarts
- FAQ
- Sources
What vitamin K1 and K2 actually are
Vitamin K is a family of fat-soluble compounds with a shared chemical core. The NIH Office of Dietary Supplements describes it as naturally present in some foods and sold as a dietary supplement. The family splits in two by structure:
- Phylloquinone (K1) is made by plants.
- Menaquinones (K2) are a series of compounds, MK-4 through MK-13, typically produced by bacteria. The number refers to the length of the side chain.
That side chain is the whole story. A short chain is cleared fast. A long chain lingers. This is why "K2" is not one thing, and why MK-4 and MK-7 behave so differently.
Food sources: where each form comes from
K1 is easy to get. Leafy greens such as kale, spinach and cabbage supply most of the vitamin K in a typical American diet. K2 is harder, because it shows up mainly in fermented foods, animal products and hard cheeses.
Natto, the fermented soybean dish, is the outlier. One analysis found about 10,985 ng/g of K2 in natto versus 321 ng/g of K1. Most people in the US do not eat natto, so their K2 comes in small amounts from cheese, eggs, poultry and pork.
| Vitamin K1 | Vitamin K2 | |
|---|---|---|
| Chemical name | Phylloquinone | Menaquinones (MK-4 to MK-13) |
| Made by | Plants | Bacteria (and converted in some animal tissue) |
| Main foods | Kale, spinach, collards, broccoli, cabbage | Natto, hard cheese, egg yolk, poultry, pork |
| Main role | Blood clotting (liver) | Bone and vascular proteins (outside the liver) |
| Time in blood | Short | Long-chain forms: days |
| Official intake target | Yes (Adequate Intake) | None set |
One practical wrinkle: food databases often lack reliable menaquinone data, so estimating your K2 intake is difficult. Anyone telling you exactly how many micrograms of K2 you get from a normal diet is guessing.
What each form does in your body
K1 and clotting
K1 is essential for blood coagulation. It acts as a cofactor in the liver, where it helps activate clotting factors. Researchers describe K1 as rapidly sequestered by the liver for that job and then cleared.
The official intake figure applies to vitamin K as a whole. For adults 19 and older, the Adequate Intake is 120 mcg a day for men and 90 mcg for women, including pregnant and lactating women. An Adequate Intake is used when evidence is not enough to set a Recommended Dietary Allowance.
K2 and tissues outside the liver
K2 does the same carboxylation chemistry, but longer-chain forms stay in circulation and reach other tissues. Vitamin K helps activate proteins involved in the maturation of about 11 or 12 proteins beyond the clotting ones, including those tied to bone and calcium handling. Osteocalcin, for instance, needs vitamin K to produce healthy bone tissue.
Cleveland Clinic's dietitian Susan Zumpano frames the difference simply: K1 is filtered out quickly to meet immediate clotting needs, while K2's longer structure lets it linger and influence bone and cardiovascular processes.
Can the gut convert K1 to K2?
Partly. Researchers note that all forms of vitamin K can in theory be converted to MK-4 in the body. That is a reason not to treat the two as completely separate nutrients. It is not a reason to assume K1 from salad does what a K2 supplement does. The tissue-reaching effect depends on how long the compound stays in your blood.
MK-4 vs MK-7: the supplement question
Most people searching this are standing in a supplement aisle looking at two labels. Here is the difference.
MK-4 has a short side chain. At nutritional doses, dietary MK-4 has poor bioavailability and often fails to raise serum levels in human studies. In a crossover-style comparison in healthy women, a single 420 µg dose of each form was given with breakfast. MK-7 was well absorbed and peaked at about 6 hours.
MK-7 has a longer chain, is absorbed efficiently and stays in the blood for several days. Comparative studies report postprandial serum concentrations about 10-fold higher than K1. Researchers also note MK-7 may be a better supplier of MK-4 to tissues than MK-4 itself.
MK-9 is a footnote: very long half-life, but often poorly absorbed because it is so fat-soluble.
| MK-4 | MK-7 | |
|---|---|---|
| Side chain | Short | Long |
| Absorption at nutritional doses | Poor in human studies | Good |
| Time in circulation | Short | Several days |
| Typical source | Animal foods; some supplements | Natto; most K2 supplements |
| Dosing implication | Frequent, larger doses used in some research | Once daily is typical on labels |
Two cautions. First, bioavailability is not the same as proven clinical benefit. A form that reaches the blood is a necessary step, not a result. Second, supplement quality varies: look for a stated MK-7 amount per serving in micrograms, ideally an all-trans form, and fat-containing meals to help absorption.
What the evidence says about bones and arteries
The headline claims are that K2 builds bone and keeps calcium out of arteries. The reality is more modest.
On the encouraging side, a 2017 review concluded that Level I and II evidence supports K1 and K2 in osteoporosis ... and Level II evidence supports K2 in preventing coronary calcification.
Observational data point the same way. Cleveland Clinic cites a study in which consuming at least 32 micrograms of K2 daily was linked to a 50% lower risk of heart-disease mortality, and another suggesting a 9% lower cardiovascular risk for every 10 micrograms a day.
But observational links can reflect diet and lifestyle rather than K2 itself. A more cautious reading from a 2022 review of vitamin K is that clinical trial evidence is still evolving and cannot yet support firm conclusions about preventing osteoporosis or cardiovascular disease in all populations.
So: K2 has a plausible mechanism and good early signals. It is not a proven treatment. If bone density is your concern, our write-up of what the science says about K2 and bone goes through the trials, and our vitamin D3 and K2 analysis explains why biomarker improvements are not the same as fewer fractures.
Vitamin K, K2 and warfarin
This is the section to read twice if you take a blood thinner.
Warfarin works by opposing vitamin K. The NIH advises that people taking warfarin and similar anticoagulants need to maintain a consistent intake of vitamin K from food and supplements, because sudden changes can affect how the drug works. That applies to K1 in a big kale salad and to K2 in a capsule alike.
What that means in practice:
- Do not start a K2 supplement on your own if you take warfarin. Talk to the prescriber or anticoagulation clinic that manages your INR first.
- Do not cut out greens either. The goal is steady intake, not zero.
- Tell your clinic about any change, including a new multivitamin that contains vitamin K.
Newer anticoagulants such as apixaban and rivaroxaban work by a different mechanism, but the safe move is the same: ask your prescriber or pharmacist before adding a supplement.
So which should you take?
It depends on your goal and your situation.
- Most healthy adults: a diet with regular leafy greens covers K1. Deficiency in healthy adults is uncommon, though it can occur with fat malabsorption or certain medications. Our deficiency guide lists risk groups and symptoms.
- Interested in K2 for bone or vascular health: the best-studied supplement form is MK-7. Consider it a modest, long-term addition, not a fix. Food sources such as hard cheese and eggs help too.
- Taking D3 or calcium for bones: K2 is often paired with vitamin D. See our vitamin D vs K2 comparison for how they differ, and our bone density supplement reviews for the wider picture.
- On warfarin, pregnant, or managing liver or kidney disease: speak to your clinician before any change.
Our perspective
We read the vitamin K literature the way we read every ingredient: by asking what was measured. Serum levels, bone turnover markers and calcification scores are real measurements. Fractures and heart attacks are outcomes people care about. For K2, the first group of results is stronger than the second, and we rate it that way.
We also think the K1 vs K2 framing is a little unfair to K1. It is the form with an established requirement and a clear, proven job. K2 is the form with the interesting open questions. When a label promises more than the trials show, we say so.
For MK-7 specifically, the pharmacokinetic data are the most convincing part of the case: it is absorbed, it persists, and it reaches the blood in a way short-chain MK-4 does not at nutritional doses. That is a reason to prefer it among K2 supplements, not a promise of results.
— NutraSmarts
Compare K2 ingredients on NutraSmarts
If you want to see how specific MK-7 ingredients are documented, we list clinical dosage benchmarks and evidence ratings for K2VITAL® (all-trans MK-7) and K2Quest® (fermentation-derived MK-7), plus a general vitamin K profile. You can browse the full ingredient database with no sign-up. If you formulate products, our free Supplement Facts label creator handles the FDA label format for you.
FAQ
Is vitamin K2 better than K1?
Not across the board. K1 is the form with an established intake target and a proven role in clotting. K2, especially MK-7, stays in the blood longer and is studied for bone and vascular roles, but trials have not yet confirmed those benefits as outcomes.
Is MK-7 better than MK-4?
In human absorption studies, yes. MK-7 is well absorbed and circulates for days, while MK-4 at nutritional doses often fails to raise serum levels. Better absorption does not prove better health results, though.
Can I take vitamin K2 with warfarin?
Only with your prescriber's approval. Warfarin works against vitamin K, so any change in vitamin K intake from food or supplements can shift your INR. The recommended approach is a consistent intake, and a K2 supplement is a change.
Does K2 remove calcium from arteries?
The proposed mechanism is that K2 activates proteins that help direct calcium away from soft tissue and toward bone. Some evidence supports reduced coronary calcification, but reviews say the clinical evidence is still evolving. It is not a proven way to clear existing plaque.
How much vitamin K do I need?
The Adequate Intake for adults is 120 mcg a day for men and 90 mcg for women. There is no official intake figure for K2 on its own.
Sources
- Vitamin K: Double Bonds beyond Coagulation, differences between K1 and K2 (PMC, 2019)
- Comparison of menaquinone-4 and menaquinone-7 bioavailability in healthy women (PMC, 2012)
- Vitamin K: sources, physiological role, kinetics, deficiency, detection, therapeutic use, and toxicity (PMC)
- Vitamin K Health Professional Fact Sheet (NIH Office of Dietary Supplements)
- Vitamin K (Harvard T.H. Chan School of Public Health, The Nutrition Source)
- Vitamin K2: What It Is and Its Benefits (Cleveland Clinic)